Question

Why are there two elements in the first period?

Answer 1

1. The modern periodic table consists of $118$elements, with 18 groups and 7 periods.
2. The vertical columns are called groups whereas horizontal rows are called periods.
3. The periodic table is divided into four major block namely: s, p, d, f.
4. The s-block contains a maximum of two electrons.
5. There are only two elements present in the periodic table, i.e. Hydrogen and Helium.
6. Period 1 follows the duet rule in which only two elements are needed to finish the valence shell of electrons.
7. The elements of this period can fill up to two electrons only both are present in $1\mathrm{s}$ shell.
8. Therefore first period have only two electrons.