# Second Ionisation Energy Confusion

It is intuitive that the second ionisation will require more energy than the first. However, I'm having trouble explaining exactly why. I came across this other question on a similar topic: Why second ionisation energy greater than the first?

Technically, there would be an increased attractive force ($$\propto kq_1q_2/r^2$$) from the nucleus, because the electrons on average will now be closer to the nucleus.

Considering, say, chlorine, when a valence electron is removed, it makes sense that the electrostatic repulsion between valence electrons would decrease. However, by removing 1 electron, why does this increase the attractive force? I don't see how would increase the coulombic force between the nucleus and a valence electron, since the magnitude of both charges are constant.

If it was the radius changing, why should the electrons be closer to the nucleus just because 1 electron is removed?

• Just to get a sense: Think in term of net charge. There is no charge for the first ionisation. After your test charge, ie the second electron to be removed, sense a net one – Alchimista Oct 28 '19 at 8:25
• @Alchimista but why does having a net charge matter? Why would that affect the energy to remove 1 electron? The electron still has the same force of attraction to the nucleus essentially right? – John Hon Oct 28 '19 at 8:43
• Beside that quantum mechanics applies, just think of a distribution of n positive and n negative charges. Upon removal of the first one you get a net charge. Obviously the field is now different and the force experienced by each charge is now greater. Don't look at protons and the electron to be removed only, look at the entire distribution. It can be an exercise in the electrostatic chapter. – Alchimista Oct 28 '19 at 9:13
• @Alchimista No I don't see why the force experienced by each charge would be greater. Say we have 10 protons and 10 electrons in your arrangement. Each electron will experience 10 times the coloumbic force of 1 proton attracting it and 9 times the coloumbic force of 1 electron repelling it. If we remove 9 electrons, such that there's only one, then the attractive force felt must be the same. All that has decreased is the repulsion force. Is this what you mean by an "increase in force felt"? – John Hon Oct 28 '19 at 22:40
• Well yes. Force is the resulting vector. That is why I invite you to look at a bunch of charges. Two plus and two minus suffice, they don't have to be specifically arranged. Just do it. – Alchimista Oct 29 '19 at 8:32

In classical models of the atom, the radius of an electron subshell changes with the number of electrons in it (and with nuclear charge). This is essentially because the radius at which a set of electrons "settle" is a result of a balance involving the electrical forces, the magnetic forces and centrifugal forces. For example, the radius of $$\ce{He}$$ is 0.567 (in Bohr units). When it ionizes to $$\ce{He+}$$, one electron moves to infinity and the other settles at a radius of 0.500 because the force balance has changed with the departure of the first electron.