Let's find the number of moles first.
Number of moles = mass/molar mass
Molar mass = 51.9961
Number of moles = 13/ 51.9961 = 0.2500.
1 mole of any particle contains 6.02 * 10^(23) atoms.
Hence 0.2500 moles contains X
Hence X = 0.25 * 6.022 * 10^(23) = 1.5055 * 10^(23)
