In the ground state, an atom of each element in Group 2 (alkaline earth metals) has a different number of electron shells.

Here's why:

* Group 2 elements all have two valence electrons (electrons in their outermost shell).

* Electron shells represent energy levels where electrons are located. As you move down a group (column) on the periodic table, the number of electron shells increases.

Example:

* Beryllium (Be): Has 2 electrons in the first shell (1s²)

* Magnesium (Mg): Has 2 electrons in the second shell (2s²)

* Calcium (Ca): Has 2 electrons in the third shell (3s²)

* Strontium (Sr): Has 2 electrons in the fourth shell (4s²)

Therefore, the number of electron shells, which corresponds to the period number on the periodic table, is the key difference in ground state atoms of Group 2 elements.