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Chemical Bonding 131
9-3 Ionic Bonds and Lattice Energy
Don’t Forget!
Quick Tip
Ionic bonding involves the transfer of electrons from one atom to another. The
more electronegative element gains electrons. The less electronegative element
loses electrons. This results in the formation of cations and anions. Usually, an
ionic bond forms between a metal and a nonmetal. The metal loses electrons to
form a cation. The nonmetal gains electrons to become an anion. The attraction
of the opposite charges forms an ionic solid.
Cations have a positive charge and anions have a negative charge.
Elements on the left side of the periodic table (the metals) react with elements
on the right side (the nonmetals) to form ionic compounds (salts).
The number of electrons lost or gained depends on the number of electrons
necessary to lead to an octet. In general, an atom can gain or lose one or two
and on rare occasions three electrons, but not more than that. Potassium, K, has
one valence electron in energy level 4. If it loses that one, it only has three filled
shells remaining. The outer shell has an octet. Bromine, Br, has seven valence
electrons, so if it gains one electron it will have its octet. A chemical reaction
takes place between the potassium and bromine with an electron moving from
the potassium atom to the bromine atom—completing the octet for both. In this
manner, the ionic compound potassium bromide, KBr, forms.
The reaction of magnesium, with two valence electrons, and chlorine, with
seven valence electrons will produce magnesium chloride. The magnesium
must donate one valence electron to each of two chlorine atoms. This leaves a
magnesium ion and two chloride ions. All the ions have a complete octet. The
ions form the ionic compound magnesium chloride, MgCl 2.