CHEMICAL BONDING
Not surprisingly, the SAT Chemistry Subject Test will want you to know
something about bonding—that is, the way atoms join to form molecular or ionic
compounds.
You’ll need to remember, first of all, that bonding usually occurs because every
atom in the bond would like to end up with 8 electrons (a stable octet) in its
outermost shell.
There are three main types of bonds: ionic, covalent (nonpolar covalent and
polar covalent), and metallic.
The Ionic Bond
When an atom in a bond gives up 1 or more electrons to the atom it bonds with,
an ionic bond is formed. Ionic bonds generally form between atoms that differ
significantly in their electronegativity values. The atom that gives up the electron
becomes a positively charged ion, and the one that accepts the electron becomes
a negatively charged ion. The positively charged atom attracts the negatively
charged atom, and this draws the two atoms together and results in the release of
energy.
One term you should be familiar with for the test is lattice energy—
the lattice (binding) energy of an ionic solid is a measure of the energy
required to completely separate a mole of a solid ionic compound into
its separate ions. So, the higher the lattice energy, the stronger the ionic
bond.Now let’s look at an example of an ionic bond. When sodium (Na) bonds with
chlorine (Cl), the sodium atom gives up its outermost electron to become Na+
and the chlorine atom receives it to become Cl−. The Na+ ion has 8 electrons in
its outermost shell. (Its electron configuration looks like neon’s.)