As well as 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 electrons, weakening the attraction of the nucleus to the valence electron. Shielding can also causes repulsion between the electrons pushing them 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.
The nucleus is the term given to the centre of the atom comprising of the proton and neutron
Nuclear charge is the attraction exerted by the nucleus on electrons due to the positive charge of the protons and negative charge of the electron
The inner shell are the electrons closest to the nucleus – usually the S and P shells due to their lower energy sub orbitals
Outer electrons is the term given to the shell/energy level furthest from the nucleus containing the electron furthest from the nucleus
A proton is a positive particle that makes up the atom in the nucleus with a positive charge
An orbital is the circular motion that the electrons follow around the nucleus. Much like the planets orbiting the sun
Shielding is the effect of inner shell electrons close to the nucleus reducing the nuclear charge on the valence electron.
Repulsion is when two charges that are the same come into contact they prefer to be further away from each other so push each other away. For example when two electrons come close to each other the negative charges cause repulsion