Based on the activity series, which metals could x represent in the reaction below? (note: the equation is not balanced.) x + mgso4 —> mg + xso4 a) sr b) al c) sn d) au

Answer is: A) Sr (strontium).
The reactivity series is a series of metals from highest to lowest reactivity. Metal higher in the reactivity series will displace another.
Strontium is only higher in this group from magnesium. Strontium is stronger reducing agent than magnesium, gives electrons easier.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.