The weight of 100g H2 and 100 g He is same yet the number of elements is not same.

The mole and atonmicity of both the gases are different, the number of atoms is not same.

Explanation:

The number of atoms in a molecule (compound) depends on mole number and atomicity.

↬ Mole of 100 g H₂ = 100g ÷ 2u = 50 mole

∴Number of atoms in 100 g H₂

= 2 x 50 x 6.022 × 10²³

= 6.022 x 10²⁴ atoms

↬ Mole of 100 g He = 100g ÷ 4u = 25 mole

∴ Number of atoms in 100 g He

= 1 × 25 × 6.022 × 10²³

= 150.55 × 10²³

= 1.5055 x 10²⁵ atoms

Thus, The mole and atonmicity of both the gases are different, the number of atoms is not same.

-TheUnknownScientist 72

the atomic mass is the bottom number on the element.

-steel jelly

i'm assuming that the correct answer is indeed the first one/ "a"..