MOLECULAR SHAPES
Some questions on the SAT Chemistry Subject Test might ask you about the
shapes of molecules. Although we can represent molecules in two dimensions on
paper, they are actually three-dimensional. If you are given a molecular formula
and asked to determine its shape, follow these preliminary steps:
Step 1: Assume the first atom in the formula is the central atom of the
structure (unless it is hydrogen, which is never a central atom).
Step 2: Using dots to indicate the valence electrons of each atom, surround
the central atom with the others, trying to give each atom an octet.
Remember hydrogen needs only 2, not 8, valence electrons to be satisfied.
It is important to realize that electrons shared between two atoms count
toward the total for both.Completing steps 1 and 2 will give you the structural formula of a molecule. To
determine the shape of the molecule, you must consider the number of sites in
which valence electron pairs surround the central atom. Since all electrons have
the same negative charge, they repel each other. The valence electron sites will
arrange themselves around the central atom to be as far from each other as
possible. There are two types of electron pair sites: those that contain electron
pairs in a bond and those that contain unbonded electron pairs (also called lone
pairs). The number of total electron pair sites and number of lone pairs will
dictate the molecule’s shape.
Suppose we have a molecule of carbon tetrachloride, CCl 4 . The structural
formula of CCl 4 is as follows: