How much energy is required to make an ion?

So say that I have a normal hydrogen atom (not an isotope, just regular normal hydrogen atom) and I want to make it into an ion $\text{H}^+$. Since to make an ion of that formula I need to take away an electron, it is just the same as giving the electron energy for it to transition from quantum energy level 1 to level $\infty$, so I use the energy level formula for the hydrogen atom $E\_n = -\frac{1312}{n^2}$ and substitute $n$ for $1$ and $\infty$ to get $-1312 \text{ kJ/mol}$ and $0\text{ kJ/mol}$ respectively. So if I add 1312 $\text{kJ/mol}$ of energy to the electron it should leave the atom, right? But my textbook says it is wrong.


