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Boiling Points of Hydrides and Intermolecular forces

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. The hydrides in a period have the same number of electrons, so their London forces are similar.
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: OCR Chemistry A (H032), AS-level, 2.2.2 Bonding and structure. Topic: Shapes and Intermolecular Forces.

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 an instantaneous dipole inducing a dipole in a neighbouring molecule.
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.
Periodicity
The repeating trend in the properties of elements across each period of the periodic table.

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