I'm working on chemistry homework and I don't understand how to figure this problem out.

You need to find the amount of moles of Calcium you have in 51.0 g of Calcium.

The atomic weight of Calcium is 40.

So, 40 g of calcium has 1 mole.

Find the number of moles 51 g of Calcium contains.

Then, 1 mole contains 6.02x10^23 particles, in this case, 1 mole contains 6.02x10^23 atoms.

When you have the number of moles, you can find the number of atoms! :)

Post your answer. :)