Effective nuclear charge

The effective nuclear charge (short-form ENC) is the actual amount of positive nuclear charge experienced by an electron. A higher ENC indicates the outer electrons are held closer to the nucleus with a higher attractive force. ENC increases from left to right on the periodic table, so atomic radius decreases from left to right.

We calculate with:

$$\text{ENC}=\text{nuclear charge (protons in nucleus)}-\text{core electrons (\# of electrons excluding valence)}$$