Question

Calculate the $Z_{\text {eff }}$ of $39^{\text {th }}$ electron of ytrrium.

Answer 1

${ }_{39} Y:\left(1 s^{2}\right)\left(2 s^{2} 2 p^{6}\right)\left(3 s^{2} 3 p^{6}\right)\left(3 d^{10}\right)\left(4 s^{2} 4 p^{6}\right)(4 d),\left(5 s^{2}\right)$
$39^{\text {th }}$ electron of $Y$ is present in $4 d$-subshell
$Z_{\text {eff }}=Z-\sigma=39-\sigma$
$s_{4 d}=36 \times 1+0 \times 0.35 \Rightarrow 36$
$\therefore Z_{\text {eff }}=3$