Question

state one reason for placing mg and CA in the same group of periodic table

Answer 1

Both have 2 electrons in their outermost shells and both are alkaline earth metals.

Answer 2

see the Electronic configuration of Mg and Ca
Element Atomic number K L M N
Mg 12 2 8 2

Ca 20 2 8 8 2

Both Mg and Ca have 2 electron in their valence shells, so they have similar properties. Hence, they are placed in same group

they have similar electronic configuration
they have similar chemical properties
they both are alkaline earth metals