why does 3d10 provide less shielding??
The electron shielding effect is the effect where core electrons block valence electrons from the nuclear charge of the nucleus.
One can calculate the effective nuclear charge with:
Zeff = Z - S
where Z is the total number of protons in the element and Z is the total number of core electrons (electrons that are not valence electrons)
Shielding of nd or nf electron by ns or np electrons is calculated to be 100% effective. Therefore, all the other electrons on the left contribute 1.00 towards the shielding constant S. Now for a 3d10 electron:
The 9 3d electrons contribute 0.35, 9*0.35=3.15
The 18 3s, 3p, 2p, 2s and 1s electrons contribute 1.00, 18* 1.00=18.00
The total shielding constant=18.00+3.15=21.15
Therefore, the Z* value is= Z-S= 29-21.15= 7.85
Positive and negative charges attract each other so the more effective charge the electrons get, the more attraction there is between the nucleus and the outer electrons. So as the effective nuclear charge increases, the atom becomes smaller or we can say that as the shielding becomes stronger, the nuclear charge decreases and the size of the atom increase. All electrons closer to the nucleus acts as a shield for the electrons further away from the nucleus which in turn decreases the attraction between the nucleus and the distant electrons. Therefore, as the number of electrons increases, the shielding effect for the last electron decreases.