Why does vanadium have a higher melting point than calcium? Explain in terms of the type and strength of bonding.
I came across this question when I was looking at some ten-year series at Popular book shop yesterday afternoon out of boredom. I was stumped.
In light of the recent directive from Mindef, it made me sit down and think through my options. In other words, my plan Bs. I have worked out a few options and am actually quite pleased with them.
I attempted the question in my head on the spot...
Vanadium and Calcium are both metals and have metallic bonds present in both atoms. However the former is a transition metal. Metallic bonding is described as an attraction between the positive cations and a sea of delocalised electrons. Its strength is determined by firstly, the number of valence of electrons the atom contributes to its sea of delocalised electrons and secondly, the charge of the positive ions.
Vanadium, because of the 3d and 4s orbitals that have similar energy levels, can therefore allow its valence electrons to come from any of the two orbitals. This explains why vanadium can exhibit variable oxidation states. This, when compared to calcium, means that vanadium atom contributes more valence electrons to its sea of delocalised electrons and hence its higher melting point.
Furthermore, this also means that the charge of the positive ions for vanadium is greater than than of the +2 for calcium and given that the size of the vanadium atom is smaller than that of a calcium atom. Hence the charge density of vanadium is comparatively higher than that of calcium which translates to a greater attraction between the positive ions and delocalised electrons in vanadium than in calcium. This means that there is a stronger metallic bonding in vanadium than calcium thereby contributing to the higher melting point in vanadium as more energy is required to break the metallic bonds in vanadium than in calcium.
I checked against my notes that have been acting coy for these past months. I had a grin on my face.
I will leave for Taiwan tomorrow morning. I will be back in a month's time. If there are no changes which I very much doubt so, I will board the plane on 29 April 2006 at 11.15pm at Chang Kai Sheik airport.
---till later---
