As described in the previous chapter, metals, which are found in the left part of the periodic table,
generally form positive ions, whereas nonmetals, which are found in the right part of the periodic
table, generally form negative ions. Note, however, the existence of anions that contain metallic
elements, such as MnO 4 – (permanganate) and CrO 4 2– (chromate). All elements in a given group tend
to form monatomic ions with the same charge. Thus ions of alkali metals (Group I) usually form
cations with a single positive charge, the alkaline earth metals (Group II) form cations with a double
positive charge, and the halides (Group VII) form anions with a single negative charge.
Ionic compounds have characteristic physical properties. They have high melting and boiling points
due to the strong electrostatic forces between the ions. They can conduct electricity in the liquid
and aqueous states, though not in the solid state. Ionic solids form crystal lattices consisting of
infinite arrays of positive and negative ions in which the attractive forces between ions of opposite
charge are maximized, while the repulsive forces between ions of like charge are minimized.