Question

The $Z_{effective}$ for $He$ is?

• A. 2
• B. 1.7
• C. 1.85
• D. 1.65

Answer 1

The correct option is A 2

The effective nuclear charge experienced by a 1s electron in helium is +1.70.

The effective nuclear charge $Z_{eff}$ is the net positive charge experienced by an electron in a multi-electron atom.

A given electron does not experience a full nuclear charge because the other electrons are sometimes between it and the nucleus and shield it from the nucleus.

The formula for effective nuclear charge is-

$Z_{eff}=Z-S$

where,

Z is the number of protons in the nucleus, and S is the shielding constant, the average number of electrons between the nucleus and the electron in question.

The American physicist John Slater derived a number of rules to determine the shielding constant.

He found that for electrons in a 1s orbital, the second electron shields the first by 0.30 units.

$Z_{eff}=Z-S=2-0.30-1.70$

Hence, the correct option is B.