Final answer:
Atomic nuclei could not exist when the Universe was less than about 2 minutes old due to high temperatures and densities. Deuterium and helium were formed when the Universe cooled down to around 3 to 4 minutes old. Lithium also formed in small amounts during this time.
Step-by-step explanation:
When the Universe was less than about 2 minutes old, atomic nuclei could not exist because of the high temperatures and densities at that time. At higher temperatures, the atomic nuclei would be immediately blasted apart by interactions with high-energy photons, preventing their formation. It wasn't until the Universe cooled down to about 900 million K, when it was around 3 to 4 minutes old, that the temperatures and densities allowed for the formation of atomic nuclei.
During this time, deuterium, a proton and neutron, lasted long enough to convert some of it into helium through collisions. This process is similar to how stars fuse new elements from simpler components. Additionally, a small amount of element 3, lithium, could also form during this period.