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Boiling Point of Hydrides

The boiling points of hydrides generally increase across a period from groups 4 to 7. Group 4 hydrides are tetrahedral, non-polar covalent molecules such as methane. From group 5 to 7 the electronegativity increases and the molecules become more polar.There are stronger dipole-dipole interactions. London forces also become stronger because there are more electrons.Water, hydrogen fluoride and ammonia do not follow the trends in boiling points of the hydrides of groups 4 to 7. This is because of hydrogen bonding between the molecules.

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Specification: AQA Chemistry (7404), AS-level, 3.1.3.7 Forces between molecules. Topic: Bonding.

Key terms in this lesson

Electronegativity
The attraction of a bonded atom for the pair of electrons in a covalent bond, measured on the Pauling scale.
Dipole
A separation of opposite partial charges caused by an uneven distribution of the bonding electron pair.
London forces
Weak intermolecular forces caused by instantaneous dipoles inducing dipoles in neighbouring molecules.
Hydrogen bond
A strong permanent dipole-dipole interaction between a hydrogen atom bonded to N, O or F and a lone pair on an electronegative N, O or F atom of another molecule.
Group
A vertical column of the periodic table, containing elements with the same number of outer-shell electrons.

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