When atoms of nonmetal elements form ions, they generally gain enough electrons to give them the same number of electrons as an atom of the next noble gas in the periodic table. The name of a metal ion is the same as the name of the metal atom from which it forms, so Ca 2+ is called a calcium ion. It has the same number of electrons as atoms of the preceding noble gas, argon, and is symbolized Ca 2+. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. For example, a neutral calcium atom, with 20 protons and 20 electrons, readily loses two electrons. To illustrate, an atom of an alkali metal (group 1) loses one electron and forms a cation with a 1+ charge an alkaline earth metal (group 2) loses two electrons and forms a cation with a 2+ charge, and so on. Atoms of many main-group metals lose enough electrons to leave them with the same number of electrons as an atom of the preceding noble gas. You can use the periodic table to predict whether an atom will form an anion or a cation, and you can often predict the charge of the resulting ion.
(b) A sodium cation (Na +) has lost an electron, so it has one more proton (11) than electrons (10), giving it an overall positive charge, signified by a superscripted plus sign. (a) A sodium atom (Na) has equal numbers of protons and electrons (11) and is uncharged. During the formation of some compounds, atoms gain or lose electrons, and form electrically charged particles called ions ( Figure 1). The transfer and sharing of electrons among atoms govern the chemistry of the elements. Electrons, however, can be added to atoms by transfer from other atoms, lost by transfer to other atoms, or shared with other atoms. In ordinary chemical reactions, the nucleus of each atom (and thus the identity of the element) remains unchanged.