LIGHTEST atoms of NITROGEN, hydrogen, carbon etc. formed the primitive atmosphere. Free atoms combined to form MOLECULES and simple inorganic compounds. Hydrogen atoms were most numerous and most reactive in primitive atmosphere. First hydrogen atoms combined with all oxygen atoms to form WATER and leaving no free oxygen. Thus primitive atmosphere was reducing atmosphere (without free oxygen) unlike the present oxidising atmosphere (with free oxygen). Presence of huge amount of free hydrogen in primitive atmosphere ALSO proves its reducing nature.