AS-Level Group 7 (Halogens)
The Halogens (and electronegativity)
The halogens are elements found in group 7 of the periodic table. They are all non-metals and have seven electrons in their outer shell. To get a full outer shell, halogens only have to gain one electron – it is this ‘desire’ to gain one electron that dictates their chemistry.
Trend in electronegativity
An atom’s electronegativity is its tendency to attract a pair of electrons in a covalent bond towards itself. The relative electronegativities of the halogens decreases as you go down the group.
Moving down the group – the number of electron sub-shells increases, meaning that the outermost electrons have more inner sub-shells between them and the nucleus. These extra inner sub-shells effectively dilute the positive charge from the nucleus that reaches the outermost electrons. This is known as shielding.
As the positive charge felt on the surface of the atom is reduced, the halogen is less able to attract electrons in a covalent bond towards itself. Remember electrons are negatively charged and are attracted to the positive nucleus of an atom or ion.
The smaller the halogen atom, the more electronegative it is; the larger the halogen atom, the less electronegative it is.
When two atoms with a large difference in electronegativity are covalently bonded together, the covalent bond becomes polar. This means that the electrons in the bond are not evenly shared between the two atoms.
As electronegativity decreases as you go down group 7, the polarity of halogen bonds also decreases.