Question: why dose gallyum have a very low mellting point

Sheila Castilho answered on 14 Nov 2018:

Jesus, I have no idea! So I had to do some research what I found is that there isn’t really an answer for that yet but ..

“From an simplistic/observational standpoint, it’s because of the unusual structure of solid gallium. The atoms form pairs (dimers) which are strongly bonded to each other (distance of 2.4 Å, about the same as in metallic aluminum), but weakly bonded to other neighboring atoms (2.7-8 Å distance). For this reason, it doesn’t form a very well-ordered or strong crystal structure in the first place. So it does not take much energy to break these already-diffuse bonds and move the dimers around, that is, to form a liquid. Although completely breaking the bonds still requires a lot of energy; so gallium has a low melting point and yet a boiling point that’s similar to other metals. In most metals, the atoms are bound to each other more equivalently, so the melting point is higher. Or to put it another way, the crystal structure of gallium is more similar to that of a molecular compound than most metals, it’s a ‘metallic molecular crystal’.

This isn’t a very good answer though. If something has a low melting point, it’s because the bonds within the solid are weak – so it doesn’t say much. It doesn’t answer the question of why gallium forms this particular structure rather than (for instance) the stronger and more ordered structure that aluminum has. It can actually do that under extreme pressures (the Gallium-IV allotrope), so it’s actually a quantitative question of why this particular crystal structure has the lowest energy (by a quite significant amount).”

Hope it helps!