Question

Explain effective nuclear charge

Answer 1

Effective nuclear charge – The attractive positive charge of nuclear protons acting on valence electrons.

1. The effective nuclear charge is always less than the total number of protons present in a nucleus due to the shielding effect.
2. Effective nuclear charge is behind all other periodic table tendencies.

The effective nuclear charge may be approximated by the equation:

Z_eff = Z - S

Where Z is the atomic number and S is the number of shielding electrons.
Trends

The periodic table tendency for effective nuclear charge:

1. Increase across a period (due to increasing nuclear charge with no accompanying increase in shielding effect).
2. Decrease down a group (although nuclear charge increases down a group, shielding effect more than counters its effect).

Examples:

Li(3) - 1s²2s¹

• A 2s lithium can have 2 electrons, 1s electrons between itself and the lithium nucleus. So, these 1s electrons shield 2s electrons from the nucleus.
• Measurements indicate the effective nuclear charge experienced by a 2s lithium electron is 0.43 times the charge of the lithium nucleus.