Many factor determine the atomic size of an element
When protons and electrons increase(their rate of increasing is equal) protons have more control on electrons than electrons have on them. Though electron shielding tries to increase the the atomic radii, because protons have more control they pull the electrons towards them and thus this explains the atomic trend
Atomic radii increases as you go down a group.
This is because of electron shielding.Electrons and protons are like magnets. Electrons repel each other. Protons also repel each other and electron and proton get attracted. When an electron from the outermost shell tries to push itself from another electron in the innershell it then called electron shielding
The electron shielding reduces the effect of protons on the nucleus and also increase the atomic radii.
Principal quantum number
The valence electrons occupy higher levels due to the increasing quantum number (n). As a result, the valence electrons are further away from the nucleus as ‘n’ increases. Electron shielding prevents these outer electrons from being attracted to the nucleus; thus, they are loosely held, and the resulting atomic radius is large.
azimuthal quantum number
As the type of electron shell effects the shielding, for example f and d shells do very bad shielding. In fact if d shell does bad shielding f shell does worse. This is because of their structure.