Question

What are the elements in the second third, fourth, and fifth periods that have the same number of highest-energy-level electrons as bariu.?

Answer 1

Surely you have got a Periodic Table in front of you.....?

Explanation:

And barium is in Group 2; i.e. `Be`, `Mg`, `Ca`, `Sr`, `Ba`, and `Ra`........and all of these form `M^(2+)` ions.