Explain the concept of the screening effect of electrons?

Answer 1

When I was in school, we called it 'shielding', but it's just a different name for the same thing:

It is the protons in an atom that attract the electrons, of course. But electrons HATE to be near each other, and inner groups of electrons force the outer groups of electrons further and further away from the protons. Since electrical attraction, like most of the forces, decreases in strength with distance, this kind of pushing away from the nucleus make a big difference!

This is why every element has a lower ionization energy and elecetronegativity than the element directly above it in the periodic table: H+ is just a naked proton, while an electron encountering Li+ is going to have to share with two others, and by the time you get to Fr+ (which has 86 electrons in the way) there may be virtually no incentive for electrons to even try to squeeze in there.

All +3 ions are NOT the same! Hope that helps!