How do i find what first ionization energies for atoms is greater? and what does that mean?

well with a higher effective nuclear charge, Cl will be harder to ionize. 8. Using Slater's rules, calculate effective nuclear charge on a 3p electron in (a) silicon & (b) sulfur. Explain how results relate to (i) the relative atomic radii of the 2 atoms; (ii) the relative 1st ionization energies of the 2 atoms. Si(1s)(2s2p)*(353p) S (1s) (2s2p)*(353p)