# Question 0d56e

Jul 21, 2017

""_1H and ""_2He

#### Explanation:

It's ""_1H and ""_2He#

Jul 21, 2017

They are the only elements with only 1 occupied electron shell.

#### Explanation:

First, you need to understand how elements are arranged in a Periodic Table (they are not arranged randomly)

Elements are arranged:
in groups (vertical column) by their valence electrons number.
in periods (horizontal row) by their number of shells occupied by electrons.

First row of periodic table consists of only elements which have only 1 shell $\left({1}_{s}\right)$ occupied by electrons.