An atom is the smallest unit of matter. Atoms are made of three subatomic particles- protons, neutrons, and electrons. Protons are positively charged, neutrons are neutrally charged (meaning they have neither a positive nor a negative charge), and electrons are negatively charged. The nucleus (center) of the atom is made of protons and neutrons. Thus, the nucleus has an overall positive charge. The negative electrons orbit the nucleus. Thus, the outer layer of an atom contains no protons nor neutrons.
The first orbit (that which is closest to the nucleus) will hold two electrons. All other orbitals want eight valence electrons in order to be stable.
The formula for finding the number of electrons that can occupy an energy level is:
Electron Capacity = 2n^2 (where n represents the number of the energy level in question).