The first shell needs 2 electrons. The second and third shells need 8 electrons. If an electron is missing, it is borrowed from the bonding atom. A pair of electrons is then shared by both atoms. Carbon is the basis of biology, because its 4 bonds create structural diversity.
Organic chemistry is the study of molecules containing carbon.
Such molecules can get very complex. When molecules do not have carbon bonds, they are called minerals or inorganic molecules. On the left side of each molecule is a positive charged atom or group called a cation. On the right side, is a negative charged atom or group called an anion. The cations and anions are drawn to each other like magnets, but they don't form covalent bonds from electrons spinning around both.