Atoms are the smallest unit of matter. They are comprised of (made of) three subatomic particles - protons, neutrons, and electrons. Protons are positively charged, neutrons are neutral (have neither positive nor negative charge), and electrons are negatively charged. The protons and neutrons are housed within the central nucleus of an atom. Thus, the overall charge of the nucleus is positive. The negative electrons travel around the nucleus. Some scientists claim the electrons travel within orbits around the nucleus while other use the terms "electron cloud", "electrons shell", or "energy level" to describe the location of electrons. Regardless, electrons travel at speeds so fast that it is impossible to identify their exact location at any given moment.
A universal theme in science is that "opposites attract". The opposite charges of the electrons and nucleus causes an electromagnetic force that keeps the electrons in orbit.
Electron shells are "layers' around the nucleus in which electrons can be located. Electron shells are designated by K, L, M, N, O, P and Q- with K being the shell closest to the nucleus and Q being the shell farthest away. Electrons in the outer shells have higher average energies. These electrons are those involved in chemical reactions. Electron capacity is the number of electrons that will fit into each shell, which can be determined using the following equation:
Electrons capacity = 2n^2
(where n= the energy level number).