Which element's ionic radius is smaller than its atomic radius? a) oxygen b) nitrogen c) calcium

Calcium, because when it becomes an ion, it loses two electrons and gets rid of its M shell, decreasing its ionic radius. The rest are anions, which increase in ionic radius because the effective nuclear charge decreases.

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

Must be negative.substance increases