Atomic Hydrogen 1.1 2008/06/20 17:16:29.336 GMT-5 2008/06/25 18:01:31.796 GMT-5 Andrew R Barron arb@rice.edu Carissa J Smith cs2@rice.edu Andrew R Barron arb@rice.edu Carissa J Smith cs2@rice.edu electron spin resonance esr hydrogen Periodic Table photolysis radical reducing agent Solution solvation thermolysis Vapor The synthesis and reactivity of atomic hydrogen Atomic hydrogen has the electron configuration of 1s 1 size 12{ {} rSup { size 8{1} } } {} and as such represents the simplest atomic configuration. However, as a consequence there is dispute as to its proper position within the Periodic Table. Its electron configuration is similar to the valence electron configuration of the alkali metals (ns 1 size 12{ {} rSup { size 8{1} } } {}) suggesting it be listed at the top of Group 1 (1A). However, its reaction chemistry is dissimilar to the alkali metals. Hydrogen is also one electron short of a Nobel gas configuration, and therefore it is possible to think of its relationship to the halogens.

Vapor Phase Atomic hydrogen (H size 12{ cdot } {}) is highly reactive and consequently has a short lifetime due to its reaction chemistry. Consequently, in order to generate and observe the reactivity they must be generate at low pressures. Thermolysis of hydrogen compound (commonly halide) or photolysis at an energy above the bond dissociation energy results in the homoleptic cleavage of the H-X bond to generate the appropriate radical species. Alternatively, atomic hydrogen can be generated from elemental hydrogen. The reverse reaction (recombination of two hydrogen atoms) is highly exothermic (-434 kJ.mol −1 size 12{ {} rSup { size 8{ - 1} } } {}) and forms the basis of the heat generated in arc welding.

Solution Atomic hydrogen may be generated in aqueous solution through the solvation of electrons. The formation equilibrium constant (Keq) is very small resulting in very low concentrations being generated (10 −5 size 12{ {} rSup { size 8{ - 5} } } {} M). As expected solvated atomic hydrogen is a strong reducing agent. Ag + size 12{ {} rSup { size 8{+{}} } } {} + H size 12{ cdot } {} → Ag + H + size 12{ {} rSup { size 8{+{}} } } {} 2I − size 12{ {} rSup { size 8{ {} rSup { size 6{ - {}} } } } } {} + 2H size 12{ cdot } {} → 2H − size 12{ {} rSup { size 8{ {} rSup { size 6{ - {}} } } } } {} + I 2 size 12{ {} rSub { size 8{2} } } {}

Solid State Hydrogen atoms may be trapped in the solid state lattice upon generation by photolysis of HX. Observation by electron spin resonance (esr) of a signal split by s = 1/2 nucleus (i.e., 1 size 12{"" lSup { size 8{1} } } {}H) results in a doublet with a coupling of 1428 MHz.