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When more electrons are in the inner orbitals, shielding takes place. This is true for all atoms above Helium. It affects ionization energy because the effective nuclear charge the valence electrons essentially feel is less than it would be if the inner orbitals didn't have any electrons. The ionization energy would be lower for any valence electron that experiences shielding.
Electrons repel each other because of their negative charge. Electrons are attracted to the positively charged nucleus, but outer subshells experience shielding due to electron-electron repulsion. Valence shell electrons experience the most shielding.
Shielding is essentially the reduction in the effect of the nuclear charge from the nucleus felt by electrons in outer orbitals because they are blocked or "shielded" from the nucleus by the electrons in the inner orbitals
Think of it like people around a campfire. The people close to the campfire will receive the most warmth, will people farther away won't receive as much. Therefore, the people farther away are more likely to go to a different fire where they get more warmth. The people close to the campfire are the inner-shell electrons, and the people getting the least warmth are the valence shell electrons. The valance shell electrons are more likely to leave the atom to go to a different atom or be bonded to another atom.
Inner electrons can shield valence electrons from the inward pull of the nucleus because of the electron-electron repulsion between these inner electrons and valence electrons. The valence electrons experience a weaker effective nuclear charge, making it easier to remove the electron from the atom, so ionization energy is lower.
Since some people already responded to your question on what shield is, I'll just say how it affects ionization energy. Basically, you will see a trend of decreasing ionization energy down the periodic table and a trend of increasing ionization energy across the period.
Everyone above explained the concept that electrons closer to the nucleus have a stronger pull towards it than the further valence electrons nicely. The important thing to take away is that electrons in further shells are more easy to form bonds with other atoms because there's less pull!
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