y the melting point of Mg is the lower than Be,Ca,Sr,& Ba. ?

Question

can any1 explain y this happen in detail? thanks...

Answer 1

Weaker intermolecular forces allows for a lower melting point

Answer 2

Metallic bond is weak in Mg because of the manner in which atoms are arranged in its crystal lattice. Moreover, the enthalpy of fusion of Mg is approximately 8.48 KJ/ mol ( It is the amount of thermal energy which must be absorbed or evolved for 1 mole of a substance to change states from a solid to a liquid or vice versa ).

But, honestly, it would be tempting to say that (apart from magnesium) the lower melting points as you go down the Group reflect weaker metallic bonds, but this is dangerous. The metallic bond isn't fully broken on melting. Boiling point is usually a better guide to the strengths of the forces involved.

You will sometimes come across an explanation for the low melting point of magnesium in terms of the way the atoms pack in the crystal. It isn't very difficult to pick holes in this argument and, in any case, the boiling point of magnesium is also surprisingly low - and packing is completely irrelevant when you are thinking about a liquid. In the case of magnesium, there must be some factor producing a relatively weak metallic bond. One reason could be the low conductivity and the high ionization enthalpy of Mg.

Answer 3

Because of the higher ionization energies limit delocalization of the lattice structure of the metallic bonds. (THis also results in lower conductivity )

For details see:
http://www.madsci.org/posts/archives/oct99/939213786.Ch.r.html