# How do you calculate calcium and potassium ionization energy?

##### 1 Answer

In general chemistry, you are never asked to calculate them from first principles, but to use them. Their first ionization energies are:

#"K":# #DeltaH_(IE_1) = "4.341 eV"#

#"Ca":# #DeltaH_(IE_1) = "6.113 eV"#

**ESTIMATION USING KNOWN ORBITAL POTENTIAL ENERGIES**

If you did want to try, you would need the orbital potential energies of the valence orbitals.

Then, by **Koopman's approximation theorem**, the energy needed to cancel out the valence orbital potential energy of a singly-occupied orbital gives the first ionization energy.

Indeed, the orbital potential energies shown here (Appendix B.9) are

#V_(4s,K) = -"4.34 eV"#

#V_(4s,Ca) = -"6.11 eV"#

So indeed, *the energy put into the atom to ionize it* (i.e. the ionization energy) is the magnitude of these given orbital potential energies.