Question

Following question is based on $Sc$ (Z$=$ 21).

For any $1s$ electron, $Z^{\ast }$ (effective nuclear charge) is :

• A. $21.0$
• B. $20.65$
• C. $16.85$
• D. $9.75$

Answer 1

Answer: (B)

$20.65$

Effective nuclear charge $Z_{eff}=Z-S$

where, Z $=$ atomic number and S $=$ screening constant

The value of $S$ for different orbits-

$=$ 0.35 per electron for an electron in nth orbit.
$=$ 0.85 per electron for electrons 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 1s-orbital (when alone)
So, for $1s$ electron

$Z_{eff}=21-0.35=20.65$