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An inversion centre (element i) may be located either:

1. In a single atom at the centre of a molecule; or
2. In space at the centre of a molecule, often at the midpoint of a bond.

If the centre is in space, then all the atoms must be present in pairs, located either side of the centre. If it is at an atom, then that atom only must be present in an odd number. Thus AB₃ molecules, for example, can never have an inversion centre.

Indicate which of the following molecules have inversion centres:

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