1.2 Electronegativity and ionic bonding

Electronegativity

Electronegativity is a measure of how strongly an atom pulls a shared electron pair towards it. The table below shows the electronegativities (obtained from www.thecatalyst.org/electabl.html) of a number of elements:

Electronegativity

Electronegativity is a chemical property which describes the power of an atom to attract electrons towards itself.

The greater the electronegativity of an element, the stronger its attractive pull on electrons. For example, in a molecule of hydrogen bromide (HBr), the electronegativity of bromine (2.8) is higher than that of hydrogen (2.1), and so the shared electrons will spend more of their time closer to the bromine atom. Bromine will have a slightly negative charge, and hydrogen will have a slightly positive charge. In a molecule like hydrogen ( $H_2$ ) where the electronegativities of the atoms in the molecule are the same, both atoms have a neutral charge.

Non-polar and polar covalent bonds

Electronegativity can be used to explain the difference between two types of covalent bonds. Non-polar covalent bonds occur between two identical non-metal atoms, e.g. H ${}_2$ , Cl ${}_2$ and O ${}_2$ . Because the two atoms have the same electronegativity, the electron pair in the covalentbond is shared equally between them. However, if two different non-metal atoms bond then the shared electron pair will be pulled morestrongly by the atom with the highest electronegativity. As a result, a polar covalent bond is formed where one atom will have a slightly negative charge and the other a slightly positive charge. This is represented using the symbols ${\delta }^{+}$ (slightly positive) and ${\delta }^{-}$ (slightly negative). So, in a molecule such as hydrogen chloride (HCl), hydrogen is $H^{{\delta }^{+}}$ and chlorine is $C{l}^{{\delta }^{-}}$ .

Polar molecules

Some molecules with polar covalent bonds are polar molecules , e.g. H ${}_2$ O. But not all molecules with polar covalent bonds are polar. An example is $C{O}_2$ . Although $C{O}_2$ has two polar covalent bonds (between C ${}^{\delta +}$ atom and the two O ${}^{\delta -}$ atoms), the molecule itself is not polar. The reason is that CO ${}_2$ is a linear molecule and is therefore symmetrical. So there is no difference in charge between the two endsof the molecule. The polarity of molecules affects properties such as solubility , melting points and boiling points .

Polar and non-polar molecules

A polar molecule is one that has one end with a slightly positive charge, and one end with a slightly negative charge. A non-polar molecule is one where the charge is equally spread across the molecule.