Atomic Number 2 on the Periodic Table Atomic Number 2 on the Periodic Table Atomic Number 2 on the Periodic Table What Element Is Atomic Number 2? Helium is element atomic number 2 on the periodic table. Science Picture Co, Getty Images By Anne Marie Helmenstine, Ph.D. Updated October 02, 2016. Helium is the element that is atomic number 2 on the periodic table. Each helium atom has 2 protons in its atomic nucleus. The atomic weight of the element is 4.0026. Atomic Number 2 Facts The element is named for the Greek god of the sun, Helios, because it was initially observed in a previously unidentified yellow spectral line during the 1868 solar eclipse. Two scientists observed the spectral line during this eclipse: Jules Janssen (France) and Norman Lockyer (Britain). The astronomers share credit for the element discovery. Direct observation of the element did not occur until 1895, when Swedish chemists Per Teodor Cleve and Nils Abraham Langlet identified helium emanations from cleveite, a type of uranium ore.

A typical helium atom contains 2 protons, 2 neutrons, and 2 electrons. However, atomic number 2 can exist without any electrons, forming what is called an alpha particle . An alpha particle has an electrical charge of 2+ and is emitted during alpha decay . continue reading below our video Trends in the Periodic Table At ordinary temperature and pressure, helium is an extremely light, colorless gas. Helium is one of the noble gases or inert gases, which means it has a complete electron valence shell so it's not reactive. Unlike gas of atomic number 1 (hydrogen), helium gas exists as monatomic particles. The two gases have comparable mass (H2 and He). Single helium atoms are so small they pass between many other molecules. This is why a filled helium balloon deflates over time -- the helium escapes through tiny pores in the material. Atomic number 2 is the second most abundant element in the universe, after hydrogen. However, the element is rare on Earth (5.2 ppm by volume in the atmosphere) because nonreactive helium is light enough that it can escape Earth's gravity and be lost to space. Some types of natural gas, such as that from Texas and Kansas, contain helium.

The primary source of the element on Earth is from liquefaction from natural gas. The largest supplier of the gas is the United States. The source of helium is a non-renewable resource, so there may come a time when we run out of a practical source for this element. Atomic number 2 is used for party balloons, but it's primary use is in the cryogenic industry for cooling superconducting magnets. The principal commercial use of helium is for MRI scanners. The element is also used as a purge gas, to grow silicon wafers and other crystals, and as a protective gas for welding. Helium is used for research into superconductivity and the behavior of matter at a temperature approaching absolute zero . One distinctive property of atomic number 2 is that this element cannot be frozen into a solid form unless it is pressurized. Helium remains liquid down to absolute zero under normal pressure, forming a solid at temperatures between 1 K and 1.5 K and 2.5 MPa pressure. Solid helium has been observed to possess a crystalline structure.

Element Atomic Number 2 Facts and Projects noble gas | Definition, Properties, Uses, & Facts | Britannica.com neon (Ne) Noble gas, any of the seven chemical elements that make up Group 18 (VIIIa) of the periodic table . The elements are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), and oganesson (Og). The noble gases are colourless, odourless, tasteless, nonflammable gases . They traditionally have been labeled Group 0 in the periodic table because for decades after their discovery it was believed that they could not bond to other atoms ; that is, that their atoms could not combine with those of other elements to form chemical compounds . Their electronic structures and the finding that some of them do indeed form compounds has led to the more appropriate designation , Group 18. When the members of the group were discovered and identified, they were thought to be exceedingly rare, as well as chemically inert, and therefore were called the rare or inert gases.

It is now known, however, that several of these elements are quite abundant on Earth and in the rest of the universe , so the designation rare is misleading. Similarly, use of the term inert has the drawback that it connotes chemical passivity, suggesting that compounds of Group 18 cannot be formed. In chemistry and alchemy , the word noble has long signified the reluctance of metals , such as gold and platinum , to undergo chemical reaction ; it applies in the same sense to the group of gases covered here. The abundances of the noble gases decrease as their atomic numbers increase. Helium is the most plentiful element in the universe except hydrogen . All the noble gases are present in Earth’s atmosphere and, except for helium and radon, their major commercial source is the air , from which they are obtained by liquefaction and fractional distillation . Most helium is produced commercially from certain natural gas wells. Radon usually is isolated as a product of the radioactive decomposition of radium compounds. The nuclei of radium atoms spontaneously decay by emitting energy and particles, helium nuclei ( alpha particles ) and radon atoms.

