Does electron shielding increase or stay constant moving left to right across a period?
I have read about both, and I just want to know which one is right.

I believe that electron shielding remains constant because when you move across a period, you are essentially adding more valence electrons, not shielding electrons, in your valence shell. Therefore, your valence-electron-count increases from left to right in a period, but your shielding-electron-count stays the same.

So the overall shielding remains constant, just the amount of valence electrons increases

However, my explanation has a flaw:
It does not support the reason why an atom's radius increases when it becomes an anion.

When an atom becomes an anion, it adds more valence electrons to its outermost shell. Now this is controversial to what I said previously, because a gain in valence electrons, but not shielding electrons, wouldn't increase the electron shielding of the atom. So the atomic radius wouldn't increase either!

The only way for an atom's radius to increase when it becomes an anion is if the electrons that are added to the valence shell have some sort of effect on the shielding, right?

This supports the first thought in my question that if we move left to right across a period, shielding increases.

But the idea that electron shielding increases as we move left to right has a flaw as well:
It is controversial to another periodic trend: ionization energy

If electron shielding did increase across a period from left to right, then this would mean that ionization energy is lower as we move from left to right, which is not true.

The only way for ionization energy to increase across a period is if the only the number of protons and valence electrons have effect on the amount of energy required, not the shielding electrons. So the shielding-effect must stay the same all throughout a period.

This goes with the what I had initially said about electron-shielding remaining constant.

So now we have a flaw for each possible answer to my question! Please correct me if I am wrong in any of my context.

  • $\begingroup$ 1) Valence electrons repel each other and this is independent of shielding and nuclear charge. 2) In terms of ionization account for the fact that generally more ionizations occur to form the anion moving from left to right. $\endgroup$ – Joseph Hirsch Dec 3 '16 at 23:26
  • $\begingroup$ And 3) as for ionization energy rising with the same degree of electron shielding, account for the fact that the nuclear charge is greater so the core charge is also greater moving from left to right. $\endgroup$ – Joseph Hirsch Dec 3 '16 at 23:38
  • $\begingroup$ @JosephHirsch so does electron shielding increase or stay constant across a period? $\endgroup$ – Aniket Dec 4 '16 at 0:20
  • $\begingroup$ @JosephHirsch And also, how does valence electrons repelling each other affect anions and make them larger than their neutral atom? $\endgroup$ – Aniket Dec 4 '16 at 0:30
  • $\begingroup$ Electron shielding (as can be seen in the rise in TRUE net effective charge of less than 1 per additional proton) rises from left to right. This MIGHT be due to repulsive forces between valance electrons. $\endgroup$ – Joseph Hirsch Dec 4 '16 at 16:45

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