ELECTRONEGATIVITY

Electronegativity In Molecules

Electronegativity in Simple Molecules

Electronegativity is very important when looking at bonds in molecules because it tells us what type of bonding exists between the atoms.

Bonds are classified by the difference in electronegativity (which has the notation Den) between the elements. Bond polarities are a continuum, i.e. they are on a sliding scale, therefore bond classification is actually a continuum. Covalent and ionic bonds are theoretical extremes with most bonding occuring somewhere between the two.

For bonds between atoms of the same element, like those in molecules such as O₂, H₂ and Cl₂, and bonds between elements which have the same electronegativity, Den=0. This means that the electron density of the pair of electrons is shared totally equally between the atoms, and the bond is classed as being a pure covalent bond. However, generally a bond is considered covalent if Den<0.7. The picture below is a representation of the electron density of a purely covalently bonded two atom molecule.

As Den increases, electron density becomes less equally shared between the atoms and the electron cloud becomes distorted. More electron density is drawn closer to the more electronegative atom meaning that there is an unequal distribution of charge. This makes the bond polar, i.e. it has a slightly positive(d⁺) end and a slightly negative(d ^-) end, with the more electronegative atom being d ^- and the less electronegative atom being d⁺. The bond has a dipole moment.
A bond is generally classed as being polar covalent when 0.7<Den<1.7.

As Den becomes increasingly large, the electron density becomes even less equally shared, to such an extent that nearly all of the electron density is on the more electronegative atom. The partial charges on the atoms increase until they are so big that the atoms no longer have d⁺and d ^-, but a full + and - charge, with the - on the more electronegative atom and the + on the less electronegative atom. A bond is generally considered ionic when Den>1.7. The most ionic bond that could be formed is the bond between Francium and Fluoride where Den would be 3.28.

Double and triple bonds between two atoms are even more polar than single bonds between two atoms. This is because with double and triple bonds, there is twice or three times as much electron density to withdraw as in a single bond, so the distribution of electron density becomes even less equal, and the charge separation between the atoms is bigger with d⁺ and d ^- being bigger.

Why Does This Happen?

This happens because of the Principle of Electronegative Equalisation, which states that when two or more atoms (initially different in electronegativity) combine chemically, they adjust to have the same intermediate electronegativity. This intermediate electronegativity can be calculated by taking the geometric mean of the individual electronegativities of the component atoms. So, the electron density flows from the more electropositive atom to the more electronegative atom, creating a d⁺ charge on the former and ad ^- charge on the latter. As d⁺ increases on the electropositive atom, its effective nuclear charge increases, and so its electronegativity increases. The same trend happens in the opposite direction for the more electronegative atom until the two have the same intermediate electronegativity.

Move on to the next page: Electronegativity in complicated molecules