The wonders of gallium

 A friend recently showed me that solid gallium can melt in your hand.

I did not know this. I was quite familiar with liquid mercury, but not gallium. 

The existence of elemental gallium was predicted by Mendeleev in 1869 after he constructed the periodic table. It was discovered within six years. He was able to predict that it would have a low-melting temperature, based on extrapolations from the known melting temperatures of elements close to it in the periodic table.

Solid gallium is soft enough to be cut with a knife.

Three different stable crystal structures for solid gallium are shown below.

The phase diagram of pure gallium is shown below.

Note the negative slope of the phase boundary between the liquid and the solid alpha-Ga. This is like water. It follows from the Clausius-Clapeyron equation that the solid state has lower density than the liquid state. Gallium is the only elemental metal with this property. (The semi-metals antinomy and bismuth also do).

Gallium remains liquid over a wider range of temperatures (2373 K) than any other known substance.

The figures above are taken from the following paper from 2020.

Ab initio phase diagram and nucleation of gallium  Haiyang Niu, Luigi Bonati, Pablo M. Piaggi, and Michele Parrinello

Unfortunately, that paper does not provide much insight into the low melting temperature. The key is that the solid state contains dimers of Ga, that are weakly bonded to each other. A helpful discussion is the introduction to the following paper.

On the bonding of Ga2, structures of Gan clusters and the relation to the bulk structure of gallium 

N. Gaston and A.J. Parker

The image above is from the entry on Gallium in the beautiful book The Elements by Theodore Gray.

I thank my young friend Alexey for introducing me to the wonders of gallium.