Question

How does the nuclear charge of a atom differ from group and period in the trends in modern periodic table

Answer 1

Effective nuclear charge – the attractive positive charge of nuclear protons acting on valence electrons.
The effective nuclear charge is always less than the total number of protons present in a nucleus due to shielding effect.
Effective nuclear charge is behind all other periodic table tendencies
Calculating the effective nuclear charge:
An estimate of effective nuclear charge can be obtained from Zeff = Z - S, where Zeff = effective nuclear charge, Z = atomic number, and, S = the screening constant. ""Consider aluminum: [Ne]3s23p1 "" Z = 13 S = 10 Zeff = Z - S = 13 - 10 = 3+
Don’t forget that Zeff is only an estimate. Actual shielding effect is always greater that the screening constant S because core electrons are much closer to the nucleus than are valence electrons.
Trends
The periodic table tendency for effective nuclear charge:
Increase across a period (due to increasing nuclear charge with no accompanying increase in shielding effect).
Decrease down a group (although nuclear charge increases down a group, shielding effect more than counters its effect).