2.4 Periodic Trends

Sheilding & Effective Nuclear Charge

When dealing with valence electrons, they will be attracted to the nucleus due to electrostatic forces. However, the core electrons, which are the non-valence electrons, will repel the valence electrons. This is called the shielding effect, since the core electrons are basically shielding some of the attraction from the valence electrons.

The net effect of this is found through the effective nuclear charge. You can subtract the amount of core electrons to the amount of protons in the nucleus to find the “effective” charge on the nucleus. For example, Oxygen has an electron configuration of ${1s^2 2s^2 2p^4}$, from which you can see that it has 2 core electrons and 6 valence electrons. Since Oxygen has 8 protons in its nucleus, the effective nuclear charge that the valence electrons actually feel will be 8 - 2 = 6. Effective nuclear charge is important to understanding many periodic trends.