http://www.ck12.org Chapter 7. Chemical Nomenclature
FIGURE 7.2
A crystal of table salt, sodium chloride,
is a large array of alternating positive
and negative ions. The purple spheres
represent the Na+ions, while the green
spheres represent the Cl−ions.The most straightforward way to describe this structure with a chemical formula is to give the lowest whole-number
ratio between the two ions. The formulas for ionic compounds are always empirical formulas. In the case of NaCl,
there are equal numbers of sodium ions and chloride ions in the salt crystal. In contrast, a crystal of magnesium
chloride has twice as many chloride ions as magnesium ions, so it has a formula of MgCl 2.
Monatomic Ions
Many ionic compounds consist of a positively charged metal cation and a negatively charged nonmetal anion that
are combined in whatever ratio will produce a compound that is electrically neutral. In other words, the total
positive charge is exactly canceled out by the total negative charge. In order to accurately write formulas for ionic
compounds, it is necessary to know the charges of the ions involved. We will begin our study withmonatomic ions,
which are ions that consist of a single atom with either a positive or negative charge.
Cations
Cations are positive ions that are formed when a metal atom loses one or more electrons. For the representative
elements, cations are generally formed by removing all of the valence electrons from the atom. Since the numbers
of valence electrons for the representative elements are constant within a particular group, all we need is the group
number of a given element to know its charge when it becomes a cation. Group 1 elements form ions with a 1+
charge, Group 2 metal ions have a 2+ charge, and the ions of Group 13 elements tend to have a 3+ charge. Heavier
p-block metals such as tin and lead are special cases and will be discussed with the transition metal ions. Monatomic
cations have the same name as their parent element. For example, Na+is the sodium ion, while Al^3 +is an aluminum
ion.