As we discussed, atoms of the same element have the same num- ber of protons. The atoms of each element have a characteristic way of losing, gaining, or sharing their electrons when interacting with other atoms to achieve stability. The way that electrons behave enables atoms in the body to exist in electrically charged forms called ions, or to join with each other into com- plex combinations called molecules. If an atom either gives up or gains electrons, it becomes an ion. An ion is an atom that has a positive or negative charge because it has unequal numbers of protons and electrons. Ionization is the process of giving up or gaining electrons. An ion of an atom is symbolized by writing its chemical symbol followed by the number of its positive ( ) or negative ( ) charges. Thus, Ca2 stands for a calcium ion that has two positive charges because it has lost two electrons.