The Shapes of Molecules and Ions

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The Shapes of Molecules and Ions...
The Electron Pair Repulsion Theory...
We can predict the shape of a simple covalent molecule consisting of a central
atom, and a number of other atoms using the ideas that:
Each pair of electrons around the atom will repel all other electron
pairs...
The pairs of electrons will therefore take up positions as far apart as
possible to minimise repulsion...
This is called the electron pair repulsion theory.
Electron pairs may be:
A shared pair...
Or a lone pair...
Electron Pairs Shape Name of Shape Bond Angle Example
2 Linear 180° BeCl2
3 Trigonal Planar 120° BF3
4 Tetrahedral 109.5° CH4
90°
5 Trigonal Bipyramidal 120° PCl5
6 Octahedral 90° SF8
3 Bonding Pairs
1 Lone Pair Pyramidal 107° NH3
2 Bonding Pairs
2 Lone Pairs Angular 105° H2O
4 Bonding Pairs
2 Lone Pairs Square Planar 90° XeF4

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Two pairs of electrons...
If there are two pairs of electrons around the atom, the molecule will be
linear. The furthest away the atoms can get from one another is 180° apart.
Three pairs of electrons...
If there are three pairs of electrons around the central atom, they will be 120°
apart. The molecules shape is trigonal planar.
Four pairs of electrons...
If there are four pairs of electrons, they are furthest apart when they are
arranged in a tetrahedral shape.…read more

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Five pairs of electrons...
If there are four pairs of electrons the shape is a trigonal bipyramid with bond
angles of 120°.
Six pairs of electrons...
If there are six pairs of electrons the shape is an octahedral, and the bond
angles are 90°.
Ammonia
Ammonia has four pairs of electrons and one lone pair. When naming the shapes
of the molecules, the lone pair isn't included. Although ammonia has 4 pairs of
electrons, it is seen as three pairs and one lone pair.…read more

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Repulsion between lone pair of electrons, is greater than the repulsion
between shared electrons.
The thumb rule is 2° per lone pair.…read more

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