Question

explain why, the first period of the modern periodic table has only two elements whereas second period has eight elements?​

Answer 1

This is because in quantum physics, this period fills up the 1s orbital. Period 1 elements follows the duet rule, they only need two electrons to complete their valence shell. These elements can only hold two electrons, both in the 1s orbital. Therefore, period 1 can have only two elements.

Answer 2

because first period fills up the 1s orbital. ... (first period can hold only one or two electrons .)