Cracking the AP Biology Exam


The Chemistry of Life


When two or more different types of atoms are combined in a fixed ratio, they form a chemical compound. You’ll sometimes find that a compound has different properties from those of its elements. For instance, hydrogen and oxygen exist in nature as gases. Yet when they combine to make water, they often pass into a liquid state. When hydrogen atoms get together with oxygen atoms to form water, we’ve got a chemical reaction:

2H2 (g) + O2 (g) → 2H2O (l)

The atoms of a compound are held together by chemical bonds, which may be ionic bonds, covalent bonds, or hydrogen bonds.

An ionic bond is formed between two atoms when one or more electrons are transferred from one atom to the other. In this reaction, one atom loses electrons and becomes positively charged and the other atom gains electrons and becomes negatively charged. The charged forms of the atoms are called ions. For example, when Na reacts with Cl, charged ions, Na+ and Cl-, are formed.

A covalent bond is formed when electrons are shared between atoms. If the electrons are shared equally between the atoms, the bond is called nonpolar covalent. If the electrons are shared unequally, the bond is called polar covalent. When one pair of electrons is shared between two atoms, the result is a single covalent bond. When two pairs of electrons are shared, the result is a double covalent bond. When three pairs of electrons are shared, the result is a triple covalent bond.