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Q.
For $Be , Z _{ eff }=1.95$ and for $Be ^{x+}, Z_{ eff }=2.30$ Hence, ion is
Classification of Elements and Periodicity in Properties
Solution:
Effective nuclear charge $Z_{\text {eff }}=Z-S$
Where, $Z=$ atomic number
and $S=$ screening constant
$=0.35$ per electron for electron in $n$th orbit
$=0.85$ per electron for electron in $(n-1)$ th orbit
$=1.00$ per electron for electrons in $(n-2)$ th, $(n-3)$ th, $(n-4)$ th orbit
$=0.30$ per electron in $1 s$-orbital (when alone)
Be $1 s^2 2 s^2$ one valence-electron in $2 s$ is screened by one electron in $2 s$-orbital (nth orbit) and two electrons in $1 s$-orbital $((n-1)$ th orbit $)$
$\therefore S=0.35+2 \times 0.85=2.05$
$\therefore Z_{\text {eff }}=4-2.05=1.95$ (given)
$Be ^{+} 1 s^2 2 s^1 2 s$ electron is screened by two electrons in $1 s$-orbital $((n-1)$ th orbital $)$
$S=2 \times 0.85=1.70$
$\therefore Z_{\text {eff }}=4-1.70=2.30$
$Be ^{3+} 1 s^1$ one electron in 1s-orbital (alone exists) is screened by another electron in same orbit. Thus, $S=0.30$
$\therefore Z_{\text {eff }}=4-0.30=3.70$
$Be ^{3+} 1 s^1$ no-screening (single electron)
Thus, $Z_{\text {eff }}=4$
Thus, $Z_{\text {effective }}: Be < Be ^{+}< Be ^{2+}< Be ^{3+}$