"For the first few hundred thousand years, the matter in the universe largely consisted of ionized hydrogen, protons, and electrons, with a 10 percent admixture of helium nuclei. At high temperature, energetic photons knock electrons away preventing the two particles from uniting to form hydrogen atoms, and the natural state of the hydrogen gas is the plasma or ionized form... [A]s the universe expands and cools... hydrogen atoms are able to persist.... From then on the universe is almost entirely atomic."
Silk, Joseph. (1994: 160) A Short History of the Universe. Scientific American Library, New York.
Neutrons are captured by protons and form a deuterium nucleus. This new combination captures a second neutron to form unstable tritium nucleus which then captures a proton to form a nucleus of helium. Trace amounts of lithium nuclei also arise.
At this hydrogen plasma stage, the universe is still far too energetic for atoms or molecules to form.
