Effective Nuclear Charge - Definition and Trends
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Facts
Effective nuclear charge – the attractive positive charge of nuclear protons acting on valence electrons.
- The effective nuclear charge is always less than the total number of protons present in a nucleus due to shielding effect.
- Effective nuclear charge is behind all other periodic table tendencies.
Shielding effect – the lessening of attractive electrostatic charge difference between nuclear protons and valence electrons by partially or fully filled inner shells.
- Shielding effect increases with the number of inner shells of electrons.
- Electrons sharing the same shell do not shield one another from the attractive pull of the nucleus.
Calculating the effective nuclear charge:
An estimate of effective nuclear charge can be obtained from Zeff = Z - S, where Zeff = effective nuclear charge, Z = atomic number, and, S = the screening constant. ""Consider aluminum: [Ne]3s23p1 "" Z = 13 S = 10 Zeff = Z - S = 13 - 10 = 3+
Don’t forget that Zeff is only an estimate. Actual shielding effect is always greater that the screening constant S because core electrons are much closer to the nucleus than are valence electrons.
Trends
The periodic table tendency for effective nuclear charge:
- Increase across a period (due to increasing nuclear charge with no accompanying increase in shielding effect).
- Decrease down a group (although nuclear charge increases down a group, shielding effect more than counters its effect).