Shapes of Molecules

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What do we know about how electrons exist in orbitals and how can we use this to predict the shape of a simple covalent molecule?
Electrons in molecules exist in pairs in orbitals. Each pair of electrons around an atom will repel all other electron pairs and the pairs of electrons will therefore take up positions as far apart as possible to minimise repulsion.
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If there are two pairs of electrons around the atom in the middle of the molecule, what shape will the molecule take?
It will be a linear molecule because the furthest away from each other the two pairs can get is 180°.
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If there are three pairs of electrons around the atom in the middle what shape will the molecule take?
It will be a trigonal planar molecule because the bonding angle will be 120°.
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If there are four pairs of electrons around the atom in the middle what shape will the molecule take?
If there are four pairs of electrons, they are furthest apart when they are arranged so that they point to the four corners of a tetrahedron. This shape, with one atom positioned at the centre, is called tetrahedral.
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If there are five pairs of electrons around the atom in the middle what shape does the molecule take?
The shape usually adopted is that of trigonal bipyramid.
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If there are six pairs of electrons around the atom in the middle what shape does the molecule take?
The shape adopted is octahedral.
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In terms of lone pairs and bonding pairs, list how much the pairs will repulse each other, from the most to the least.
Lone pair - Lone pair; Lone pair - bonding pair; Bonding pair - Bonding pair.
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What is the approximate rule for the change in bonding angles where a lone pair is concerned?
It decreases by about 2° per lone pair.
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Why is bonding pair-lone pair repulsion stronger than bonding pair-bonding pair repulsion?
In ammonia, for example, the bonding pairs of electrons are attracted to the nitrogen nucleus and the hydrogen nucleus. However the lone pairs are only attracted to the nitrogen nucleus so is pulled close to it. This makes the repulsion bigger.
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What happens to a tetrahedral molecule that contains a lone pair of electrons?
The electron pairs form form a tetrahedron but the bonds form a triangular pyramid.
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Other cards in this set

Card 2

Front

If there are two pairs of electrons around the atom in the middle of the molecule, what shape will the molecule take?

Back

It will be a linear molecule because the furthest away from each other the two pairs can get is 180°.

Card 3

Front

If there are three pairs of electrons around the atom in the middle what shape will the molecule take?

Back

Preview of the front of card 3

Card 4

Front

If there are four pairs of electrons around the atom in the middle what shape will the molecule take?

Back

Preview of the front of card 4

Card 5

Front

If there are five pairs of electrons around the atom in the middle what shape does the molecule take?

Back

Preview of the front of card 5
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