[sniper]

Sir,
     What is the general formula to find the ionization energy of elements?

[Albert]

I sincerely doubt there's a formula for it. I think it's calculated with empirical data.

[nearly.alex]

an estimate can be worked out due to the trends across a period but this is still a very rough estimate especially for the first few periods, also this has to be modified to take into account whether the electron being removed is the only one in that subshell or the first electron to be -1/2 spin in the HOMO ie. oxygen which would make the IE be slightly less than expected. but i do not believe there is a formula and i from what i know the ionization energy has to be worked out experimentally.

[lemonoman]

According to Quantum Mechanics (and a few degrees of approximation), we find something called Koopman's theorem.  It states that "ionization energies of a molecule are equal to the eigenvalues of the Fock operator associated to the occupied molecular orbitals" (WikiPedia, "Koopman's Theorem").

This method is kinda complicated, and makes a lot of approximations...if you don't have a background in quantum mechanics, I don't think it's worth getting into .  You have to know the wavefunctions for the electrons, and Slater determinants, a whole slew of stuff.

It's best just to get the data by experimenting.  Or looking it up somewhere...

[lemonoman]

Oh, and by the way, if you want a REALLY simplified, extremely general and very-rough (i.e. not necessarily accurate) method...there's some electrostatic formulae.  Check out the WikiPedia article on Ionization Potentials