Ions and Ionic Charges

Atoms are neutral particles with equal numbers of protons and electrons. An element is defined by the number of protons, which does not change; however, many atoms can gain or lose one or more electrons to become a charged particle.

When an atom or molecule gains or loses electrons, it is called an ion. Electrons, being much smaller in mass than protons, do not contribute significantly to the atomic mass. Therefore, the atomic mass of an ion is essentially the same as the neutral atom’s atomic mass.

When atoms or molecules lose electrons, they are called cations. Cations have a positive charge because they have more protons, which have a positive charge, than electrons, which have a negative charge. Cations are given the same name as their element, followed by the word ion.

A calcium atom has 20 protons and 20 electrons. It loses two electrons to become a  calcium ion with 20 protons and 18 electrons.  Because it now has two more positive charges than negative charges, it has an overall charge of positive two. 

The chemical symbol for an ion is written as the chemical symbol of the atom with the charge written on the right superscript position. The chemical symbol for a calcium ion is therefore written as Ca²⁺.

When atoms or molecules gain electrons, they are called anions. Anions have a negative charge because they have more electrons than protons. Anions are named by adding the suffix '-ide' to their element name.

A fluorine atom has 9 protons and 9 electrons. Fluorine gains one electron to become a fluoride ion with 9 protons and 10 electrons. Because it now has one more negative charge than positive charges, it has an overall charge of negative one.

The chemical symbol for fluoride is written as F⁻.  The one is usually omitted for ions with a positive or negative one charge.

Atoms will lose or gain specific numbers of electrons when forming an ion. Many of the charges for ions formed by the main group elements can be predicted using the periodic table. 

The metals, on the left side of the periodic table, will lose electrons. Group 1 alkali metals all lose one electron and have a positive one charge when they become an ion. Group 2 alkaline earth metals lose two electrons and have a positive two charge. 

The nonmetals, on the right side of the periodic table, will gain electrons. Group 17 halogens gain one electron and have a negative one charge. Groups 16 elements gain two electrons and will have a negative two charge.

Many transition metals can form cations with different charges. Group 18 noble gases do not generally form ions.