Hydrogen bond
Hydrogen bonds can vary in strength from weak (1–2 kJ mol) to strong (161.5 kJ mol in the ion ). Typical enthalpies in vapor include:
Iron–hydrogen alloy
The density deficit is thought to be due to mixture of lighter elements such as silicon or carbon. Hydrogen has been thought unlikely because of its volatility, but recent studies have uncovered plausible mechanisms for its incorporation and permanence in the core. It is estimated that hcp FeH would be stable under those conditions. Iron–hydrogen alloys could have been formed in a reaction of iron with water in magma during the formation of the earth. Above 5 GPa, iron will split water yielding the hydride and ferrous ions:
Indeed, Okuchi obtained magnetite and iron hydride by reacting magnesium silicate, magnesium oxide, silica and water with metallic iron in a diamond cell at 2000 C. Okuchi argues that most of the hydrogen accreted to Earth should have dissolved into the primeval magma ocean; and if the pressure at the bottom of the magma was 7.5 GPa or more, then almost all of that hydrogen would have reacted with iron to form the hydride, which then would have sunk to the core where it would be stabilized by the increased pressure. Moreover, it appears that at those pressures iron binds hydrogen in preference to carbon.