why is hydrogen the only element in the periodic table that lacks a neutron?
Two or more protons without any neutrons are not stable - their electrostatic repulsion is larger than the nuclear force. With an additional neutron, the nuclear force is stronger (and the electromagnetic force a bit weaker), and 3He is stable.
Right after the big bang the only nucleons were free protons and neutrons (which decay into protons). Nuclear fusion started to take place resulting in a lot of helium nuclei plus a small amount of other light nuclei, deuterons and Lithium nuclei. The net result was that the universe (after cooling down enough to form atoms) was about 75% H1 and 25% He4 and trace amounts of the others. The rest of the elements got created in stars and supernovae. However most of the universe is still H1 and He4.
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