I know that first generation stars' main fuel was Hydrogen. I know the Big Bang happened at some point in time. Now if the strong force exists, then why aren't different, higher mass, number elements produced? why was there single proton nucleus like hydrogen?
Answer
The answer touches upon the concept of Big Bang Nucleosynthesis (BBN), which is excellently explained on a graduate level in Baumann's lecture notes.
The key idea is the following: In order to form metals (anything heavier than hydrogen and helium), you need deuterium nuclei. But deuterium nuclei are only formed significantly when the temperature of the primordial plasma falls far below the binding energy of deuterium (T ~ 2.2 MeV). Why? Well, the formation of deuterium has to compete with the enormous amount of high-energy photons in the universe at the time, which split up the deuterium nuclei. So the photon bath has to be cool enough, so that most of the photons don't have enough energy to split the nuclei apart again. The relevant number is the baryon-to-photon ratio $\eta\sim10^{-9}$, i.e. for each baryon we have $10^9$ photons.
Once deuterium is produced, it is almost instantly fused to helium nuclei. However, basically no elements heavier than helium are formed in BBN because they require high enough number densities of helium nuclei from which they would be fused. But by the time helium fusion has begun, the reaction rates for those are already too slow.
For a more detailed and quantitative discussion see the link to the lecture script, chapter 3 'Thermal history', the section about BBN.
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