protactinium, and below cerium. Pure thorium is very ductile and, as normal for metals, can be cold-rolled, swaged, and drawn. At room temperature, thorium metal has a face-centred cubic crystal structure; it has two other forms, one at high temperature (over 1360 °C; body-centred cubic) and one at high pressure (around 100 GPa; body-centred tetragonal). Thorium metal has a bulk modulus (a measure of resistance to compression of a material) of 54 GPa, about the same as tin's (58.2 GPa). Aluminium's is 75.2 GPa; copper's 137.8 GPa; and mild steel's is 160–169 GPa. Thorium is about as hard as soft steel, so when heated it can be rolled into sheets and pulled into wire. Thorium is nearly half as dense as uranium and plutonium and is harder than both. It becomes superconductive below 1.4 K. Thorium's melting point of 1750 °C is above both those of actinium (1227 °C) and protactinium (1568 °C). At the start of period 7, from francium to thorium, the melting points of the elements increase (as in other periods), because the number of delocalised electrons each atom contributes increases from one in francium to four in thorium, leading to greater attraction between these electrons and the metal ions as their charge increases from one to four. After thorium, there is a new downward trend in melting points from thorium to plutonium, where the number of f electrons increases from about 0.4 to about 6: this trend is due to the increasing hybridisation of the 5f and 6d orbitals and the formation of directional bonds resulting in more complex crystal structures and weakened metallic bonding. (The f-electron count for thorium metal is a non-integer due to a 5f–6d overlap.) Among the actinides up to californium, which can be studied in at least milligram quantities, thorium has the highest melting and boiling points and second-lowest density; only actinium is lighter. Thorium's boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points. The properties of thorium vary widely depending on the degree of impurities in the sample. The major impurity is usually thorium dioxide ); even the purest thorium specimens usually contain about a tenth of a percent of the dioxide. Experimental measurements of its density give values between 11.5 and 11.66 g/cm3: these are slightly lower than the theoretically expected value of 11.7 g/cm3 calculated from thorium's lattice parameters, perhaps due