Question

The Z(effective)​ for $\mathrm{He}$ is?

• A. $1.8$
• B. $1.70$
• C. $2$
• D. $0$

Answer 1

The correct option is B

$1.70$

Explanation for correct option:

B. $1.70$

Effective nuclear charge:

• "The effective nuclear charge means net positive charge experienced by an electron in a polyelectronic atom."
• It can be expressed as:

${\mathrm{Z}}_{\mathrm{eff}}=\mathrm{Z}-\mathrm{S}$

• For Helium, the atomic number is $2$ with electronic configuration $1s^{\mathit{2}}$.
• According to Slater’s rule, the shielding constant for $1s$ electron is $0.30$. Thus, an effective nuclear charge of helium is:

$$\begin{gathered} {\mathrm{Z}}_{\mathrm{eff}}=\mathrm{Z}-\mathrm{S} \\ {\mathrm{Z}}_{\mathrm{eff}}=2-0.3 \\ {\mathrm{Z}}_{\mathrm{eff}}=1.70 \end{gathered}$$

Thus, the effective nuclear charge of Helium is $1.70$.

Explanation for incorrect options:

The effective nuclear charge of Helium is $1.70$. Thus, options A, C, and D are incorrect.

Hence, the correct option is B i.e. $1.70$.