Key difference – First vs Second Ionization Energy (I1E vs I2E)
Before analyzing the difference between first and second ionization energy, let us first discuss what ionization energy is. In general, the ionization energy is referred to as the energy required to remove an electron from a gaseous atom or an ion. Since electrons are attracted to the positive nucleus, the energy needs to be supplied for this process. This is considered as an endothermic process. The ionization energies are expressed in kJ mol-1. The key difference between first and second ionization energy is best explained in their definitions; The energy absorbed by a neutral, gaseous atom to produce a +1 charged ion (to remove an electron) is called first ionization energy whereas the energy absorbed by a positively charged (+1) gaseous ion to produce ion with a +2 charge is called the second ionization energy. The ionization energy is calculated for 1 mol of atoms or ions. In other words; first ionization energy relates to neutral gaseous atoms and second ionization energy relates to the gaseous ions with a (+1) charge. The magnitude of the ionization energy varies depending on the charge of the nucleus, the distance of the electron form of the nucleus and the number of electrons between the nucleus and the outer shell electrons.
What is First Ionization Energy (I1E)?
The first ionization energy is defined as the energy absorbed by 1 mol of neutral gaseous atoms to remove the most loosely bound electron from atom to produce 1 mol of gaseous ions with +1 charge. The magnitude of first ionization energy increases along a period in the periodic table and decreases along a group. First ionization energy has periodicity; it has the same pattern repeatedly along the periodic table.
What is Second Ionization Energy (I2E)?
The second ionization energy is defined as the energy absorbed by 1 mol of positively charged gaseous ions to produce 1 mol of gaseous ions with a +2 charge, by removing the loosely bound electron from the +1 ion. Second ionization energy also shows periodicity.
What is the difference between First and Second Ionization Energy (I1E and I2E)?
Definition of First and Second Ionization Energy
First ionization energy (I1E): The energy required to remove the most loosely bound electron from 1 mol of gaseous atoms to produce 1 mol of gaseous ions with a positive charge (+1).
X (g) X+ (g) + e–
(1 mol) (1 mol) (1 mol)
Second ionization energy (I2E): The energy required to remove the most loosely bound electron from 1 mol of gaseous ions with a +1 charge to produce mol of gaseous ions with +2 charge.
X+ (g) X2+ (g) + e–
(1 mol) (1 mol) (1 mol)
Characteristics of First and Second Ionization Energy
Energy requirement
Usually expelling the first electron from a ground state gaseous atom is easier than expelling the second electron from a positively charged ion. Therefore, the first ionization energy is less than the second ionization energy and the energy difference between first and second ionization energy is significantly large.
| Element | First ionization energy (I1E) /kJ mol-1 | Second ionization energy (I2E) / kJ mol-1 |
| Hydrogen (H) | 1312 | |
| Helium (He) | 2372 | 5250 |
| Lithium (Li) | 520 | 7292 |
| Beryllium (Be) | 899 | 1757 |
| Boron (B) | 800 | 2426 |
| Carbon (C) | 1086 | 2352 |
| Nitrogen (N) | 1402 | 2855 |
| Oxygen (O) | 1314 | 3388 |
| Fluorine (F) | 680 | 3375 |
| Neon (Ne) | 2080 | 3963 |
| Sodium (Na) | 496 | 4563 |
| Magnesium (Mg) | 737 | 1450 |
Trends of the ionization energy in the periodic table
First ionization energy (I1E): First ionization energy values of atoms in every period show the same variation. Magnitude is always less than the second ionization energy values
Second ionization energy (I2E): Second ionization energy values of atoms in every period show the same variation; those values are always higher than the first ionization energy values.