Some properties of the noble gases helium polonium (Po) In 1785 Henry Cavendish , an English chemist and physicist, found that air contains a small proportion (slightly less than 1 percent) of a substance that is chemically less active than nitrogen . A century later Lord Rayleigh , an English physicist, isolated from the air a gas that he thought was pure nitrogen, but he found that it was denser than nitrogen that had been prepared by liberating it from its compounds. He reasoned that his aerial nitrogen must contain a small amount of a denser gas. In 1894, Sir William Ramsay , a Scottish chemist, collaborated with Rayleigh in isolating this gas, which proved to be a new element— argon . Apparatus used in the isolation of argon by English physicist Lord Rayleigh and chemist Sir William … Encyclopædia Britannica, Inc. After the discovery of argon, and at the instigation of other scientists, in 1895 Ramsay investigated the gas released upon heating the mineral clevite, which was thought to be a source of argon.

Instead, the gas was helium , which in 1868 had been detected spectroscopically in the Sun but had not been found on Earth . Ramsay and his coworkers searched for related gases and by fractional distillation of liquid air discovered krypton , neon , and xenon , all in 1898. Radon was first identified in 1900 by German chemist Friedrich E. Dorn; it was established as a member of the noble-gas group in 1904. Rayleigh and Ramsay won Nobel Prizes in 1904 for their work. In 1895 the French chemist Henri Moissan , who discovered elemental fluorine in 1886 and was awarded a Nobel Prize in 1906 for that discovery, failed in an attempt to bring about a reaction between fluorine and argon. This result was significant because fluorine is the most reactive element in the periodic table. In fact, all late 19th- and early 20th-century efforts to prepare chemical compounds of argon failed. The lack of chemical reactivity implied by these failures was of significance in the development of theories of atomic structure.

In 1913 the Danish physicist Niels Bohr proposed that the electrons in atoms are arranged in successive shells having characteristic energies and capacities and that the capacities of the shells for electrons determine the numbers of elements in the rows of the periodic table. On the basis of experimental evidence relating chemical properties to electron distributions, it was suggested that in the atoms of the noble gases heavier than helium, the electrons are arranged in these shells in such a way that the outermost shell always contains eight electrons, no matter how many others (in the case of radon, 78 others) are arranged within the inner shells. Britannica Stories Ringling Bros. Folds Its Tent In a theory of chemical bonding advanced by American chemist Gilbert N. Lewis and German chemist Walther Kossel in 1916, this octet of electrons was taken to be the most stable arrangement for the outermost shell of any atom. Although only the noble-gas atoms possessed this arrangement, it was the condition toward which the atoms of all other elements tended in their chemical bonding. Certain elements satisfied this tendency by either gaining or losing electrons outright, thereby becoming ions ; other elements shared electrons, forming stable combinations linked together by covalent bonds .

The proportions in which atoms of elements combined to form ionic or covalent compounds (their “ valences ”) were thus controlled by the behaviour of their outermost electrons, which—for this reason—were called valence electrons . This theory explained the chemical bonding of the reactive elements, as well as the noble gases’ relative inactivity, which came to be regarded as their chief chemical characteristic. (See also chemical bonding: Bonds between atoms .) Shell atomic model Periodic Table of the Elements Screened from the nucleus by intervening electrons, the outer (valence) electrons of the atoms of the heavier noble gases are held less firmly and can be removed (ionized) more easily from the atoms than can the electrons of the lighter noble gases. The energy required for the removal of one electron is called the first ionization energy . In 1962, while working at the University of British Columbia , British chemist Neil Bartlett discovered that platinum hexafluoride would remove an electron from (oxidize) molecular oxygen to form the salt [O2+][PtF6−]. The first ionization energy of xenon is very close to that of oxygen; thus Bartlett thought that a salt of xenon might be formed similarly.

In the same year, Bartlett established that it is indeed possible to remove electrons from xenon by chemical means. He showed that the interaction of PtF6 vapour in the presence of xenon gas at room temperature produced a yellow-orange solid compound then formulated as [Xe+][PtF6−]. (This compound is now known to be a mixture of [XeF+][PtF6−], [XeF+] [Pt2F11−], and PtF5.) Shortly after the initial report of this discovery, two other teams of chemists independently prepared and subsequently reported fluorides of xenon—namely, XeF2 and XeF4. These achievements were soon followed by the preparation of other xenon compounds and of the fluorides of radon (1962) and krypton (1963). Connect with Britannica Facebook Twitter YouTube Instagram Pinterest In 2006, scientists at the Joint Institute for Nuclear Research in Dubna , Russia , announced that oganesson , the next noble gas, had been made in 2002 and 2005 in a cyclotron .

(Most elements with atomic numbers greater than 92—i.e., the transuranium elements —have to be made in particle accelerators.) No physical or chemical properties of oganesson can be directly determined since only a few atoms of oganesson have been produced. General properties of the group Each noble-gas element is situated in the periodic table between an element of the most electronegative group, the halogen elements (Group 17, the atoms of which add electrons to achieve the octet and thereby become negative ions ), and an element of the most electropositive group, the alkali metals (Group 1, the atoms of which lose electrons to become positive ions). Several important uses of the noble gases depend on their reluctance to react chemically. Their indifference toward oxygen , for example, confers utter nonflammability upon the noble gases. Although helium is not quite as buoyant as hydrogen , its incombustibility makes it a safer lifting gas for lighter-than-air craft.

