Period 3 chemistry
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How is it possible for the sulphur in sulphur trioxide to have 3 double bonds? If this were the case it would have an outer shell containing 12 electrons.
Jenkin repliesYou have hit the nail on the head!
The ‘octet rule’ works well for compounds of elements in the second period, but as you say, in sulphur trioxide there are 12 electrons in sulphur’s outer shell. There are other well-known examples of this sort of thing: phosphorus pentachloride (PCl5), sulphur hexafluoride (SF6) and iodine heptafluoride (IF7).
In compounds such as these, there is a so-called ‘expanded octet’. This is possible because the elements in the third period have empty 3d subshells only slightly higher in energy than the 3s and 3p subshells and these can accommodate the extra electrons. There are no 2d subshells, so the period 2 elements, N, O and F cannot expand their octets.
Another factor in some compounds may be the size of the central atom; a phosphorus atom is big enough for five Cl atoms to fit around it but an N atom is not, so NCl5 does not exist.
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updated: 06 June 2005
