$\mathrm{K} > \mathrm{Ca} > \mathrm{Mg} > \mathrm{Al}$ .The reactivity of elements depends upon their tendency to lose or gain electrons to complete their outer most shell. On moving left to right in a period reactivity decreases and down the group increases. So $\mathrm{Ca}$ and $\mathrm{Mg}$ both belongs to $2^{\text{nd}}$ group but have different periods. $\mathrm{Mg}$ is in $3^{\text{rd}}$ period and $\mathrm{Ca}$ is in $4^{\text{th}}$ period.