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Ionization Energy Definition and Trend


Ionization Energy is a process, involves the removal of an electron present in an outermost orbit of an atom. After ionization, the resulting atom is known as an ion.

A simple ionization reaction:

A (g) + energy → A+ (g) + e

Where A is any atom or molecule, A+ is that atom or molecule with an electron removed, and e is the removed electron. 

Positively charged ions are often referred to as cations.

Negatively charged ions are often referred to as anions.

A (g) + e → A(g) + energy

A+ representing cation.

A–   representing  anion

Ionization energy: It is defined as the minimum amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated neutral gaseous atom or molecule.

Ionization energy is typically measured in kJ/mol, it is the amount of energy it takes for all the atoms in a mole to lose one electron each.

Ionization energies of a particular atom depend upon the average electron distance from the nucleus and effective nuclear charge

Ionization Energy Trends in the Periodic Table

1st ionization energy – the amount of energy required to remove the highest energy electron from a neutral gaseous atom.

For Example:         A (g) → A+(g) + e–  

 The ionization energy is always positive because it requires energy to remove an electron.

2nd ionization energy– the amount of energy required to remove a second electron from a singly charged gaseous cation.

For Example:A+(g) → A2+(g) + e

The second ionization energy is nearly ten times that of the first because the number of electrons causing repulsions is reduced.

3rd ionization energy– the amount of energy required to remove the third electron from a doubly charged gaseous cation.

For Example:A2+(g) → A3+(g) + e

Third ionization energy is even higher than the second ionization energy.

Ionization energy depends on the following two factors:

First, the force of attraction between electrons and the nucleus.

Second, force of repulsion between electrons.

The effective charge of the nucleus felt by the outermost electrons will be less than the actual nuclear charge. This is because of the inner electrons will shield the outermost electrons by hindering the path of nuclear charge. This effect is known as the shielding effect

Ionization Energy Trend in the Periodic Table

General periodic trends:

1. In a group, while moving from top to bottom ionization energy decreases.

2. Ionization energy increases from left to right across a period.

1. Trends in ionization in a group:

Ionization: ionization energy, ionization energy trend

The first ionization energy of elements in a group decreases as we move from up to down. While stirring downward in a group, the atomic number increases and the number of shells also increases. Outermost electrons are very far from the nucleus and thus can be detached easily. One more factor that decreases the ionization energy is the shielding effect due to an increasing number of shells as we move downward in a group.

2. Trends in ionization energy across a period:

As moving from left to right across any period, the ionization energy of elements increases. This is because of the decrease in the size of atoms across a period. The valence electrons get closer to the nucleus as moving from left to right due to increased nuclear charge. The force of attraction between nucleus and electrons increases and hence more energy is required to remove an electron from the valence shell.

Ionization: ionization energy, ionization energy trend

Fig. Periodic Trends in ionization energy of elements

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