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Rather than atoms losing and gaining electrons to form compounds, compounds can also be formed when atoms share electrons.

The resulting bonds are called covalent bonds.

When this happens, molecules are formed.

“A molecule is an assembly of two or more atoms tightly bound together.”

- Brown and LeMay -

The molecule behaves as a single, distinct object.

An example would be when hydrogen reacts with oxygen.

In the resulting molecule, there is a shared pair of electrons between each hydrogen and the oxygen.

This gives each hydrogen a share in two electrons - a complete outer shell.

And, it gives oxygen a share in 8 electrons - a complete outer shell.

A shared pair of electrons between two atoms is called a single covalent bond.

We could also represent this as a single line.

H H

OThis would be the structural formula.

In reality, the electron clouds would surround the nuclei of both atoms, and the molecule would probably look more like the model below:

For convenience, we simply write the formula for water as H2O.

Most molecular substances are formed between nonmetals.

Several elements exist in nature as diatomic molecules.

H2 F2 Cl2 I2 O2 N2

Formulas that show the actual number of atoms in a molecule are called molecular formulas.

Formulas that indicate the simplest whole number ratios of atoms present are called empirical formulas.

For ionic compounds, we are generally using the empirical formulas.

CHARACTERISTICS OF COVALENT COMPOUNDS

1. Have low melting points and boiling points - many would be gases or liquids at room temperature.

2. Would not conduct electricity.

3. Many are not soluble in water or other polar solvents.

Rules for naming binary molecular compounds:

1. The name of the element farthest to the left in the periodic table is given first.

2. If both elements are in the same group, the lower one is named first.

3. The name of the second element is given an -ide ending.

4. Greek prefixes are used to indicate the number of atoms present.

Examples: Cl2O dichlorine monoxide

NF3 nitrogen trifluoride

N2O4 dinitrogen tetroxide

Name the following compounds:

a. SO2

b. PCl5

c. N2O3

Give the chemical formula for:

a. Silicon tetrabromide

b. Disulfur dichloride

ACIDS

For the time being, acids are defined as substances that yield hydrogen ions (H+) when dissolved in water.

For compounds that end in -ide, the ending will change to -ic.

HCl hydrogen chloride (gas)

HCl hydrochloric acid as a water solution

H2S hydrogen sulfide (gas)

H2S hydrosulfuric acid as a water solution

HCN hydrogen cyanide (gas)

HCN hydrocyanic acid as a water solution

Other common acids are:

H2SO4 sulfuric acid

HNO3 nitric acid

H3PO4 phosphoric acid

H2CO3 carbonic acid

Bases will be defined, for the time being, as substances that yield hydroxide ion (OH-) when dissolved in water.

Some common bases are:

NaOH sodium hydroxide

KOH potassium hydroxide

Ca(OH)2 calcium hydroxide

NH4OH ammonium hydroxide

Note: when you react an acid with a base, the products will be water (H+ + OH- --> H2O) and a salt.

HCl + NaOH --> H2O + NaCl