# What does it mean that an electron quantum numbers are n1= 4 - l1?

I'm trying to solve some exercises from the 2017 IChO, and the problem 6-A1 states that

in first ionization, an electron with the quantum numbers $$n_1 = 4 - l_1$$ is removed.

Now, I've never seen this way of expressing quantum numbers, and I don't know what it's supposed to mean, so I would appreciate it if someone could dumb it down to me.

• Could you provide a link to the exercises? As is, I can't make out what you writing. If not, it may help if you rewrite the equation in the question using [Mathjax]( chemistry.meta.stackexchange.com/q/86/41556) so it is more readable. Welcome to the site! @Camila – Tyberius Dec 29 '18 at 21:16
• I've edited your question including the link to the problem I found (please check out whether it is the correct one). Still, to me it's not quite clear what you are trying to find out since $n_i$ and $l_i$ are standard notations for principal and azimuthal Q.N. – andselisk Dec 30 '18 at 1:27
• @Camila you seem to just be getting confused by the subscripts. Those are just their to label that $n_1$ and $l_1$ are the quantum numbers of the 1st electron ionized. – Tyberius Dec 30 '18 at 14:22

$$n_1 = 4 - l_1$$ means the same as $$n_1 + l_1 = 4$$, so the $$n$$ and $$l$$ quantum numbers of the first electron added up should equal 4. This could refer to 3p or 4s.
For the second electron removed, the $$n$$ and $$l$$ add up to 5. This could refer to 3d, 4p or 5s.