The noble gases—most often helium and argon , the least expensive—are used to provide chemically unreactive environments for such operations as cutting, welding , and refining of metals such as aluminum (atmospheric oxygen and, in some cases, nitrogen or carbon dioxide would react with the hot metal). The helium-filled balloon Bubble, manned by a scientist … James Aldred/Nature Picture Library Editor Picks: Exploring 10 Types of Basketball Movies The noble gases absorb and emit electromagnetic radiation in a much less complex way than do other substances. This behaviour is used in discharge lamps and fluorescent lighting devices: if any noble gas is confined at low pressure in a glass tube and an electrical discharge is passed through it, the gas will glow. Neon produces the familiar orange-red colour of advertising signs; xenon emits a beautiful blue colour. Neon signs along Nanking Road, Shanghai. Paul Souders/Corbis Noble gases have uses that are derived from their other chemical properties. The very low boiling points and melting points of the noble gases make them useful in the study of matter at extremely low temperatures .

The low solubility of helium in fluids leads to its admixture with oxygen for breathing by deep-sea divers : because helium does not dissolve in the blood , it does not form bubbles upon decompression (as nitrogen does, leading to the condition known as decompression sickness , or the bends ). Xenon has been used as an anesthetic ; although it is costly, it is nonflammable and readily eliminated from the body. Radon is highly radioactive ; its only uses have been those that exploit this property (e.g., radiation therapy ). (Oganesson is also radioactive, but, since only a few atoms of this element have thus far been observed, its physical and chemical properties cannot be documented.) Only krypton , xenon, and radon are known to form stable compounds. The compounds of these noble gases are powerful oxidizing agents (substances that tend to remove electrons from others) and have potential value as reagents in the synthesis of other chemical compounds . noble gases - Student Encyclopedia (Ages 11 and up) Six elemental gases are composed of such exceptionally stable atoms that they almost never react with other elements.

They are the gases that make up Group 0 (the rightmost column) of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). When they were first discovered, these gases were thought to be exceedingly rare and completely inactive; thus they were named the rare gases, inert gases, or noble gases (the term noble, in chemistry and alchemy, had long signified a lack of chemical reactivity). Article Contributors Chemical Elements.com - Helium (He) Bentor, Yinon. Chemical Element.com - Helium. < For more information about citing online sources, please visit the MLA's Website . This page was created by Yinon Bentor. Use of this web site is restricted by this site's license agreement . Copyright © 1996-2012 Yinon Bentor. All Rights Reserved. Atomic number 2 | definition of atomic number 2 by Medical dictionary Atomic number 2 | definition of atomic number 2 by Medical dictionary helium (He) [he´le-um] a chemical element, atomic number 2, atomic weight 4.003.

(See Appendix 6.) Helium is a chemically inert element that is odorless, tasteless, and noncombustible. Because of its low density it is easily moved through the air passages and therefore requires little effort in breathing on the part of the patient in respiratory distress. Although helium itself has no chemical therapeutic value, when combined with oxygen it facilitates delivery of oxygen to the lungs (see helium-oxygen therapy ). It should be noted that helium causes the voice to be high-pitched and the spoken word difficult to understand. This should be explained to the patient with the assurance that the effect is harmless and temporary. helium-oxygen therapy the administration of a mixture of helium and oxygen (commonly 80 per cent He and 20 per cent O2, or 70 per cent He and 30 per cent O2); used in the management of airway obstruction associated with bronchospasm or bronchial asthma. The He-O2 mixture is about one third the density of air. This reduces turbulent flow and the patient effort required for ventilation.

he·li·um (He), (hē'lē-ŭm), A gaseous element present in minute amounts in the atmosphere (0.000524% of dry volume); atomic no. 2, atomic wt. 4.002602; used as a diluent of medicinal gases; used as a diluent of oxygen principally in nonmedical applications, and in its liquid form as the coolant for super-conducting magnets (as in magnetic resonance imaging). [G. hēlios, the sun] helium /he·li·um/ (He) (hēl´e-um) chemical element, at. no. 2. It is obtained from natural gas. Used as a diluent for other gases, particularly with oxygen in the treatment of certain cases of respiratory obstruction, and as a vehicle for general anesthetics. helium (He) [hē′lē·əm] Etymology: Gk, helios, sun a colorless, odorless gaseous element; the second lightest element. Its atomic number is 2; its atomic mass is 4.00.

