What is the effective nuclear charge (Zeff) of Li?

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First, recall the concept: effective nuclear charge Zeff is the net positive charge experienced by the valence electron after screening by inner electrons. For lithium (Li), the atomic number Z is 3, and the electron configuration is 1s2 2s1. The two electrons in the 1s shell shield the outer 2s electron from the full pull of the nucleus.

Option a: +3. This would imply no shielding at all, i.e., the valence electron feels the full nuclear charge. In Li, there are inner electrons that screen, so +3 is too high.

Option b: +5. This is greater than the actual nuclear charge Z=3, which is impossible for Zeff. It disregards shielding entirely and exceeds the atom’s positive charge.

Option c: +2. If the two 1s electrons provided some shielding, Zeff could be reduced to +2, but in Li specifically the shielding of the single valence electron by the two 1s electrons typically results in a Zeff closer to +1 rather than +2.

Option d: +4. This is also greater than Z and ignores shielding, making it physically implausible for Li’s single valence electron to experience such a strong net charge.

Option e: +1. This aligns with the standard calculation: Zeff ≈ Z − S, with Z = 3 and S ≈ 2 from the 1s electrons, giving Zeff ≈ 1. The outer electron in Li experiences only a modest net attraction due to partial shielding by the inner shell.

In summary, considering the electron configuration and the shielding effect of the inner electrons, the most accurate Zeff value for Li’s valence electron is +1. The other options either neglect shielding entirely, overshoot the nuclear charge, or assume unrealistic levels of shielding for this simple atom.

CHM1052 - MUM S2 2025 Week 7: Preparation quiz

What is the effective nuclear charge (Zeff) of Li?

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