Answer:
Strotonium (Sr)
Step-by-step explanation:
The element in Group 2 of the periodic table with 3 electron shells is Strontium (Sr). It has the electron configuration [Kr] 5s², which means it has three electron shells: the first shell (K) with 2 electrons, the second shell (L) with 8 electrons, and the third shell (M) with 18 electrons.