Ionization Energy Trend

What Is Ionization Energy?

Ionization energy or Ionization enthalpy of elements is defined as

the amount of energy required by an isolated gaseous atom to lose an electron in its ground state.

Losing of electrons results in cation formation.

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The first ionization energy of element A is defined as the energy required by an atom to form A⁺ ions.  The unit of ionization energy is given as KJ mol^-1.

A (g) → A⁺ (g) + e^–

 In the same way, second ionization energy is described as the energy needed to remove the second electron from its valence shell. It can be explained by the equation given below:

A⁺ (g)  →   A²⁺ (g) + e^–

Whenever an electron is removed from an atom, a specific amount of energy is required to remove it, hence the ionization enthalpies of chemical elements are always positive.  The second most outer electron will be more attracted by the nucleus than the first outer electron. Therefore, the second ionization energy will be greater than the first ionization energy. In the same way, third ionization enthalpy will be greater than the second one.

Factors affecting Ionization Energy

Ionization energy depends on two factors:

1. The force of attraction between electrons and the nucleus.
2. The force of repulsion between electrons.

The effective nuclear charge felt by the outermost electrons will be less than the actual nuclear charge. This is because the inner electrons will shield the outermost electrons by hindering the path of nuclear charge. This effect is known as the shielding effect. For example, In Na, the 3s¹ electrons will be shielded by its core electrons (1s², 2s² and 2p⁶). Generally, the shielding effect is more prominent when the inner orbitals are completely filled.

Ionization Energy Trend in the Periodic Table

General periodic trends:

1. In a group, while moving from top to bottom it decreases.
2. It increases from left to right across a period.

1. Trends in ionization enthalpy in a group:

The first ionization enthalpy of elements decreases as we move down in a group. While moving down in a group, the atomic number increases and the number of shells also increases. Outermost electrons are far away from the nucleus and thus can be removed easily. The second factor that decreases the ionization energy is the shielding effect due to an increasing number of shells as we move down a group.

2. Trends in ionization enthalpy across a period:

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

This was just a brief layout of the ionization energy and its trend in the periodic table. To know more about the properties of elements (such as ionization enthalpy) and their trends in the periodic table, please download BYJU’S – the learning app.