Molecular Compounds

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Many compounds are linked together in electrically neutral particles called molecules. Some may contain just two atoms (diatomic molecules). For example, CO is carbon monoxide, a poisonous gas. Most molecules are complex and can contain many more atoms. Water has three atoms, two hydrogens and one oxygen: H2O. Sucrose, or sugar has many more atoms with the formula C12H22O11. The molecules that occur in living organisms and plastics can contain millions of atoms.
The attractions that hold the atoms together are called chemical bonds. They arise from the sharing of electrons between one atom and another (will be discussed further in Atomic Structure and Bonding). Compounds that exist as molecules are called molecular compounds and the formulas that describe their compositions are called molecular formulas.
The properties of molecular compounds vary greatly, but they usually have little or no electrical conductivity properties.

Some simple molecular compounds are binary covalent compounds. They are formed between two nonmetals.

These are the rules to name covalent compounds:
  1. The first element in the formula is named first, using the full element name.
  2. The second element is named using the suffix -ide.
  3. Prefixes are used to denote the numbers of atoms present (see table below).
  4. The prefix mono- is only used to name the second element. For example, CO is carbon monoxide, not monocarbon monoxide.
Prefixes
Prefix Number Indicated
mono- 1
di- 2
tri- 3
tetra- 4
penta- 5
hexa- 6
hepta- 7
octa- 8
nona- 9
deca- 10
Examples
Compound Name
NO Nitrogen monoxide
N2O Dinitrogen monoxide
NO2 Nitrogen dioxide
N2O3 Dinitrogen trioxide
N2O4 Dinitrogen tetraoxide
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