Mark the species among the following that contains the highest number of atoms.

1. 4 g He

2. 46 g Na

3. 0.40 g Ca

4. 12 g He

Hint: 1 mol of an element is equal to it atomic mass 
Step 1: 
For comparing number of atoms, first we calculate the moles as all are monoatomic and hence, 
moles×NA=number of atoms.
Step 2:
Calculate the moles of given elements as follows:
Moles of 4 g He=44=1 mol
46 g of Na=4623=2 mol
0.40 g Ca=0.4040=0.1 mol
12 g He=124=3 mol
Hence, 12 g He contains greater number of atoms as it possesses maximum number of moles.