\(\therefore\) \(\frac{1}{{{\lambda _{{K_{\min }}}}}} = R{(Z - 1)^2}\left( {\frac{1}{{{1^2}}} - \frac{1}{{{\infty ^2}}}} \right) = R{(Z - 1)^2}\)

\( \Rightarrow \,\,{\lambda _{{K_{\min }}}} = \frac{1}{{R{{(Z - 1)}^2}}}\,\)

\( \Rightarrow \,\,{\text{0}}{\text{.7Å}} = \frac{{{\text{91 - {\AA}}}}}{{{{{\text{(Z - 1)}}}^{\text{2}}}}}\)

\( \Rightarrow \,{(Z - 1)^2} = \frac{{91}}{{0.7}} \Rightarrow Z - 1 = 36\)

\(\therefore\) \(Z = 37\)