The atom is the smallest fundamental unit of matter. Each element is made up of specific atoms, with each solid, gas, liquid, and plasma being made up of atoms. You can think of them as the LEGOs of life - they're the building blocks of everything you see around you.

The atom is made of even smaller particles, known as subatomic particles. Physicists have discovered a great number of subatomic particles, however, biochemists are only concerned with three - the proton, neutron, and electron - as they are the only three that contribute to the chemical behavior of the atom.

According to the nuclear model of the atom, the atom consists of a very dense, positively-charged inner region, which has negatively-charged electrons moving around it. 

The nucleus consists of protons and neutrons. Protons are positive particles, while neutrons are neutral particles. Both of these particles have an approximate mass of 1 atomic mass unit, or amu (1.0073 and 1.0087, respectively). 

Electrons are negatively charged, with a charge equal in magnitude to a proton. Electrons have a significantly smaller mass than a proton or neutron (5.486 x 10^-4 amu) and are continually moving around the nucleus. An attractive electrostatic force is responsible for keeping the negative electron bound to the positive nucleus. Due to the constant movement, we look at electron location in terms of orbitals, which refer to their probable location in space.

Atoms of a particular element have a characteristic number of protons, which is represented by the element's atomic number.

The number of neutrons does not have to be the same for each atom of a particular element. As the atomic mass of an element is affected by the number of protons and neutrons, this means that atoms of the same element can have different atomic masses. Atoms with the same atomic number, but different atomic masses, are known as isotopes of each other. In other words, atoms with the same number of protons but different a number of neutrons, are isotopes. As an example, carbon-12, which has 6 protons and 6 neutrons, is an isotope of carbon-14, which has 6 protons and 8 neutrons.

While nuclear reactions will affect the nucleus of an atom, often changing the element, chemical reactions will involve the gaining or losing of electron(s). When the number of electrons of an atom is changed, an ion, or charged atom, is formed.

A cation is positively charged, while an anion is negatively charged. One way to remember the difference is to think of "anion" as "a negative ion."

Ionic bonds are characterized by the transfer of electrons. Atoms of each element have their own unique properties, including how likely they are to give away an electron (ionization energy) and how readily an atom will accept additional electrons (electron affinity). When one atom gives away an electron, or multiple electrons, to another which readily accepts them, two ions are formed. These ions are held together in rigid, 3D structures due to strong electrostatic forces.

Molecules that resulted from ionic bonds dissociate into ions when dissolved in water.

A type of bonding that occurs when atoms share a pair of electrons is known as a covalent bond. The way that this works is that the positively charged nuclei are attracted to the concentration of negative charge from the electrons between the atoms. This electrostatic attractive force is strong enough to overcome the repulsive force that occurs from the two positive nuclei becoming closer to each other, essentially making the shared electrons work as glue to hold the atoms together.

A single covalent bond comes from a single shared electron pair. Multiple electron pairs can be shared, resulting in double bonds (two shared electron pairs) and triple bonds (three shared electron pairs).

The electrons are not always shared equally; bond polarity measures how equally they are shared.

Hydrogen Bonds have a slightly misleading name, as they are not technically bonds, but rather a force of attraction. 

They occur between polar molecules that contain a hydrogen which is bound to a highly electronegative element (usually an oxygen or a nitrogen). The partially positive hydrogen and the partially negative electronegative element are attracted to each other. Essentially, the bond looks like a hydrogen being sandwiched between two highly electronegative atoms.

Individual hydrogen bonds are weak and can be broken fairly easily. However, they often occur in rather large numbers, and as such play a very important role in an incredible number of biochemical molecules.