Electrons are by far the smallest of the three atomic particles, their mass and size is too small to be measured using current technology. They have a negative charge. Protons and neutrons are of similar size and weight to each other, protons are positively charged and neutrons have no charge.
Most atoms have a neutral charge; because the number of protons (positive) and electrons (negative) are the same, the charges balance out to zero. However, in ions (different number of electrons) this is not always the case, and they can have a positive or a negative charge. Protons and neutrons are made out of quarks, of two types; up quarks and down quarks. A proton is made of two up quarks and one down quark and a neutron is made of two down quarks and one up quark.
The nucleus is in the middle of an atom. It is made up of protons and neutrons. Usually in nature, two things with the same charge repel or shoot away from each other. So for a long time it was a mystery to scientists how the positively charged protons in the nucleus stayed together. They solved this by finding a particle called a "gluon". Its name comes from the word "glue" as gluons act like atomic glue, sticking the protons together using the "strong nuclear force". It is this force which also holds the quarks together that make up the protons and neutrons.
The number of neutrons in relation to protons defines whether the nucleus is stable or goes through radioactive decay. When there are too many neutrons or protons, the atom tries to make the numbers the same by getting rid of the extra particles. It does this by emitting radiation in the form of alpha, beta or gamma decay. Nuclei can change through other means too. Nuclear fission is when the nucleus splits into two smaller nuclei, releasing a lot of stored energy. This release of energy is what makes nuclear fission useful for making bombs and electricity, in the form of nuclear power.
The other way nuclei can change is through nuclear fusion, when two nuclei join together, or fuse, to make a heavier nucleus. This process requires extreme amounts of energy in order to overcome the electrostatic repulsion between the protons, as they have the same charge. Such high energies are most common in stars like our Sun, which fuses hydrogen for fuel.
Electrons orbit, or travel around, the nucleus. They are called the atom's "electron cloud". They are attracted towards the nucleus because of the electromagnetic force. Electrons have a negative charge and the nucleus always has a positive charge, so they attract each other.
Around the nucleus, some electrons are further out than others, in different layers. These are called "electron shells". In most atoms the first shell has two electrons, and all after that have eight. Exceptions are rare, but they do happen and are difficult to predict. The further away the electron is from the nucleus, the weaker the pull of the nucleus on it. This is why bigger atoms, with more electrons, react more easily with other atoms.
The electromagnetism of the nucleus is not strong enough to hold onto their electrons and atoms lose electrons to the strong attraction of smaller atoms.
Some elements, and many isotopes, have what is called an "unstable nucleus". This means the nucleus is either too big to hold itself together or has too many protons or neutrons. When this happens the nucleus has to get rid of the excess mass or particles. It does this through radiation. An atom that does this can be called "radioactive". Unstable atoms continue to be radioactive until they lose enough mass/particles that they become stable. All atoms above atomic number 82 (82 protons, lead) are radioactive.
There are three main types of radioactive decay; alpha, beta and gamma.
Every radioactive element or isotope has what is named a "half-life". This is how long it takes half of any sample of atoms of that type to decay until they become a different stable isotope or element. Large atoms, or isotopes with a big difference between the number of protons and neutrons will therefore have a long half life, because they must lose more neutrons to become stable.
Marie Curie discovered the first form of radiation. She found the element and named it radium. She was also the first female recipient of the Nobel Prize.
Frederick Soddy conducted an experiment to observe what happens as radium decays. He placed a sample in a light bulb and waited for it to decay. Suddenly, helium (containing 2 protons and 2 neutrons) appeared in the bulb, and from this experiment he discovered this type of radiation has a positive charge.
James Chadwick discovered the neutron, by observing decay products of different types of radioactive isotopes. Chadwick noticed that the atomic number of the elements was lower than the total atomic mass of the atom. He concluded that electrons could not be the cause of the extra mass because they barely have mass.
Enrico Fermi, used the neutrons to shoot them at uranium. He discovered that uranium decayed a lot faster than usual and produced a lot of alpha and beta particles. He also believed that uranium got changed into a new element he named hesperium.
Otto Hahn and Fritz Strassmann repeated Fermi's experiment to see if the new element hesperium was actually created. They discovered two new things Fermi did not observe. By using a lot of neutrons the nucleus of the atom would split, producing a lot of heat energy. Also the fission products of uranium were already discovered: thorium, palladium, radium, radon and lead.
Fermi then noticed that the fission of one uranium atom shot off more neutrons, which then split other atoms, creating chain reactions. He realised that this process is called nuclear fission and could create huge amounts of heat energy.
That very discovery of Fermi's led to the development of the first nuclear bomb code-named 'Trinity'.
Astronomy
Astronomy (from the Greek "astron" (ἄστρον) meaning "star" and "nomos" (nόμος) meaning "law") is the scientific study of celestial bodies such as stars, planets, comets, and galaxies. The imaginary patterns in the night sky are called constellations.
The objects studied include stars, galaxies, planets, moons, asteroids, comets and nebulae. Phenomena outside the Earth's atmosphere are also studied. That includes supernovae explosions, gamma ray bursts, and cosmic microwave background radiation. Astronomy concerns the development, physics, chemistry, meteorology and movement of celestial bodies, as well as the structure and development of the Universe.
Astronomy is one of the oldest sciences. Ancient Greek people used the positions of the stars to navigate, and to find when was the best time to plant crops. Astronomy is very similar to astrophysics. A related subject, cosmology, is concerned with studying the Universe as a whole, and the way the universe changed over time. Astronomy is not the same as "astrology", the belief that motion of the stars and the planets may affect human lives.
Since the 20th century there have been two main types of astronomy, "observational" and "theoretical" astronomy. Observational astronomy uses telescopes and cameras to "observe" or look at stars, galaxies and other astronomical objects. Theoretical astronomy uses maths and computer models to explain the observations and predict what might happen. Working together, theories predict what should happen and observations show whether the predictions work. The main work of astronomy is to explain puzzling features of the universe. For thousands of years the most important issue was the motions of planets; now many other topics are studied.