In ordinary chemical reactions, the nucleus — which contains the protons and neutrons of each atom and thus identifies the element — remains unchanged. Electrons, however, can be added to atoms by transfer from other atoms, lost by transfer to other atoms, or shared with other atoms. The transfer and sharing of electrons among atoms govern the chemistry of the elements. During the formation of some compounds, atoms gain or lose electrons to form electrically charged particles called ions.
When an atom loses electrons, it forms a positive ion called a cation. Cations have more protons than electrons. Likewise, when an atom gains electrons, it forms a negative ion called an anion. Anions have more electrons than protons.
The periodic table helps to predict whether an atom will form an anion or a cation, and also the charge of the resulting ion. Moving from the far left to the right on the periodic table, atoms of main-group metals lose enough electrons to leave them with the same number of electrons as an atom of the preceding noble gas. They form cations with a charge equal to the group number. 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. For example, a neutral calcium atom, with 20 protons and 20 electrons, readily loses two electrons. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. It has the same number of electrons as atoms of the preceding noble gas, argon, and is symbolized Ca2+. The name of a metal ion is the same as the name of the metal atom from which it forms, so Ca2+ is called a calcium ion.
Moving from the far right to the left on the periodic table, atoms of nonmetal elements generally gain enough electrons to give them the same number of electrons as an atom of the next noble gas in the periodic table. They form anions with a negative charge equal to the number of groups moved left from the noble gases. For example, atoms of group 17 elements (one group left of the noble gases) gain one electron and form anions with a 1− charge; atoms of group 16 elements (two groups left) gain two electrons and form ions with a 2− charge, and so on. For example, the neutral bromine atom, with 35 protons and 35 electrons, can gain one electron to provide it with 36 electrons. This results in an anion with 35 protons, 36 electrons, and a 1− charge. It has the same number of electrons as atoms of the next noble gas, krypton, and is symbolized Br−.
The trend between the charge of the ion and position of the group on the periodic table can be used as a guide in many cases. However, its predictive value decreases when moving toward the center of the periodic table. Transition metals and some other metals often exhibit variable charges that are not predictable by their location in the table. For example, copper can form ions with a 1+ or 2+ charge, and iron can form ions with a 2+ or 3+ charge.
This text is adapted from Openstax, Chemistry 2e, Section 2.6: Molecular and Ionic Compounds.
