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CH2 (WJEC Syllabus Topic 4.3): shapes of molecules and ions
(a) Explain what is meant by the terms lone pairs and bonding pairs of electrons and recall
The sequence of repulsions between: two bonding pairs a bonding pair and a lone pair two
Lone pair: a pair of electrons in the valence shell (i.e. outermost shell) not involved in
Bonding pair: two electrons shared between atoms to form a covalent bond.
Greatest repulsion lone pair - lone pair
lone pair - bond pair
bond pair - bond
(b) Explain the VSEPR principle in terms of minimising the total repulsions between electrons
in the Valence shell of a given molecule or ion, giving examples where appropriate
VSEPR is the Valence Shell Electron Pair Repulsion theory
It states that pairs of electrons repel one another so that there is minimum repulsion
This will cause the centres of the atoms in the molecule to define a particular shape.
Working out the shape of a molecule
1. Work out the number of valence electron pairs (both lone pair and bonding pair)
surrounding the central atom. Remember to allow for the formation of bonds and ions.
2. Consider how these will arrange themselves around the atom. The easiest way of
doing this is a dot and cross diagram.
3. Distinguish between lone pairs and bonding pairs to calculate bond angles
4. Describe the shape in terms of the atoms involved (lone pairs are now "invisible" on
the diagram. The pairs will space themselves out as far as possible but a double
bond should be treated as a single bond.
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CH4, SF6 and NH4
+) and apply the VSEPR principle to predict or explain the shapes of other specified
simple species involving up to six electron pairs in the valence shell of the central atom.
Boron is in group 3, so starts off with 3 electrons. It is forming 3 bonds, adding
another 3 electrons. There is no charge, so the total is 6 electrons - in 3 pairs.
Because it is forming 3 bonds there can be no lone pairs.…read more
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