Here's how to determine this:

* Magnesium (Mg): 1 atom

* Sulfur (S): 1 atom

* Oxygen (O): 4 atoms

Total atoms in one molecule of MgSO₄ = 1 + 1 + 4 = 6

However, atomicity refers to the total number of atoms in the *smallest unit* of a substance. Since MgSO₄ is an ionic compound, the smallest unit is a formula unit. Therefore, the atomicity is 7 (one formula unit contains 7 atoms).