Electronegativity

Electronegativity is a measure of the ability of an atom to attract and keep electrons. The greater the electronegativity of an atom, the greater the pull the nucleus of that atom exerts on electrons (its own and others). This pull on the electron is somewhat proportional to the degree of fullness of the atom's valence shell. If the valence shell is almost full then the electronegativity is great and the atom exerts great pull on any available electrons to try and fill the valence shell.

If however the valence shell is almost empty then the atom will have a very low electronegativity and the pull on the electron is such that the electron is easily lost.

Remember the gain or loss of an electron that leads to a full valence shell will make the atom less reactive.

If two atoms have an attraction for the same electron, then the atom with the greater electronegativity will win "the tug-of-war" for that electron.

If the difference in electronegativity between the two atoms is great enough, then the electron will be completely transferred and two ions will be formed.

In most cases the difference in electronegativity between two atoms is not great enough to lead to the transfer of an electron but instead simply results in the unequal sharing seen in a polar covalent bond.

There are some tendencies which are seen when looking at electronegativities of elements in the periodic table.

1. As you move down a column (or family) in the table the electronegativity decreases.

2. As you move across a row (left to right) the electronegativity increases UNTIL you reach the last column. The elements of the last column have a full valence shell and do not usually attract any additional electrons.

A good "rule of thumb" for determining relative electronegativity between to atoms in a bond pair, is to count the electrons in the outer shell and is how full it is.

Example 1

Oxygen has 6 electrons in its valence shell. The shell is 6/8ths full.

Hydrogen has 1 electron in its valence shell. The shell is 1/2 full (remember the first electron shell takes 2 electrons to fill it).

Oxygen is more electronegative than hydrogen. In a bond between oxygen and hydrogen the electrons are shared unequally with the oxygen getting more of the share.

Example 2

Carbon has 4 electrons in its valence shell. The shell is 1/2 full.

Hydrogen has 1 electron in its valence shell. The shell is 1/2 full (remember the first electron shell takes 2 electrons to fill it).

Carbon and hydrogen have similar electronegtivities and therefore tend to share the electrons in their bond equally

Example 3

This same process holds even in the case of multiple bonds. Consider the carbonyl group (Carbon double bonded to an oxygen).

Carbon has 4 electrons in its valence shell. The shell is 1/2 full.

Oxygen has 6 electrons in its valence shell. The shell is 6/8ths full.

Oxygen is more electronegative than Carbon. The double bond of the carbonyl group (between oxygen and carbon) has the electrons shared unequally, with the oxygen getting more of the share. This equal sharing is a polar bond.

Back to Chemical Bonds.