Question

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?

Answer 1

Anion ionic radius is larger than the parent atom's covalent radius due to increased electron-electron repulsion and decreased effective nuclear charge (Zeff) per electron. For cations, a decrease in electron-electron repulsion and an increased Zeff on the remaining electrons result in a smaller ionic radius.

Step-by-step explanation:

The intuition that electrons being pulled towards a positively charged nucleus should decrease the ionic radius is incorrect when it comes to anions. When an atom gains electrons to become an anion, the increased electron-electron repulsion and the decrease in effective nuclear charge per electron (Zeff) cause the ionic radius to be larger than the parent atom.

Conversely, when electrons are removed from an atom to form a cation, the cationic radius decreases due to reduced electron-electron repulsion and greater Zeff felt by the remaining electrons.

For example, a sulfur atom has a covalent radius of 104 pm, but when it gains electrons to become the sulfide anion, its radius increases to 170 pm. This is due to the greater number of electrons causing increased repulsion between them, as well as a larger principal quantum number in anions proceeding down a group.