Electronegativity – a measure of the tendency of an atom to attract a shared pair (bonding pair) of electrons.
Covalent bonds involve two atoms sharing electrons. The more electronegative an atom is the greater its ability to attract the shared electrons.
In a molecule of hydrogen both hydrogen atoms are exactly the same. In the covalent bond that forms between them the shared pair of electrons are equally attracted to the nuclei of both atoms. On average the shared pair of electrons can be found in the middle of both atoms.
However, many bonds involve atoms of different elements. When a bond involves two different types of atom, the shared electrons are likely to be attracted to one of the atoms more than the other. The atom that attracts the electrons more is said to be more electronegative than the other atom.
Electronegativity and The Periodic Table
In general the electronegativity of atoms increases as you move from left to right across a period. For example, Oxygen is more electronegative than Lithium.
Electronegativity tends to decrease as you move down a group. For example, Caesium is less electronegative than Sodium.
This means that if we look at the table as a whole the least electronegative elements can be found in the bottom left of the table (such as Caesium, Francium and Barium) and the most electronegative atoms can be found in the top right (such as Oxygen, Chlorine and Fluorine).
The Pauling Scale
The Pauling Scale is used to measure the electronegativity of an atom.
Fluorine is the most electronegative element and the Pauling Scale assigns it a value of around 4. The other elements are scaled relative to Fluorine all the way down to Francium with a value of around 0.7.
Electronegativity and Bonding
The type of bond two atoms form is determined by the difference in their electronegativity.
Covalent Bonds
When there is no difference in electronegativity between two atoms (two atoms of the same element) or only a small difference a covalent bond forms.
For example, Hydrogen and Chlorine form a covalent bond. Chlorine is slightly more electronegative than Hydrogen. Although the shared pair of electrons are slightly more attracted to the Chlorine atom than the Hydrogen atom the attraction isn’t great enough to completely pull them away from the Hydrogen atom.
A small difference in electronegativity can make a covalent bond polar. Find out more here.
Ionic Bonds
A large difference in electronegativity results in the formation of an ionic bond. The more electronegative atom has the negative charge and the less electronegative atom has the positive charge.
As an example Sodium and Chlorine form an ionic bond. Chlorine is much more electronegative than Sodium and effectively takes the electron from Sodium forming two oppositely charged ions.
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