Non-covalent interactions
A hydrogen bond (H-bond), is a specific type of interaction that involves dipole-dipole attraction between a partially positive hydrogen atom and a highly electronegative, partially negative oxygen, nitrogen, sulfur, or fluorine atom (not covalently bound to said hydrogen atom). It is not a covalent bond, but instead is classified as a strong non-covalent interaction. It is responsible for why water is a liquid at room temperature and not a gas (given water's low molecular weight). Most commonly, the strength of hydrogen bonds lies between 0 - 4 kcal/mol, but can sometimes be as strong as 40 kcal/mol In solvents such as chloroform or carbontetrachloride one observes e.g. for the interaction between amides additive values of about 5 kJ/mol. According to L. Pauling the strength of a hydrogen bond is essentially determined by the electrostatic charges. Measurements of thousands complexes in chloroform or carbontetrachloride have led to additive free energy increments for all kind of donor-acceptor combinations.
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.