Tell the relation between effective nuclear charge $\left({\mathrm{Z}}_{\mathrm{eff}}\right)$, atomic number $\left(\mathrm{Z}\right)$, and shielding constant $\left(\mathrm{\sigma }\right).$ Explain it qualitatively.

Arrange the following in correct order of stability:

(i) ${\mathrm{Ga}}^{+},{\ln }^{+},{\mathrm{Tl}}^{+}$

(ii) ${\mathrm{As}}^{+5},{\mathrm{Sb}}^{+5},{\mathrm{Bi}}^{+5}$

(i) Which element has the highest first ionisation enthalpy?

(ii) Which element has the most metallic character?

Justify your answer in each case.