Why are there only two elements in the first period of the periodic table?

The first period has less elementsthan any other periods in the periodic table. There are only two elements in the first period: hydrogen and helium. We can explain why there is lesselements in the first row in modern theories of atomic structure ... Theseelements can only hold two electrons, both in the 1s orbital

The reason why there are only two elements in the first period of the periodic table is because hydrogen is the main reactant while helium is the main non-reactant.