Electronegativity is the power of an atom in a molecule to attract electrons toward itself – we are typically talking about two atoms that are covalently bonded, with the more electronegative of the atoms pulling the shared pair of bonding electrons away from the less electronegative atom.
Electronegativity cannot be directly measured but it is essentially a result of the overall atomic core charge of an atom and the atom’s size. The charge on the atomic core takes into account the number of protons in the nucleus and the electrons in the inner shell e.g. fluorine has a core with a charge of +7 (9 p+ – 2e–) . Small atoms with a high atomic core / nuclear charge will attract electrons to themselves strongly.
The Pauling scale is commonly used to compare the electronegativity of different atoms. Generally, electronegativity increases across a period and decreases down a group.
If we form a covalent bond between two atoms of different electronegativity, as in hydrogen fluoride, the shared pair of bonding electrons will be attracted far more strongly to the fluorine atom and away from the hydrogen atom because fluorine is considerably more electronegative than hydrogen. The resulting bond is described as polar.
As a guide …
- C-H, C-I and H-I bonds are considered to be non-polar
- H-Br and C-Br bonds are slightly polar
- H-O, H-N, H-Cl, H-F, C-O, C-N, C-Cl and C-F bonds are polar
All of this has a huge effect on the physical properties of molecules (in terms of intermolecular bonding) and in determining how they react chemically.