Chemistry

External link: Periodic table at ptable.com

An **atom** is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small; typical sizes are around 100 picometers (a ten-billionth of a meter, in the short scale).

## Componests

Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and typically a similar number of neutrons. Protons and neutrons are called nucleons. More than 99.94% of an atom's mass is in the nucleus.

The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. If the number of protons and electrons are equal, that atom is electrically neutral. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion.

The electrons of an atom are attracted to the protons in an atomic nucleus by this electromagnetic force. The protons and neutrons in the nucleus are attracted to each other by a different force, the nuclear force, which is usually stronger than the electromagnetic force repelling the positively charged protons from one another.

The electron is by far the least massive of these particles. Under ordinary conditions, electrons are bound to the positively charged nucleus by the attraction created from opposite electric charges.

Protons have a mass 1,836 times that of the electron. The number of protons in an atom is called its atomic number.

Neutrons have a free mass of 1,839 times the mass of the electron.

## Bohr model

Main articles: Bohr model
In 1913 the physicist Niels Bohr proposed a model in which the electrons of an atom were assumed to orbit the nucleus but could only do so in a finite set of orbits, and could jump between these orbits only in discrete changes of energy corresponding to absorption or radiation of a photon. This quantization was used to explain why the electrons orbits are stable (given that normally, charges in acceleration, including circular motion, lose kinetic energy which is emitted as electromagnetic radiation, see *synchrotron radiation*) and why elements absorb and emit electromagnetic radiation in discrete spectra.

Later in the same year Henry Moseley provided additional experimental evidence in favor of Niels Bohr's theory. These results refined Ernest Rutherford's and Antonius Van den Broek's model, which proposed that the atom contains in its nucleus a number of positive nuclear charges that is equal to its (atomic) number in the periodic table. Until these experiments, atomic number was not known to be a physical and experimental quantity. That it is equal to the atomic nuclear charge remains the accepted atomic model today.

## Valence

Valency is the combining power of an element. It is equal to number of hydrogen atoms that atom can combine or displace in forming compounds. The outermost electron shell of an atom in its uncombined state is known as the valence shell, and the electrons in
that shell are called valence electrons. The number of valence electrons determines the bonding
behavior with other atoms. Atoms tend to chemically react with each other in a manner that fills (or empties) their outer valence shells.

For example, a transfer of a single electron between atoms is a useful approximation for bonds that form between atoms with one-electron more than a filled shell, and others that are one-electron short of a full shell, such as occurs in the compound sodium chloride and other chemical ionic salts.
However, many elements display multiple valences, or tendencies to share differing numbers of electrons in different compounds.

The chemical elements are often displayed in a periodic table that is laid out to display recurring chemical properties, and elements with the same number of valence electrons form a group that is aligned in the same column of the table. (The horizontal rows correspond to the filling of a quantum shell of electrons.) The elements at the far right of the table have their outer shell completely filled with electrons, which results in chemically inert elements known as the noble gases.

## Elements

A red circle is an unpaired electron in the outermost shell.

A blue circle is a lone pair of electrons.

Each electron shell is twice the radius of the previous shell.

Note: The metallic forms of the elements have electronic structures that differ slightly from the one the Aufbau principle predicts.

## Molecules

Each unpaired electron in the outermost shell (called the valence shell) forms a chemical bond with an unpaired electron in the outermost shell of a neighboring atom.

Pure Hydrogen gas consists of diatomic molecules.

Hydrogen (H2), Fluorine (F2), Chlorine (Cl2), Bromine (Br2), and Iodine (I2) all form diatomic molecules.

So does Nitrogen (N2) and Oxygen (O2).

Atoms that have no unpaired electrons in their valence shell are called noble gases. Noble gases do not form chemical bonds and are therefore Monatomic gases.

Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn) are noble gases.

A single molecule of Hydrogen Fluoride consists of one Fluorine atom and one hydrogen atom and is therefore diatomic.

Fluorine is a powerful oxidizer. In fact, Fluorine is the most electronegative element known. (It really likes electrons).

