By Harold McGee
The electrons in atoms are arranged around the nucleus in orbits that determine how strongly any particular electron is held there. Some electrons are held close and tightly to the nucleus, while others range far away and are held more weakly. The behavior of the outermost electrons largely determines the chemical behavior of the elements. For example, the elements classified as metals—copper, aluminum, iron— hold their outermost electrons very weakly, and easily give them away to the atoms of other elements—oxygen, chlorine—that are hungrier for electrons, and that tend to grab up any that are loosely held. This imbalance in electrical pulls among different elements is the basis of most chemical reactions. Reactions are encounters among atoms and molecules that result in the loss, gain, or sharing of electrons, and thus changes in the properties of the atoms and molecules involved.