There are three types of metals which absorb hydrogen. One is the group of salt-like hydride forming metals, which absorb exothermically large, equivalent quantities of hydrogen, other is the group of so called "poor absorber" metals, which absorb endothermically very small quantities, and another is the medium of the both groups.
The typical metals of the former group are the alkali and alkali-earth metals, the group of contrast, the poor absorber are the Fe, Ni, Cu, Pt, etc., and the medium are the group of such as Pd, Ti, V, Zr, Th etc. Authors found that the heat of solution of hydrogen in alkali and alkali-earth metals or in poor absorbers are proportional or in inverse ratio respectively to the ionization energy of metals. These facts, together with other phenomena, can be the proof of the negative or positive character of the dissolved hydrogen in the metals.
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