Why is the ionization energy of a sodium atom greater than that of a potassium atom?
Sodium has a higher ionization energy than potasium because the outer electron in sodium is closer to the effective (inner electron shielded) nuclear charge of +e than the potassium atom’s outer electron is to it’s effective (inner electron shielded) nuclear charge of +e because it is in the N=4 level as opposed to the sodium being in the N=3 level, approximating the single electron outer shell atoms. The coulomb potential and quantum mechanics dictate that this be so.
