Period 3 chemistry
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The Nuffield Chemistry students’ book (on page 389) mentions that the introduction of some amount of covalency into an lattice is likely to make the lattice energy more negative than one might expect (in other words, it might make the lattice ‘stronger’). This, if I understand correctly, is because the deformation in electron clouds allow ions to attract each other more strongly.
On page 391, the book states that Period 3 chlorides get less and less soluble as we go across period 3 - this makes sense to me, because the lattice energies get stronger, and the energy of hydration isn't likely to be able to overcome this large lattice energy. One page 390, however, the book states that as we go across period 3, the melting points of period 3 chlorides decrease - how is that possible if the lattice energies get more and more negative? Surely, melting involves breaking the lattice, and lattices with more covalent character (which are stronger) should be harder to melt...
Ulex says
I think it is important not to consider the lattice energy argument in isolation. It is legitimate to argue that, for a particular substance, one might calculate a lattice energy theoretically, and find that in practice the lattice energy is more negative than expected because of the ionic distortion which you noted from p383. It would be wrong to extend this to a comparison of one substance with another, however, because the extent of covalency is not the only factor which varies, in fact this is only a subtle effect causing small differences.
Chlorides get less soluble as you go across a Period (so long as hydrolysis reactions don’t occur) because the process of dissolving changes from being essentially ions separating and hydrating (Born-Haber Cycle argument) to being a matter of molecules separating and being attracted to water molecules (intermolecular forces argument).
Melting points are tricky because when a substance melts the forces between the molecules or ions it contains are only partially broken. There is a decrease of melting point as you go across a Period but lattice energy is not the only key to its explanation.
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Thanks, that does clarify things!
Daniel G. 02 April 2005
updated: 03 March 2005
