Atomic and ionic properties of hydrogen

A resemblance between hydrogen and the halogens. Hydrogen forms alkali metal hydrides like NaH, which has the same structure as the sodium halides. 

Whether portrayed as a Lewis structure (Structure 5.1) or by molecular orbital theory, the bond holding together the two atoms in the H2 molecule turns out to have a bond order of 1. 

The hydrogen atom contains only a single electron moving around a single proton. This suggests that the atom should be small, and the covalent radius of only 37 pm, shown in Table 5.1, supports this. Table 5.1 compares some atomic and molecular properties of hydrogen with those of fluorine, the smallest of the halogen atoms. Let us look at some of the data. 

Another important difference is provided by the electron affinities. The value for hydrogen is much less than that of fluorine. The small electron affinity of hydrogen becomes especially important when set alongside the much higher bond dissociation energy of the H2 molecule. 

Such differences explain why metal hydrides are much more easily decomposed into their elements than metal fluorides, and indeed all other halides why, for example, sodium fluoride melts at about 1 000 °C, and boils at 1 700 °C without decomposition, whereas sodium hydride breaks down into sodium and hydrogen at only 400 °C. 

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