Which has a greater number of atoms a gram of helium or a gram of neon?

              1 g of He contains greater number of atoms.

Proof:

Number of Atoms in 1 g of He:

Step 1: Calculate Moles of 1 g He as,

                         Moles  =  Mass ÷ M.Mass

                         Moles  =  1 g ÷ 4 g.mol⁻¹

                        Moles  =  0.25 mol

Step 2: Calculate Number of Atoms of He as,

                         # of Atoms  =  Moles × 6.022 × 10²³

Putting value of moles,

                         # of Atoms  =  0.25 × 6.022 × 10²³

                         # of Atoms  =  1.50 × 10²³ Atoms of He

Number of Atoms in 1 g of Ne:

Step 1: Calculate Moles of 1 g He as,

                         Moles  =  Mass ÷ M.Mass

                         Moles  =  1 g ÷ 20.18 g.mol⁻¹

                        Moles  =  0.0495 mol

Step 2: Calculate Number of Atoms of Ne as,

                         # of Atoms  =  Moles × 6.022 × 10²³

Putting value of moles,

                         # of Atoms  =  0.0495 × 6.022 × 10²³

                         # of Atoms  =  2.98 × 10²² Atoms of Ne