Helium is one of the rare or inert gases and does not combine with other elements. It occurs in the atmosphere at concentrations of five parts per million. Because of its lightness and lack of flammability, it is also used to lift airships and balloons. In the liquid state it is used for low-temperature activities. The main physiological and medical uses of helium are in respiratory therapy and testing and the prevention of nitrogen narcosis and decompression sickness in hyperbaric environments. Helium is one third as soluble in lipids as is nitrogen. That characteristic accounts for its preferred use in hyperbaric atmospheres, such as those associated with deep-sea diving. A mixture of 80% helium and 20% oxygen is commonly breathed by deep-sea divers to prevent gas emboli and by patients undergoing treatment to clear obstruction of the respiratory tract. Problems associated with such uses involve the high velocity of acoustic transmission in helium and the high thermal conductivity of the gas. These characteristics produce voice distortions and hypothermia in persons who inhale it. The low density of helium reduces the effort of breathing any gas mixture of which it is a component. Helium is used in pulmonary function testing to calculate the diffusion and residual capacities of the lungs.

he·li·um (hē'lē-ŭm) A gaseous element present in minute amounts in the atmosphere (0.000524% of dry volume); atomic no. 2, atomic wt. 4.002602; used as a diluent of medicinal gases, particularly oxygen. [G. hēlios, the sun] he·li·um (He) (hē'lē-ŭm) A gaseous element present in minute amounts in the atmosphere used as a diluent of medicinal gases. [G. hēlios, the sun] helium (He) (hē´lēum), n a colorless, odorless, tasteless gas; one of the inert gaseous elements. Atomic number, 2; atomic weight, 4.003. Used in medicine as a diluent for other gases. helium a chemical element, atomic number 2, atomic weight 4.003, symbol He. See Table 6. Helium is a chemically inert element that is odorless, tasteless and noncombustible.

Because of its low density it is easily moved through the air passages and therefore requires little effort in breathing on the part of the patient who is in respiratory distress. Although helium itself has no chemical therapeutic value, when combined with oxygen it facilitates the delivery of this gas to the lungs (see helium-oxygen therapy ). Atomic Number and Mass Number - Video & Lesson Transcript | Study.com Atomic Number and Mass Number Watch short & fun videos Start Your Free Trial Today An error occurred trying to load this video. Try refreshing the page, or contact customer support. You must create an account to continue watching Register for a free trial Are you a student or a teacher? I am a student Start Your Free Trial To Continue Watching As a member, you'll also get unlimited access to over lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed. Coming up next: Early Atomic Theory: Dalton, Thomson, Rutherford and Millikan You're on a roll. Keep up the good work! Your next lesson will play in 10 seconds 0:19 The Atom Add to Add to Add to Want to watch this again later? Log in or sign up to add this lesson to a Custom Course.

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Only Study.com members will be able to access the entire course. Create an account to start this course today Try it free for 5 days! Lesson Transcript Instructor: Kristin Born Kristin has an M.S. in Chemistry and has taught many at many levels, including introductory and AP Chemistry. Atoms are the basic building blocks of everything around you. In order to really understand how atoms combine to form molecules, it's necessary to be familiar with their structure. In this lesson, we'll dissect atoms so we can see just what really goes into those little building blocks of matter. Subatomic Particles The periodic table of elements Did you know that you are made up of mostly empty space? That is not meant to be an insult; more of an example of how you are made up of atoms and most of the volume of an atom is completely empty space. An atom is the smallest particle of an element that still has the same properties of that element. Remember that an element is a pure substance made up of only one type of atom. Thomson, Rutherford, and Millikan (all scientists) discovered that atoms were made up of even smaller particles called subatomic particles.

This lesson is going to focus on the three main subatomic particles: the proton, neutron, and electron. As we move through this lesson, I would highly recommend having a periodic table in front of you as a reference. The periodic table is an organized reference of all of the known elements arranged according to their properties. You will almost always have access to one of these, so it would be a good idea to learn how to use it. Protons Protons are probably one of the single most important parts of an atom. The number of protons an atom has determines what type of element it is. For example, all atoms that have only one proton are going to be atoms of hydrogen. Find hydrogen on the top-left side of your periodic table. You should notice a few things about this box. First, it may have a large H printed in it. This is the chemical symbol for hydrogen. Each element has a different chemical symbol. You should also notice that the number one is prominently displayed in the box. This indicates the number of protons that atoms of that element have. You may quickly notice that this number is increasing by one as you move from left to right and top to bottom on the periodic table.