Fluorine is much more electronegative than hydrogen. This causes Hydrogen Fluoride to be a polar molecule.

A single molecule of Water (H2O) consists of one oxygen atom and two hydrogen atoms.

Water is a polar molecule.

A single molecule of Ammonia (NH3) consists of one nitrogen atom and three hydrogen atoms.

Ammonia is a polar molecule.

A single molecule of Methane (CH4) consists of one carbon atom and four hydrogen atoms.

Methane is a hydrocarbon. Hydrocarbons are non-polar molecules and therefore not soluble in water.

Methane is a gas. It boils at -161 °C.

A single molecule of Ethane (C2H6) consists of two carbon atoms and six hydrogen atoms.

Ethane is a gas. It boils at -88 °C.

A single molecule of Ethanol (C2H6O), also known as drinking alcohol, consists of two carbon atoms, six hydrogen atoms, and one oxygen atom.

One part of the molecule is polar and the other part is non-polar.

A single molecule of Octane (C8H18) consists of eight carbon atoms and eighteen hydrogen atoms.

Octane is a liquid. It boils at 125 °C.

If the chain were even longer and the hydrogen atoms were replaced with fluorine atoms then you would have teflon.

But Benzene is a little bit different.

A single molecule of Benzene consists of six carbon atoms and six hydrogen atoms arranged in a ring. Each carbon atom bonds with 2 other carbon atoms and with one hydrogen atom thus accounting for three unpaired electrons per carbon atom. But, as we already know, each carbon atom has four unpaired electrons so one electron per carbon atom is unaccounted for. There are six carbon atoms therefore a total of six electrons per benzene molecule are unaccounted for. Those six delocalized electrons form the blue circle around the molecule. See also: Aromaticity, Superatom, and Solvated electron. Benzene molecules are shaped like flat plates. (See Discotic liquid crystal.)

A single molecule of Pyridine consists of five carbon atoms, five hydrogen atoms, and one nitrogen atom arranged in a ring. Six unpaired electrons form the blue ring around the molecule.

Pyridine is a nitrogen heterocycle.

Graphene consists of carbon atoms densely packed in a regular hexagonal pattern:

Carbon nanotubes consist of a two-dimensional hexagonal lattice of carbon atoms, bent and joined in one direction so as to form a hollow cylinder:

A single molecule of Buckminsterfullerene is a Truncated icosahedron of carbon atoms:

## Acids and Bases

The terms nucleophile and electrophile are more or less interchangeable with Lewis base and Lewis acid, respectively.

Boron trifluoride is a Lewis acid:

Its Boron atom has a vacant orbital which can form a covalent bond by sharing a lone pair of electrons on an atom in a base (in this case ammonia). See Coordinate covalent bond and Adduct.

A molecule of Antimony pentafluoride consists of one Antimony atom and five Fluorine atoms.

Its Antimony atom has a vacant orbital which can form a covalent bond by sharing a lone pair of electrons with a sixth Fluorine atom (actually its an ion) thus creating Antimony hexafluoride.

Note that the molecule in the image above has a net negative charge of -1.

I would guess that the reason the six pairs of electrons are able to share the outer electron shell in antimony is because of sd5 Orbital hybridisation.

Antimony hexafluoride ions form a salt with Fluoronium ions creating Fluoroantimonic acid which is the strongest acid known with H0=-28.

Thats 1016 times stronger than 100% sulfuric acid which has H0=-12.
It has been shown to protonate even hydrocarbons (which cannot normally be "protonated"). It can only be stored in containers lined with Teflon.

From Wikipedia:Grotthuss mechanism:

**Proton jumping** is the process by which an 'excess' proton diffuses through the hydrogen bond network of water molecules or other hydrogen-bonded liquids through the formation and concomitant cleavage of covalent bonds involving neighboring molecules. The details of the hopping and transport mechanism are still debated.

A **superbase** is a compound that has a high affinity for protons. A "superbase" has been defined as an organic compound whose basicity is greater than that of proton sponge (pictured below), which has a conjugate pKa of 12.1