1. Calculate the number of moles:

* Oxygen (O):

* Molar mass of oxygen = 16.00 g/mol

* Moles of oxygen = (10.0 g) / (16.00 g/mol) = 0.625 mol

* Calcium (Ca):

* Molar mass of calcium = 40.08 g/mol

* Moles of calcium = (10.0 g) / (40.08 g/mol) = 0.249 mol

2. Calculate the number of atoms:

* Avogadro's number: 6.022 x 10^23 atoms/mol

* Oxygen:

* Atoms of oxygen = (0.625 mol) * (6.022 x 10^23 atoms/mol) = 3.76 x 10^23 atoms

* Calcium:

* Atoms of calcium = (0.249 mol) * (6.022 x 10^23 atoms/mol) = 1.50 x 10^23 atoms

Conclusion:

10.0 g of oxygen has more atoms than 10.0 g of calcium.