How does the nuclear charge relate to the energy of the 1s orbital?

Effective nuclear charge is defined as he net positive charge experienced by an electron in an atom. It is termed "effective" because the shielding effect of electrons prevents higher orbital electrons from experiencing the full nuclear charge of the nucleus due to the repelling effect of inner-layer electrons.

The 1s is the closest shell to the nucleus of an therefore maximum nuclear charge is experienced. The formula for effective nuclear charge is:

Zeff = Z - S

where

Z = the number of protons in the nucleus, and

S = the shielding constant, the average number of electrons between the nucleus and the electron.

Hence, the energy required to remove an electron from the 1s orbital is the strongest.