So, helium has the number two, lithium has the number three, and so on. This means that all atoms of helium have two protons and all atoms of lithium have three protons. The number of protons is called the atomic number, and it is really special because it gives the atom its identity. Keep in mind that not all periodic tables are exactly the same so some may have the atomic number for each element in a different location. Each box in the period table shows the chemical symbol and atomic number of the element Another interesting feature of the proton is that it carries an electrical charge of positive one. Don't worry about the units of this charge - just remember that it has a charge of positive one. Protons are also relatively heavy. They have a mass of about one amu, or atomic mass unit. Because atoms are so tiny, they require a different type of unit to measure their mass. We will be using the atomic mass unit (or amu) when we discuss the masses of atoms and molecules. The final feature of a proton is that it is tucked away inside the nucleus of the atom. The nucleus is the very tiny, dense region in the center of an atom.

The nucleus is so tiny, that if the nucleus were the size of a basketball, the rest of the atom would be the size of a large city. One thing that gets a little confusing is that the word nucleus can mean different things depending on which branch of science you are in. In chemistry, the word nucleus refers to the nucleus (or center) of an atom. In biology, the word nucleus refers to the nucleus of a cell. To give you a visual idea of the difference between an atom and a cell, there are trillions upon trillions of atoms that make up each living cell. And atoms are not living - they are the building blocks of everything. Neutrons The next particle inside the atom is called the neutron. Just like protons, neutrons are also located inside the nucleus of the atom. Neutrons also have a mass of about one amu. So protons and neutrons are both located inside the nucleus and they both have a mass of about one amu. Unlike protons, however, neutrons do not carry an electrical charge. They are neutral particles. This should be very easy to remember because the words neutron and neutral are so similar. Remember that all atoms of the same element have the same number of protons.

Does this mean that they have to have the same number of neutrons? Nope! Atoms of the same element can have different numbers of neutrons. These atoms with the same number of protons, but different numbers of neutrons, are called isotopes. Think of isotopes like siblings - they are related, but have some differences. For example, all carbon isotopes have the same number of protons: six. However, some carbon atoms are going to have six neutrons, some will have seven, and some may have eight neutrons. These isotopes will all chemically react the same way, but they will have differences in their nuclear stability, which means that some will be able to undergo a nuclear reaction more easily than others. This is because they all have different amounts of neutrons in their nucleus. The mass number is equal to the atomic number plus the number of neutrons × ELEMENTS | COMPOUNDS | CHEMISTRY | DEFINITIONS ELEMENTS - DEFINITION What is an element? According to modern view, in simpler terms, an element can be defined as a substance made up of atoms of same atomic number. For example, hydrogen gas, H2 is an element since the atoms in it have the same atomic number i.e.

1. Recall that atomic number, Z is the number of protons in an atom. Whether elements are pure substances or mixtures? They may be either pure substances or mixtures! However, an old definition of an element is - it is a pure substance that cannot not be separated into simpler substances either by physical or by chemical means. But an element may consists of two or more isotopes which can be separated by techniques like mass spectrometry, diffusion etc. Isotopes are the atoms (or nuclides) with same atomic number but different mass numbers. For example, hydrogen gas consists of different isotopes like: Protium (1H1), Deuterium (1H2 ), Tritium (1H3), which can be separated, in principle, by diffusion. Note that the subscript represents atomic number, Z and the superscript indicates mass number, A = #protons + #neutrons. Thus an element may be either a pure substance consisting of atoms of same isotope or a mixture of different isotopes. It is not possible to find an element in the nature in its pure form since every element is associated with at least two isotopes.

It is also strictly not true to say that an element contains identical atoms due to above reasons. What makes an element different from other element? Atomic number. Yes. Different elements have different atomic numbers i.e. they differ by number of protons. E.g. The atomic number of hydrogen element is 1. Only one proton. Whereas, the atomic number of oxygen is 8. There are eight protons in the oxygen atom. Whether element exists in the free atomic state or in the molecular state? Noble gas elements, e.g., Helium, Neon, Argon etc., exist in the free uncombined atomic state. However, most of the elements are in the combined molecular form. E.g. H2 molecules are present in hydrogen gas. O2 molecules are present in dioxygen gas. What is the physical state of an element? It may exist in gaseous or liquid or solid state depending on the strength of intermolecular forces and conditions like temperature, pressure etc. How many elements have been discovered until now? Till to date, 117 elements are reported. In the near future the number may increase. You can see the names, symbols and atomic numbers of these elements in the modern long form of periodic table .

How many elements are present in the nature? According to one view, there are 92 naturally occurring elements and the rest are artificial. However, this number is debatable. According to another view there are only 88 naturally occurring elements since the elements like Technetium, Promethium, Francium and Astatine are present only in very minute quantities and are formed as short lived isotopes during the decay of other radioactive elements. If above elements are regarded as naturally occurring elements, then one has to consider the short lived Plutonium could occur naturally since traces of this element are discovered in uranium deposits. COMPOUNDS A chemical compound consists of two or more different types of atoms in combined state. For example, water is a compound made up of hydrogen and oxygen. Its formula is H2O i.e., there are two hydrogen atoms per every oxygen atom. Each oxygen atom is combined with two hydrogen atoms. Some other examples of compounds are: * common salt - NaCl * carbon dioxide - CO2 * Glucose - C6H12O6 There is a subtle difference between mixtures and compounds. For example, a few atoms of helium and neon in a container is a mixture.

