The relative ability of an atom to attract electrons and form a covalent bond is known as electronegativity.

The electronegativity of atoms is dependent on how well they can attract electrons and hold on to them. As the nuclear charge of an atom increases but its shielding remains the same, its electronegativity increases. Electronegativity decreases down a group as the number of shells increases. This is because shielding increases so the electrons do not have as strong an attraction to the atom.

  • When an atom has a high electronegativity it is called electronegative.
  • When an atom has a low electronegativity it is said to be electropositive.

Electronegatives are relative. Electronegativity has no units and is measured on a scale from 0.7 to 4.0.

Electronegativity is a useful way in which to predict whether the bonds formed between two atoms will be ionic, covalent or metallic.

For example in a covalent bond between two atoms:

  • If both atoms are similar in terms of electronegativity then the electrons will be attracted by both equally and remain midway between them, forming a covalent bond.
  • If one atom has a higher electronegativity than the other then it will attract the electrons more strongly and so, even though the atoms still share the electrons, one has a slight deficit of electrons and is slightly positively charged while the other has a slight surplus of electrons and so is slightly negatively charged. This bond is called a polar covalent bond. A slightly positive charge is represented as d+ while a slightly negative charge is represented as d.
  • If the difference between the two atoms is very large then the sharing of the electrons is so uneven that atom with the highest electronegativity has basically complete possession of the elections. In this case, the electrons have basically been passed from one atom to the other.
  • If both atoms are electropositive then neither of them can attract electrons effectively and so these electrons do not stay localised within the bond. Instead, they are free to move, forming a metallic bond.

In general:

  • Metallic bond: the atoms have electronegativies lower than 1.6 – 1.9.
  • Covalent bond: one atom has an electronegativity higher than 1.9 and the difference between the atoms is lower than 0.5.
  • Polar covalent: one atom has an electronegativity higher than 1.9 and the difference between the atoms is higher than 0.5 but lower than 2.1.
  • Ionic bond: the difference between the atoms is higher than 2.1.

There are exceptions to these rules but these basic guidelines are very useful as long as their limitations are taken into consideration.