6 Electronegativity and intermediate bonding

Electronegativity and intermediate bonding Definition Electronegativity is the relative tendency of an atom in a covalent bond in a molecule to attract electrons in a covalent bond to itself. F, O, N and Cl are the most electronegative atoms Factors affecting electronegativity Electronegativity increases across a period as the number of protons increases and the atomic radius decreases because the electrons in the same shell are pulled in more. It decreases down a group because the distance between the nucleus and the outer electrons increases and the shielding of inner shell electrons increases A compound containing elements of similar electronegativity and hence a small electronegativity difference will be purely covalent Formation of a permanent dipole – (polar covalent) bond A polar covalent bond forms when the elements in the bond have different electronegativities . (Of around 0.3 to 1.7) When a bond is a polar covalent bond it has an unequal distribution of electrons in the bond and produces a charge separation, (dipole) δ+ δ- ends. The element with the larger electronegativity in a polar compound will be the δ- end H – Cl + – A compound containing elements of very different electronegativity and hence a very large electronegativity difference (> 1.7) will be ionic e.g. CCl4 will be non-polar whereas CH3Cl will be polar A symmetric molecule (all bonds identical and no lone pairs) will not be polar even if individual bonds within the molecular ARE polar. Symmetric molecules The individual dipoles on the bonds ‘cancel out’ due to the symmetrical shape of the molecule. There is no NET dipole moment: the molecule is NON POLAR C H H H Cl δ+ δ- Electronegativity is measured on the Pauling scale (ranges from 0 to 4) The most electronegative element is fluorine and it is given a value of 4.0 Factors affecting electronegativity Electronegativity increases across a period as the number of protons increases and the atomic radius decreases because the electrons in the same shell are pulled in more. It decreases down a group because the distance between the nucleus and the outer electrons increases and the shielding of inner shell electrons increases