CHM120H5 Lecture Notes - Lecture 23: Effective Nuclear Charge, Electron Configuration
Document Summary
Consider an atom of helium containing two protons surrounded by two electrons both occupying the 1s orbital. We need to consider not just the proton - electron attractions, but also the electron - electron repulsions. Because of this repulsion, each electron experiences a nuclear charge that is somewhat less than the actual nuclear charge. Essentially, one electron shields, or screens the other electron from the nucleus. The positive charge that an electron actually experiences is called the effective nuclear charge, zeff, and zeff is always somewhat less than the actual nuclear charge. Zeff = z - s, where s is the shielding constant. Considering helium, it may appear that zeff would be one for each electron (i. e. , each electron is attracted by two protons and shielded by one electron, 2 - 1 = 1), but. An electron in an s orbital has a finite, albeit very small, probability of being located quite close to the nucleus.
