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Here's my understanding:

  1. An ion with a negative charge has gained electrons. Hence the negative charge.
  2. With a greater negative charge, there should be more attraction towards the positive charge.
  3. As a result the electrons should be pulled towards the positively charged electrons.
  4. Ionic radius should decrease.
  5. Further an ion gains an electron to achieve a full shell. So the number of shells remains constant.

I understand that this is wrong. However, what is wrong with this intuition? Why does the ionic radius increase when the charge becomes more negative and decrease when the charge becomes more positive?

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    $\begingroup$ You are ignoring the electron repulsion, that's what is wrong. $\endgroup$
    – Ivan Neretin
    Feb 18 at 0:25
  • $\begingroup$ The addition of electrons does not mean attraction increases on each electron.. if you know about slater's rule, you will have a better understanding. When you add electrons an atom forms ions, which in fact become greater in size than an elemental atom because of electronic repulsion between the electrons. Hence, Ionic Radius Increase with the increase in negative charge $\endgroup$
    – Piyush Baweja
    Feb 18 at 6:00
  • $\begingroup$ This would be correct if you could increase the charge of an electron from -1 to -2. That is not how ions acquire a negative charge, though. (In contrast, nuclei come with different charges, which is why the inner electrons in sodium are closer to the nucleus than those in lithium, for example.) $\endgroup$
    – Karsten ♦
    Feb 18 at 13:37

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With a greater negative charge, there should be more attraction towards the positive charge

The pulling and pushing doesn't happen collectively to all electrons at the same time, it happens to each electron individually. So adding an electron to an atom doesn't increase the pushing and pulling to other electrons. On the other hand the nucleus is attracted more to the electrons, but the symmetries of the orbitals of an atom prevent the nucleus from moving right-left, up-down or inside-outside.

And of course the electron-electron repulsion pushes electrons away from each other so the overall radius is increased.

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The number of protons and neutrons remain the same when you add an electron to an ion. So the Effective nuclear charge doesn't change, only the number of electrons changes. So, the effective nuclear charge which can hold 'n' number of electrons isn't sufficient hold 'n+1' number of electrons. So the nucleus does not attract the electrons. But due to the inter-electronic repulsions, the electrons get away from each other, so they get farther from each other.

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