What is "ionization energy"

n. (context physics English) The energy needed to remove an electron from an atom or molecule to infinity

The ionization energy (IE) is qualitatively defined as the amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated gaseous atom to form a cation. It is quantitatively expressed in symbols as:

where is any atom or molecule capable of being ionized, is that atom or molecule with an electron removed, and is the removed electron. This is an endothermic process.

Generally, the closer the electrons are to the nucleus of the atom, the higher the atom's ionization energy.

The units for ionization energy are different in physics and chemistry. In physics, the unit is the amount of energy required to remove a single electron from a single atom or molecule: expressed as an electron volt. In chemistry, the units are the amount of energy it takes for all the atoms in a mole of substance to lose one electron each: molar ionization energy or enthalpy, expressed as kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol).

Comparison of IEs of atoms in the Periodic table reveals two patterns:
# IEs generally increase as one moves from left to right within a period.

1. IEs decrease as one moves from down any given group.
   

(The latter is due to the outer electron shell being progressively further away from the nucleus with the addition of one inner shell per row as one moves down the column.)

The nth ionization energy refers to the amount of energy required to remove an electron from the species with a charge of (n-1). For example, the first three ionization energies are defined as follows:

1st ionization energy 2nd ionization energy 3rd ionization energy

The term ionization potential is an older name for ionization energy, because the oldest method of measuring ionization energies was based on ionizing a sample and accelerating the electron removed using an electrostatic potential. However this term is now considered obsolete. Some factors affecting the ionization potential include:
# Nuclear charge as the greater the magnitude of nuclear charge the more tightly the electrons are held by the nucleous and hence more will be ionization energy.

1. Number of electron shells as the greater the size of the atom less tightly the electrons are held by the nucleus and ionization energy will be less
2. Screening effect as the greater the magnitude of screening effect the less tightly the electrons are held by the nucleus and hence less will be the ionization energy.
3. Type of orbital ionized as the atom having stable electronic configuration has less tendency to lose electrons and consequently has high ionization energy.
4. Occupancy of the orbital matters as if the orbital is half or completely filled then it is harder to remove electrons