These atoms are not in the chemically combined state. Author: Aditya vardhan Vutturi CHEMISTRY I: ATOMS AND MOLECULES CHEMISTRY I: ATOMS AND MOLECULES Table of Contents Atoms | Back to Top Most of the Universe consists of matter and energy . Energy is the capacity to do work. Matter has mass and occupies space. All matter is composed of basic elements that cannot be broken down to substances with different chemical or physical properties. Elements are substances consisting of one type of atom , for example Carbon atoms make up diamond, and also graphite. Pure (24K) gold is composed of only one type of atom, gold atoms. Atoms are the smallest particle into which an element can be divided. The ancient Greek philosophers developed the concept of the atom, although they considered it the fundamental particle that could not be broken down. Since the work of Enrico Fermi and his colleagues, we now know that the atom is divisible, often releasing tremendous energies as in nuclear explosions or (in a controlled fashion in) thermonuclear power plants. Subatomic particles were discovered during the 1800s.

For our purposes we will concentrate only on three of them, summarized in Table 1. The proton is located in the center (or nucleus ) of an atom, each atom has at least one proton. Protons have a charge of +1, and a mass of approximately 1 atomic mass unit (amu). Elements differ from each other in the number of protons they have, e.g. Hydrogen has 1 proton; Helium has 2. The neutron also is located in the atomic nucleus (except in Hydrogen). The neutron has no charge, and a mass of slightly over 1 amu. Some scientists propose the neutron is made up of a proton and electron-like particle. The electron is a very small particle located outside the nucleus. Because they move at speeds near the speed of light the precise location of electrons is hard to pin down. Electrons occupy orbitals, or areas where they have a high statistical probability of occurring. The charge on an electron is -1. Its mass is negligible (approximately 1800 electrons are needed to equal the mass of one proton). Table 1. Subatomic particles of use in biology.

Name electron orbital 9.1095 X 10-31 kg The atomic number is the number of protons an atom has. It is characteristic and unique for each element. The atomic mass (also referred to as the atomic weight) is the number of protons and neutrons in an atom. Atoms of an element that have differing numbers of neutrons (but a constant atomic number) are termed isotopes . Isotopes, shown in Figure 1 and Figure 2, can be used to determine the diet of ancient peoples by determining proportions of isotopes in mummified or fossilized human tissues. Biochemical pathways can be deciphered by using isotopic tracers. The age of fossils and artifacts can be determined by using radioactive isotopes, either directly on the fossil (if it is young enough) or on the rocks that surround the fossil (for older fossils like dinosaurs). Isotopes are also the source of radiation used in medical diagnostic and treatment procedures. Figure 1. Note that each of these isotopes of hydrogen has only one proton. Isotopes differ from each other in the number of neutrons, not in the number of protons.

Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Some isotopes are radioisotopes, which spontaneously decay, releasing radioactivity . Other isotopes are stable. Examples of radioisotopes are Carbon-14 (symbol 14C), and deuterium (also known as Hydrogen-2; 2H). Stable isotopes are 12C and 1H. Figure 2. Carbon has three isotopes, of which carbon-12 and carbon-14 are the most well known. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The Periodic Table of the Elements, a version of which is shown in Figure 3, provides a great deal of information about various elements. An on-line Periodic Table is available by clicking here , Figure 3. The Periodic Table of the Elements. Each Roman numeraled column on the label (at least the ones ending in A) tells us how many electrons are in the outer shell of the atom.

Each numbered row on the table tells us how many electron shells an atom has. Thus, Hydrogen, in column IA, row 1 has one electron in one shell. Phosphorous in column VA, row 3 has 5 electrons in its outer shell, and has three shells in total. Image from James K. Hardy's chemistry site at the University of Akron. Electrons and energy | Back to Top Electrons, because they move so fast (approximately at the speed of light), seem to straddle the fence separating energy from matter. Albert Einstein developed his famous E=mc2 equation relating matter and energy over a century ago. Because of his (and others) work, we think of electrons both as particles of matter (having mass is a property of matter) and as units (or quanta) of energy. When subjected to energy, electrons will acquire some of that energy, as shown in Figure 4. Figure 4. Excitation of an electron by energy, causing the electron to "jump" to another electron (energy) level known as the excited state. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

An orbital is also an area of space in which an electron will be found 90% of the time. Orbitals have a variety of shapes. Each orbital has a characteristic energy state and a characteristic shape. The s orbital is spherical. Since each orbital can hold a maximum of two electrons, atomic numbers above 2 must fill the other orbitals. The px, py, and pz orbitals are dumbbell shaped, along the x, y, and z axes respectively. These orbital shapes are shown in Figure 5. Energy levels (also referred to as electron shells) are located a certain "distance" from the nucleus. The major energy levels into which electrons fit, are (from the nucleus outward) K, L, M, and N. Sometimes these are numbered, with electron configurations being: 1s22s22p1, (where the first shell K is indicated with the number 1, the second shell L with the number 2, etc.).

