Probably too late for this answer but here it is anyway. The outer shell he is referring to is the M-shell bcos the 17th electron is occupied in the 3p orbital(which is part of the M-shell) if you follow Hund's rule. 1s2, 2s2, 2p6,3s2, 3p5 = 2+2+6+2+5=17. So, Chlorine is more than half filled and thus attracts electron. It needs only one more electron to complete the 3p orbital and thus the "outer shell". It has this tendency bcos it reduces the overall energy of the atom, but you don't need to know this as it requires QM treatment. Anyway, that is your answer, hope it helps. If it doesn't and you are still sceptical then reading a bit about the QM treatment of Hydrogen atoms and subsequently about the quantum numbers n,l,m_l and m_s should help you bridge the gap between Physics and Chemistry, at least on the atomic scale. If you can differentiate between shells and orbitals then that would also help you answer this question but then again, the fundamentals lie in the quantum numbers.
