Cracking the SAT Chemistry Subject Test

THE WORLD OF MOLECULES

As we mentioned in Chapter 4, an atom is the smallest particle of an element.
Atoms of one element often attract each other or atoms of a different element. If
this attraction is strong enough, a chemical bond can result. Chemical bonds join
two or more atoms into units called molecules. (We’ll talk more about bonding
in Chapter 7.)

Everybody knows the formula of water—it’s H 2 O. This formula tells us that, in a

molecule of water, 2 hydrogen atoms and 1 oxygen atom are bonded together
into a unit. An individual water molecule is the smallest unit of water that exists.

Diatomic Molecules

When a molecule consists of just two atoms (whether they are of identical or
different elements), it’s called a diatomic molecule. Some elements exist as
diatomic molecules at room temperature and atmospheric pressure (1 atm). For
example, oxygen in air exists in the form of O 2 . There are seven important

elements that exist as diatomic molecules: oxygen (O 2 ), iodine (I 2 ), hydrogen

(H 2 ), nitrogen (N 2 ), chlorine (Cl 2 ), fluorine (F 2 ), and bromine (Br 2 ).

Elements    as  Diatomic

Molecules

One good    way to

remember    all of  the

diatomic    molecules   is  the

following   mnemonic:

Oh  I   Have    Nice

Closets For Brooms

O 2 ,   I 2 ,   H 2 ,   N 2 ,   Cl 2 ,  F 2 ,   Br 2

Formula Weights, Empirical Formulas, and Percent Composition

For the SAT Chemistry Subject Test, you’ll need to know some simple atom-
molecule math.

1. 
   

        Formula Weight: For any molecule,   we  calculate   the formula weight

   
   

   by adding up the atomic weights of all the atoms in the molecule. It’s easy.
   Take hydrogen peroxide (H 2 O 2 ) as an example.