Electrons are the negatively charged particles that surround the nucleus of the atom. Their distance from the nucleus depends upon the amount of energy they have. Those with the most energy can travel the farthest away, while those with less are drawn closer.

These particles, as they position themselves around the center of the atom, have a tendency to pair up and form energy levels, or electron shells. Each level can contain a certain number of electrons. If a level fills and another electron is added, it forms a new level. For example, the first level, or orbital as it is sometimes misleadingly called, can contain two of these subatomic particles, and the second can contain eight.

Electrons are generally affected by three tendencies:

1. Electrons seem to pair
2. They are negatively charged and so are subject to the electrostatic attraction-repulsion rules
3. Atoms tend to fill their outermost electron energy level

These factors are the basis for the vast variety of chemical reactions and chemical bonds that occur in nature. As atoms transfer electrons to fill their outermost, or valence, electron shells and then react because of their unbalanced electrostatic charges, unique structures as simple as a water molecule or as complex as a DNA strand can form.