Q.Define excitation energy, binding energy and ionization energy of an electron in an atom.

[!TLDR] Excitation energy = energy to move an electron from the ground state to a given excited state ($E_n-E_1$); ionization energy = energy to free the ground-state electron completely (=13.6 eV for hydrogen); binding energy of the ground-state electron = numerically equal to its ionization energy …

The excitation energy of a given excited state n is the energy that must be supplied to raise an electron from the ground state (n=1) up to that state n; it equals $E_n-E_1$, and for hydrogen's first excited state (n=2) this works out to $-3.4-(-13.6)=10.2$ eV. The ionization energy is the specific, minimum amount of energy needed to remove the electron entirely from the atom starting from its ground state, i.e. to raise its energy all the way from $E_1=-13.6$ eV (for hydrogen) up to exactly 0 eV (the free-electron limit); this is a single fixed number for a given atom -- 13.6 eV for hydrogen -- not something that varies by orbit. The binding energy of the ground-state electron is the same 13.6 eV figure viewed from the opposite direction: it is the energy that would be RELEASED if a free electron and a free proton, …

Recall each definition directly in terms of $E_n=-13.6/n^2$ eV, noting ionization energy and ground-state binding energy are num …

Confusing 'excitation energy of state n' (the energy to reach state n FROM the ground state) with the energy difference between two arbitrary excited states, or treating 'binding energy' and 'ionization energy' as two …