COVALENT COMPOUNDS
Simple Compounds Ionic Compounds  Elmhurst College
Polar Covalent Review Covalent Compounds    Chemistry Department
Non polar Covalent Compare Covalent & Ionic  Virtual ChemBook

 

 Covalent Compounds

Introduction to Covalent Bonding:

Bonding between non-metals consists of two electrons shared between two atoms. Using the Wave Theory, the covalent bond involves an overlap of the electron clouds from each atom. The electrons are concentrated in the region between the two atoms. In covalent bonding, the two electrons shared by the atoms are attracted to the nucleus of both atoms. Neither atom completely loses or gains electrons as in ionic bonding.

There are two types of covalent bonding:

1. Non-polar bonding with an equal sharing of electrons.

2. Polar bonding with an unequal sharing of electrons. The number of shared electrons depends on the number of electrons needed to complete the octet.

NON-POLAR BONDING results when two identical non-metals equally share electrons between them. One well known exception to the identical atom rule is the combination of carbon and hydrogen in all organic compounds.

HYDROGEN:

The simplest non-polar covalent molecule is hydrogen. Each hydrogen atom has one electron and needs two to complete its first energy level. Since both hydrogen atoms are identical, neither atom will be able to dominate in the control of the electrons. The electrons are therefore shared equally. The hydrogen covalent bond can be represented in a variety of ways as 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.

Instead the two hydrogen atoms SHARE both electrons equally.

POLAR BONDING results when two different non-metals unequally share electrons between them. One well known exception to the identical atom rule is the combination of carbon and hydrogen in all organic compounds.

The non-metal closer to fluorine in the Periodic Table has a greater tendency to keep its own electron and also draw away the other atom's electron. It is NOT completely successful. As a result only partial charges are established. One atom becomes partially positive since it has lost control of its electron some of the time. The other atom becomes partially negative since it gains electron some of the time.

HYDROGEN CHLORIDE:

Hydrogen Chloride forms a polar covalent molecule. The graphic on the left shows that chlorine has 7 electrons in the outer shell. Hydrogen has one electron in its outer energy shell. Since 8 electrons are needed for an octett, they share the electrons.

However, chlorine gets an unequal share of the two electrons, although the electrons are still shared (not transfered as in ionic bonding), the sharing is unequal. The electrons spends more of the time closer to chlorine. As a result, the chlorine acquires a "partial" negative charge. At the same time, since hydrogen loses the electron most - but not all of the time, it acquires a "partial" charge. The partial charge is denoted with a small Greek symbol for delta.