Question

I am trying to understand successive ionisation energies. In particular, an explanation for why the second ionisation energy is greater than the first ionisation energy. I'm looking for a clear and concise explanation.

The common explanation is that it's harder for an electron to be removed from a cation than the atom or that the second electron feels a greater nuclear charge. However, these explanations do not satisfy me.

For starters, How can the nuclear charge be greater for the second electron when the number of protons within the nucleas remains unchanged. Assuming both the first and second electrons are on the same energy level, they experience the same amount of shielding from inner electrons. So it seems to me, both electrons feel the same nuclear charge. I can see the second electron, perhaps feeling repulsion from the first that results in less nuclear charge. Would this then be correct?

I don't particularly understand the explanation about it being harder to remove an electron from a cation. The way I see it, is that the atomic radius decreases when the first electron is removed, meaning the second electrons feels more electrostatic attraction from being closer. Is this explanation sufficient? Could someone please provide a clear explanation for this?

Answer 1

The second ionisation energy is greater than the first due to the reduced electron-electron repulsion resulting in higher effective nuclear charge and the closer proximity of remaining electrons to the nucleus, both of which increase the electrostatic attraction between electrons and the nucleus.

Step-by-step explanation:

The reason the second ionisation energy is greater than the first revolves around the concept of effective nuclear charge (Zeff) and changes in electrostatic attraction between the remaining electrons and the nucleus after an electron is removed.

Although the number of protons remains the same, the removal of an electron reduces electron-electron repulsion, so remaining electrons are held more tightly. Additionally, as you pointed out, the atomic radius decreases when an electron is removed, further increasing the effective nuclear charge on the remaining electrons because they are closer to the nucleus and less shielded.

The concept of Zeff highlights that while the exact nuclear charge does not change, the perceived charge by each electron increases when another electron is removed, which means greater energy is required to remove the next electron. Moreover, once a valence electron is removed, subsequent electrons may come from progressively lower energy levels, where the nuclear attraction is stronger, significantly increasing the energy required to remove further electrons.