Atomic, Molecular and Nascent Hydrogen

There are the various forms of hydrogen. Different hydrogens have different physical and chemical properties.

1. Molecular hydrogen: It is the most stable form of hydrogen. Hydrogen found in free state is always molecular hydrogen. When atomic hydrogen is kept free, it immediately revert back to molecular form producing large amount of heat.
$$\ce{H. + H. ->2H + 104 K Cal}$$

2. Atomic hydrogen: Atomic hydrogen is formed by dissociation of hydrogen molecule into atoms when subjected to high electric discharge at low pressure. The breaking of molecular hydrogen into atomic hydrogen is endothermic process. It is represented by symbol H and  a dot (⋅) .

$$\ce{H2<=>2H – 104 K Cal}$$

3. Nascent Hydrogen: Nascent hydrogen is formed during chemical reaction. So, it also called newly born hydrogen. It has very short life span and cannot exist independently. Nascent hydrogen is more reactive than molecular hydrogen but less reactive than atomic hydrogen. It is denoted by [H].

Following reactions support the statement that nascent hydrogen is more reactive than molecular hydrogen.

– Reduction of ferric chloride: When ferric chloride is reacted with ordinary hydrogen, no reaction takes place.
$$\ce{FeCl3 + H2->No reaction}$$
$$\ce{Zn + H2SO4->ZnSO4 + 2H}$$
But when zinc is treated with acidified FeCl3, it is changed into light green color which is ferrous chloride.
$$\ce{$\underset{\text{Yellowish brown}}{\ce{H2O}}$ + [H]->$\underset{\text{Light green}}{\ce{FeCl2}}$ + HCl}$$
– Reduction of potassium dichromate: Ordinary hydrogen cannot reduce potassium dichromate but when nascent hydrogen is acidified with K2Cr2O7, it is reduced to chromium sulphate.
$$\ce{K2Cr2O7 + H2SO4 + H2-> No reaction}$$
$$\ce{Zn + H2SO4->ZnSO4 + 2H}$$
$$\ce{$\underset{\text{Yellowish brown}}{\ce{K2Cr2O7}}$ + 4H2SO4 + 6[H]->K2SO4 $\underset{\text{Green}}{\ce{Cr2(SO4)3 }}$ + 7H2O}$$

Differences between Nascent hydrogen and atomic hydrogen:

Atomic Hydrogen               Nascent Hydrogen
Atomic hydrogen is produced by the dissociation of ordinary hydrogen at high pressure and low temperature. Nascent hydrogen is formed during chemical reaction.
It is strong reducing agent and highly reactive than nascent hydrogen. Nascent hydrogen is weaker reducing agent and is less reactive.
Atomic hydrogen does not exist in solution since it cannot isolated. Nascent hydrogen exists in solution as it can be isolated.
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