This nomenclature tells us that for the atom mentioned in this paragraph, the first energy level (shell) has two electrons in its s orbital (the only orbital it can have), and second energy level has a maximum of two electrons in its s orbital, plus one electron in its p orbital. Figure 5. Geometry of orbitals. S-orbitals are spherical, p-orbitals are shaped like a dumbbell or figure 8. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Chemical Bonding | Back to Top During the nineteenth century, chemists arranged the then-known elements according to chemical bonding, recognizing that one group (the furthermost right column on the Periodic Table, referred to as the Inert Gases or Noble Gases) tended to occur in elemental form (in other words, not in a molecule with other elements). It was later determined that this group had outer electron shells containing two (as in the case of Helium) or eight (Neon, Xenon, Radon, Krypton, etc.) electrons.

As a general rule, for the atoms we are likely to encounter in biological systems, atoms tend to gain or lose their outer electrons to achieve a Noble Gas outer electron shell configuration of two or eight electrons. The number of electrons that are gained or lost is characteristic for each element, and ultimately determines the number and types of chemical bonds atoms of that element can form. Atomic diagrams for several atoms are shown in Figure 6. Figure 6. Atomic diagrams illustrating the filling of the outer electron shells. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Ionic bonds are formed when atoms become ions by gaining or losing electrons. Chlorine is in a group of elements having seven electrons in their outer shells (see Figure 6). Members of this group tend to gain one electron, acquiring a charge of -1. Sodium is in another group with elements having one electron in their outer shells. Members of this group tend to lose that outer electron, acquiring a charge of +1.

Oppositely charged ions are attracted to each other, thus Cl- (the symbolic representation of the chloride ion) and Na+ (the symbol for the sodium ion, using the Greek word natrium) form an ionic bond, becoming the molecule sodium chloride, shown in Figure 7. Ionic bonds generally form between elements in Group I (having one electron in their outer shell) and Group VIIa (having seven electrons in their outer shell). Such bonds are relatively weak, and tend to disassociate in water, producing solutions that have both Na and Cl ions. Figure 7. TOP: Formation of a crystal of sodium chloride. Each positively charged sodium ion is surropunded by six negatively charged chloride ions; likewise each negatively charged chloride ion is surrounded by six positively charged sodium ions. The overall effect is electrical neutrality. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. BOTTOM: Table Salt Crystal (SEM x625). This image is copyright Dennis Kunkel at , used with permission. Covalent bonds form when atoms share electrons.

Since electrons move very fast they can be shared, effectively filling or emptying the outer shells of the atoms involved in the bond. Such bonds are referred to as electron-sharing bonds. An analogy can be made to child custody: the children are like electrons, and tend to spend some time with one parent and the rest of their time with the other parent. In a covalent bond, the electron clouds surrounding the atomic nuclei overlap, as shown in Figure 8. Figure 8. Formation of a covalent bond between two Hydrogen atoims. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Carbon (C) is in Group IVa, meaning it has four electrons in its outer shell. Thus to become a "happy atom", Carbon can either gain or lose four electrons. By sharing the electrons with other atoms, Carbon can become a happy atom,. alternately filling and emptying its outer shell, as with the four hydrogens shown in Figure 9. Figure 9. Formation of covalent bonds in methane. Carbon needs to share four electrons, in effect it has four slots.

Each hydrogen provides an electron to each of these slots. At the same time each hydrogen needs to fill one slot, which is done by sharing an electron with the carbon. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The molecule methane (chemical formula CH4) has four covalent bonds, one between Carbon and each of the four Hydrogens. Carbon contributes an electron, and Hydrogen contributes an electron. The sharing of a single electron pair is termed a single bond. When two pairs of electrons are shared, a double bond results, as in carbon dioxide. Triple bonds are known, wherein three pairs (six electrons total) are shared as in acetylene gas or nitrogen gas. The types of covalent bonds are shown in Figure 10. Figure 10. Ways of representing covalent bonds. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Sometimes electrons tend to spend more time with one atom in the bond than with the other.

