Key Difference – Excitation vs Ionization Potential
The two terms excitation potential and ionization potential are related to the energy required to move electrons, but there is a difference between them based on the destination of the electron movement. In other words, in these two situations, the destination of the electron after the movement is different. Two movements of electrons can be identified this way. Electrons can either move to a higher energy level within the atom or molecule or detach themselves from the nucleus and move away from the atom. Both of these processes require definite quantities of energy. Electrons cannot move unless the required energy is not absorbed. The key difference between excitation and ionization potential is that excitation potential is the energy required to jump from one energy level to other while ionization potential is the energy required to remove an electron from an atom.
What is Excitation Potential?
Atoms have energy levels that are called orbits. Electrons move around the nucleus in these orbits. Electrons can’t choose arbitrary orbits; they are placed in certain orbits according to their energy levels and they are restricted to move or jump to another energy level unless they absorb the required amount of energy. Moving from one orbit to another after absorbing the required amount of energy is called excitation and the energy absorbed to move from one orbit to other is called excitation potential or excitation energy. 
What is Ionization Potential?
Ionization is the process of removing an electron from the valence shell. In general, electrons are bonded to the nucleus through strong electrostatic forces. Therefore, energy is required to completely remove an electron from the atom. This is defined as removing an electron from atom or molecule to an infinite distance. The energy required for this process is called, “ionization energy” or “ionization potential”.
In other words, it is the potential difference between the initial state, in which electron is bounded to the nucleus and the final state in which electron is no longer attached to the nucleus where it is rest at the infinity.
Periodic trends for ionization energy (IE) vs. proton number
What is the difference between Excitation and Ionization Potential?
Definition of Excitation and Ionization Potential
Excitation Potential:
The energy absorbed by an electron to move from one energy level to a higher energy level is called, “excitation potential” or excitation energy. This is usually the energy difference between initial and final state.
Note: electron moves inside the atom, but in different energy levels.
Ionization Potential:
The energy required to remove an electron from an atom is called, “ionization potential” or “ionization energy”. This is the potential difference between two states where an electron is bounded to the nucleus and electron is removed from the atom. The energy when the electron is at an infinite distance is considered as zero.
Note: an electron is removed from the atom and there is no attraction with the nucleus when it is removed.
Calculation:
Excitation Potential:
When an electron jumps from ground state (n=1) to another (n=2) energy level the corresponding energy is called 1st excitation potential.
| 1st excitation potential = Energy (n=2 level) – Energy (n=1 level) = -3.4 ev – (-13.6 ev) = 10.2 ev |
When an electron jumps from ground state (n=1) to another (n=3) energy level the corresponding energy is called 2nd excitation potential.
| 2nd excitation potential = Energy (n=3 level) – Energy (n=1 level) = -1.5 ev – (-13.6 ev) = 12.1 ev |
Ionization Potential:
Consider removing an electron from n=1 energy level. The ionization potential is energy required to remove an electron from n=1 level to infinity.
| Ionization potential = E infinity – E (n=1 level) = 0 – (-13.6 ev) = 13.6 ev |
In atoms, the most loosely bound electrons are removed first and the ionization potential gradually increases as it ionizes.
Image Courtesy:
“Mean Excitation Potential” by HPaul – Own work. (Public Domain) via Wikimedia Commons
“First Ionization Energy” by User: Sponk (CC BY-SA 3.0) via Commons
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