# Effective Nuclear Charge - Definition and Trends

### Facts

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 shielding effect.
2. 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.

1. Shielding effect increases with the number of inner shells of electrons.
2. 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:

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).