In such cases a polar covalent bond develops. Water (H2O) is an example. Since the electrons spend so much time with the oxygen (oxygen having a greater electronegativity, or electron affinity) that end of the molecule acquires a slightly negative charge. Conversely, the loss of the electrons from the hydrogen end leaves a slightly positive charge. The water molecule is thus polar, having positive and negative sides. Hydrogen bonds , as shown in Figure 11, result from the weak electrical attraction between the positive end of one molecule and the negative end of another. Individually these bonds are very weak, although taken in a large enough quantity, the result is strong enough to hold molecules together or in a three-dimensional shape. Figure 11. TOP: Formation of a hydrogen bond between the hydrogen side of one water molecule and the oxygen side of another water molecule. BOTTOM: The presence of polar areas in the amino acids that makeup a protein allows for hydrogen bonds to form, giving the molecule a three-dimensional shape that is often vital to that protein's proper functioning.

Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Chemical reactions and molecules | Back to Top Molecules are compounds in which the elements are in definite, fixed ratios, as seen in Figure 12. Those atoms are held together usually by one of the three types of chemical bonds discussed above. For example: water, glucose, ATP. Mixtures are compounds with variable formulas/ratios of their components. For example: soil. Molecular formulas are an expression in the simplest whole-number terms of the composition of a substance. For example, the sugar glucose has 6 Carbons, 12 hydrogens, and 6 oxygens per repeating structural unit. The formula is written C6H12O6. Figure 12. Determination of molecular weights by addition of the weights of the atoms that make up the molecule. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Chemical reactions occur in nature, and some also can be performed in a laboratory setting.

One such reaction is diagrammed in Figure 13. Chemical equations are linear representations of how these reactions occur. Combination reactions occur when two separate reactants are bonded together, e.g. A + B -----> AB. Disassociation reactions occur when a compound is broken into two products, e.g. AB -----> A + B. Figure 13. Diagram of a chemical reaction: the combustion of propane with oxygen, resulting in carbon dioxide, water, and energy (as heat and light). This chemical reaction takes place in a camping stove as well as in certain welding torches. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Biological systems, while unique to each species, are based on the chemical bonding properties of carbon. Major organic chemicals (those associated with or formed by the actions of living things) usually include some ratios of the following elements: C, H, N, O, P, S. All forms of matter are composed of one or more elements. Be able to list the major elements in living things. Describe how protons, electrons, and neutrons are arranged into atoms and ions.

Define the terms atomic number and atomic mass and be able to describe their sugnificance. Atoms with the same atomic number but a different mass number are isotopes. List the isotopes of hydrogen and of carbon. Be able to describe radioisotopes and list three ways they are used in biology. The union between the electron structures of atoms is known as the chemical bond. Be able to list and describe the three types of chemical bonds found in living things. Be able to describe the distribution of electrons in the space around the nucleus of an atom. An atom tends to react with other atoms when its outermost shell is only partly filled with electrons. Be able to discuss why this happens. Be able to define the two types of ions and describe thow ionic bonds form between positive and negative ions. In a covalent bond, atoms share electrons. List several elements that tend to form covalent bonds. Distinguish between a nonpolar covalent bond and a polar covalent bond and give an example of each. Define hydrogen bond and describe conditions under which hydrogen bonds form and cite one example. Explain what is meant by the polarity of the water molecule, and how the polarity of water molecules allows them to interact with one another.

Review Questions | Back to Top Which of these is not a subatomic particle? a) proton; b) ion; c) neutron; d) electron The outermost electron shell of every Noble Gas element (except Helium) has ___ electrons. a) 1; b) 2; c) 4; d) 6; e) 8 An organic molecule is likely to contain all of these elements except ___. a) C; b) H; c) O; d) Ne; e) N The chemical bond between water molecules is a ___ bond. a) ionic; b) polar covalent; c) nonpolar covalent; d) hydrogen A solution with a pH of 7 has ___ times more H ions than a solutrion of pH 9. a) 2; b) 100; c) 1000; d) 9; e) 90 The type of chemical bond formed when electrons are shared between atoms is a ___ bond. a) ionic; b) covalent; c) hydrogen The type of chemical bond formed when oppositely charged particles are attrached to each other is a ___ bond.

a) ionic; b) covalent; c) hydrogen Electrons occupy volumes of space known as ___. a) nuclei; b) periods; c) wavelengths; d) orbitals Carbon has an atomic number of 6. This means it has ___. a) six protons; b) six neutrons; c) six protons plus six neutrons; d) six neuitrons and six electrons Each of the isotopes of hydrogen has ___ proton(s). a) 3; b) 1; c) 2; d) 92; e) 1/2 A molecule is ___. a) a mixture of various components that can vary; b) a combination of many atoms that will have different ratios; c) a combination of one or more atoms that will have a fixed ratio of its components; d) more important in a chemistry class than in a biology class

Question: Which gaseous element has the Atomic Number 2?
Answer: HELIUM