Covalent bonding

Atoms share pairs of electrons (rather than donating as in ionic bonding). A single pair bonding atoms is a single bond, and obviously, a double bond is formed when a double pair is involved.

A nonpolar covalent bond involves the even distribution of electrons; when the bond is polar, the electrons are shared unevenly between two atoms which creates a slight charge at the ends of the molecule. The charge is positive where the electrons spend the least time and negative where they spend the most time.

Energy-level shells

There are seven of these known to exist, they are formed concentrically around the nucleus of an atom and contain electrons. There are never more than two electrons in the shell nearest the nucleus. More electrons are in the outer shells. Only the electrons in the outer shell can participate in chemical reactions. The shell is stable when it has the maximum number of electrons possible. If the shell is unstable, it will gain, lose, or share electorns with other atoms.