BR Question: The ionization energy of H (g) is 1312 kJ/mole. A good approximation for the second ionization potential of helium (g) is: ANSWER: 5248 kJ/mole What I dont understand is part of the explanation. It states how the Zeff value of H is +1 and the Zeff value of helium is +2. I thought that in order to find Zeff then the equation would be protons- core electrons (2). Why is this equation not used in the problem to find the Zeff? Thank you!

The trick is that they are asking for the second ionization energy of helium, which only has two electrons in its neutral state. The second ionization refers to removal of the lone remaining electron from a nucleus that has a +2. There are no core electrons to consider, so it depends on +2. Because the ionization energy depends on Zexp2, the IE is four times as great for a +2 nucleus as a +1 nucleus.

Sorry to bump an old thread, but could you please explain this more in depth? Like why is the Zeff for He+ = +2? And how does the I.E. = Zexp2 equation relate the helium number to the hydrogen number? Sorry...so confused!!!