As well as effective nuclear charge and its effects on the valence electrons we also need to consider shielding very often when looking at chemical properties of elements and trends and patterns in the periodic table. The number of protons in the nucleus is always shielded by the inner shells, weakening the attraction of the nucleus to the valence electron. Shielding can also causes repulsion between the sub orbitals pushing electrons further out as the orbitals overlap and cause repulsion between each other.
For example looking at elements in the same group if we look at sodium and rubidium we can see that the sodium nucleus has 2 shells of inner electrons between the nucleus and the valence electron meaning it has 2 shells of electrons that interfere with the attraction between the nucleus and the outer electron. Where as rubidium has 4 shells of electrons between its nucleus and the valence electron, which despite having a larger nucleus and more protons has a larger number of shells reducing the effect of the nucleuses attraction to the outer electron.
For example an s shell will shield a p shell because it is closer to the nucleus and has a greater attraction to the nucleus, this weakens the attraction of the electrons in the p sub shell. So instead of experiencing the nuclear charge the p shell experiences the effective nuclear charge. We can calculate this by subtracting the atomic number (number of protons ) from the Shielding constant S.
Zeff = Z-S
And from this equation we can conclude that the effective nuclear charge of an atom increases as the number of protons increases explaining the trend across the periodic table by increasing the number of protons and decreasing the atomic radius.
The effective nuclear charge is the attraction of the nucleus to the valence electron taking into account the number of protons and the number of inner shell electrons.
The inner shell are the electrons closest to the nucleus – usually the S and P shells due to their lower energy sub orbitals
The valence electron is the outermost electron of an atom
Shielding is the effect of inner shell electrons close to the nucleus reducing the nuclear charge on the valence electron.
An S shell is a spherical shaped shell that can hold two electrons with opposite spin
A P shell is a dumbbell shaped shell in three axis creating 3 shells that can hold 6 electrons in 3 pairs
Atomic radius is the distance measured from the nucleus to the outer valence electrons – measured in pm picometres which is 1x10-12 m