Explain why atoms increase in size when the atom gains electrons.

Adding electrons creates an ion that is larger then the neutral atom; removing electrons produces an ion that is smaller then the neutral atom. When electrons are added, mutual repulsions push them apart and they occupy a larger volume so the ion increases in size.

Hope it helps

The activity series is shown in the attached picture. if the element is below the other, then you can displace the other. following the activity series as a guide, the reactions are as follows: nickel and aluminum chloride3 ni + 2 alcl₃  → 3 nicl₂ + 2 albarium and copper bromideba + cubr  →  no reaction because cu is less oxidizable than bapotassium nitrate and aluminum3 kno₃ + al  → al(no₃)₃ + 3 ksodium phosphate and zincnaso₄ + zn  → znso₄ + na