Calculate the number of moles of magnesium, chlorine, and oxygen atoms in 2.60 moles of magnesium perchlorate, mg(clo4)2. express the number of moles of mg, cl, and o atoms numerically, separated by commas.

In this question, there are  2.60 moles of magnesium perchlorate which has a formula of Mg(ClO4)2. To find the separate number of each kind of atom, you need to know the total number of atoms for every magnesium perchlorate. 
The unabbreviated formula should be Mg ClO4 ClO4. From this name, you can know that there is 1 Mg, 2Cl ad 8 O. Then the number of mols should be:

Mg= 1*2.6 moles= 2.6 moles
Cl= 2*2.6 moles= 5.2 moles
O= 8*2.6 moles= 20.8 moles