Formation of Covalent Compounds

Introduction:

Covalent compounds share two electrons in forming a bond between atoms. Covalent compounds are formed only by the interactions of non-metal atoms. The number of atoms which make up covalent molecules is determined by the number of electrons in outer levels and the Octet Rule.

HYDROGEN:

The simplest covalent molecule is the diatomic hydrogen molecule.

The Lewis Symbols are shown on the right.

The "octet" for hydrogen is only 2 electrons since the nearest rare gas is He. The diatomic molecule is formed because individual hydrogen atoms containing only a single electron are unstable. Since both atoms are identical a complete transfer of electrons as in ionic bonding is impossible -- How would you determine which atom should lose an electron and which atom should gain an electron?

Instead the two hydrogen atoms equally share both electrons.

By sharing electrons, the rare gas structure for He of 2 electrons is satisfied on each atom 50% of the time. The shared electrons in a covalent bond may be counted twice - once for each atom to determine whether the Octet Rule is satisfied.

IODINE:

A molecule of iodine can also be used to demonstrate covalent bonding. Click to see the graphic of the Lewis symbols.

There are 7 electrons in the outer shell, therefore, only one electron is needed to
complete the octet. The two iodine atoms actually share the two "7th" electrons.

Again, the two iodine atoms equally share both electrons.

OXYGEN:

Molecules of oxygen, present in about 20% concentration in air are also a covalent molecules . Click to see the graphic of the Lewis symbols.

There are 6 electrons in the outer shell, therefore, 2 electrons are needed to
complete the octet. The two oxygen atoms share a total of four electrons in two separate bonds. Both bonds are called double bonds.

The two oxygen atoms equally share the four electrons.

HYDROGEN CHLORIDE:

Example: Determine the formula for hydrogen chloride.
Solution : Click to see the graphic of the Lewis Symbols.

Determine that hydrogen needs to share one electron and chlorine needs to share one electron. Then write Lewis symbols for the molecule:.

Check to see whether the octet is complete around each atom by counting electrons including those shared;

For hydrogen: 2 e- = "octet"; For chlorine: 8 e- = octet

WATER OR HYDROGEN OXIDE:

Example: Determine the formula for water.
Solution : Click to see the graphic of the Lewis Symbols.

Determine that hydrogen needs to share one electron and oxygen needs to share two electrons. Then write Lewis symbols for the molecule and note that two hydrogens are needed to complete the octet of oxygen. This is why water is H-two-O.

Check to see whether the octet is complete around each atom by counting electrons including those shared;

For hydrogen: 2 e- = "octet"; For oxygen: 8 e- = octet

Octet Rule: Elemental atoms generally lose, gain, or share electrons with other atoms
in order to achieve the same electron structure as the nearest rare gas with eight
electrons in the outer level.

The proper application of the Octet Rule provides valuable assistance in predicting and explaining various aspects of chemical formulas.