Molecules and Molecular Compounds

A molecule consists of two or more atoms bound together.

Each molecule has a chemical formula.

The chemical formula indicates

1. Which atoms are found in the molecule, and

2. In what proportion they are found.

A molecule made up of two atoms is called a diatomic molecule.

Different forms of an element which have different chemical formulas are known as allotropes. Allotropes differ in their chemical and physical properties.

Compounds composed of molecules are molecular compounds.

• These contain at least two types of atoms.

• Most molecular substances contain only nonmetals.

Molecular formulas

Give the actual numbers and types of atoms in a molecule.

Table 5-5a - Some Molecular Formulas and the Information they Convey
Formula / Composition
H2O / Total of 3 atoms in each molecule - 1 oxygen and 2 hydrogen
O2 / Total of 2 atoms in each molecule - 2 oxygen atoms
CO2 / Total of 3 atoms in each molecule - 1 carbon and 2 oxygen
C6H12O6 / Total of 24 atoms in each molecule - 6 carbon, 12 hydrogen, and 6 oxygen.

While the molecular formula shows the molecule as it actually exists, the Empirical formula for a compound shows the simplest whole number ratio for the elements in the compounds..

Table 5-5b - Comparing Empirical and Molecular Formulas
Compound / Molecular Formula / Empirical Formula
Water / H2O / H2O
Hydrogen Peroxide / H2O2 / HO
Glucose / C6H12O6 / CH2O
Methane / CH4 / CH4
Ethane / C2H6 / CH3
Octane / C8H18 / C4H9