Metallic bonds are still present in the molten metal, and aren't entirely broken until it boils. When a metal melts, the bonds aren't completely broken – only loosened enough for the atoms to move around. Melting point isn't a good guide to the strength of the metallic bonds. You can't have it both ways! If a minor change of structure at magnesium-calcium makes a huge difference, then a major one at barium should make an even bigger difference. It just follows the general trend – suggesting that the major change of structure isn't making much difference. Although the barium melting point is lower than that of strontium, it isn't dramatically lower. That's a less efficient packing, and you might expect that to be reflected in a much weaker metallic bond. Any differences just due to the structures should only be minor.īy contrast, barium is 8-co-ordinated (like the Group 1 metals). In that case, you would expect the metallic bond to be similar in each case, because the orbitals are going to overlap and delocalise in the same sort of way. Each atom is touched by 12 surrounding atoms. Let's take this first, because that argument is relatively easy to demolish.ĭespite the fact that the first four elements have two different structures, those structures are both 12-co-ordinated. Why I don't believe this explanation The odd position of magnesium That is supposed to account for the fact that magnesium is out of line with the rest of the Group. All that matters is that there is a change in crystal structure between magnesium and calcium. Don't worry if you don't know what this means. Beryllium and magnesium are both hexagonal close-packed calcium and strontium are face-centred cubic barium is body-centred cubic. That means that the atoms are more easily separated to make a liquid and finally a gas.Īs you go down the Group, the arrangement of the atoms in the various solid metals changes. As the atoms get bigger, the nuclei get further away from these delocalised electrons, and so the attractions fall.

The atoms in a metal are held together by the attraction of the nuclei to the delocalised electrons. The oddity of magnesium has to be explained separately. The melting points get lower as you go down the Group because the metallic bonds get weaker. All of these elements are held together by metallic bonds.

Calcium atomic radius