Ionization Enthalpy and Valency

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Introduction - Ionization Energy

Ionizing energy is the energy used for withdrawing an electron from a gaseous atom or ion. An atom or molecule's first or original ionizing force, or Ei, is the energy needed to remove one mole of electrons from one mole of separated gaseous atoms or ions.

In periodic table: Ionization energy increases from left to right and ionization decreases from top to bottom.

Ionisation Energy Formula

Ionization Energy for each ion shall be calculated in the periodic table. To recognize energy ionization, it is therefore helpful to understand the calculations used in measuring the amount of energy needed to remove electrons.

The basic equation of Ionization energy is written as:

X → X+ + e-

Once the electrons are removed from the atom or molecule, the amount of energy required to remove other electrons from the same atom or molecule. Thus the equation changes. 

Equation changes, as amount of energy required to remove electron is changed.

  • 1st ionization energy equation

X → X+ + e-

  • 2nd ionization energy equation

X+ → X2+ + e-

  • 3rd ionization energy equation

X2+ → X3+ + e-

Units which are used to measure ionization are not necessarily identical. When disclosing ionization energy, chemists refer to one mole (mol) of a material. The meaning is either kJ / mol or kcal / mol. Physicists use the unit electron volt (eV).

The ionization energy determines how closely an atom is clinging onto its electrons. The closer an electron remains, the greater the potential for the ionization. The advances in energy from ionization are just the opposite of those for atomic radii. Generally, as the atomic radii grow higher, the forces of ionization get less, and vice versa.

Ionisation Energy Definition:

Ionization energy is the energy required to remove an electron from an atom or molecule to infinity.

Ionization Energy Trend in the Periodic Table

Along with atomic and ionic radius, electron affinity, electronegativity and metallicity, follows a trend on the modern periodic table of elements.

As the atomic radii decreases across the period, ionization energy increases from left to right. consequently, there is a greater effective attraction between the negatively charged electrons and positively-charged nucleus.

Ionization for the alkali metal on the left side of the table and average for the noble gas on the far-right side of a period is at its minimum value. The noble gas has a valence shell packed up, so it prevents elimination of electrons.

In a group the ionization energy decreases from top to bottom. That is because the outermost electron 's principal quantum number decreases moving down a group. There are more protons in the atoms which move down a group (greater positive charge), yet the result is to bring in the shells of electrons, rendering them smaller and filtering outer electrons from the nucleus attractive force. More shells of electrons have introduced that pass down a group so that the outermost electron is increasingly distant from the nucleus.

First, Second, and Subsequent Ionization Energies

The energy required to remove the electron of the outermost valence from a neutral atom is the first energy of ionisation. The second energy of ionization is that which is required to remove the next electron, etc.

The second frequency of ionisation is always higher than the energy of the first ionization. Take a metal-alkali atom for example.

It is fairly easy to remove the first electron, because its absence gives the atom a strong shell of electron. Removing the second electron involves a new layer containing electrons similar to the atomic nucleus and lower to it.

The first ionization energy of hydrogen may be represented by the following equation:

H(g) → H+(g) + e-

ΔH° = -1312.0 kJ/mol

Exemptions to the Ionization Energy Trend

Two anomalies are found in the pattern readily apparent in a table of first ionization energies. Boron's first ionizing value is less than beryllium's, and the oxygen's first ionizing energy is less than helium.

The reason for the discrepancy is due to these elements electron configuration, and Hund 's rule. For beryllium, the first theoretical electron for ionization falls from the 2s orbital, while boron ionization includes a 2p electron.

The electron falls from the 2p orbital for both nitrogen and oxygen, but the spin is the same for all 2p nitrogen electrons, while one of the 2p oxygen orbitals produces a number of paired electrons.


Valence refers to an atom or a group of chemically bonded atoms being able to form chemical bonds with other atoms or groups of atoms.

An element's valence is determined by the number of electrons at the outer shell (valence). The charge on the particle is the valence of polyatomic ions (such as SO42-).

Valency and its Periodic Trends

Elements are placed in groups (columns) according to the number of valence electrons in the periodic table, so of course the location of the item in the periodic table will give us an idea of its valence.

Both group 1 elements have 1 valence electron so they have a +1 valence, because they tend to give up 1 electron.

This is the same for group 2 giving up two electrons, and group 3 giving up 3 electrons.

Nevertheless, group 5 elements have 5 valence electrons and will tend to take 3 electrons, thereby providing a valence of -3.

Group 6 elements, have 6 electrons of valence which tend to take 2 electrons and have a valence of -2.

Group 7 components have 7 valence electrons and a valence of -1 would tend to take 1 electron.

Group 8 elements do not react and therefore have a value of 0.

FAQ (Frequently Asked Questions)

1. What is Enthalpy of Ionisation?

Ans. Ionization Enthalpy is defined as the amount of energy an isolated gaseous atom would take to lose an electron in its ground state. Whenever an electron is expelled from an atom, it takes a specific amount of energy to expel it, hence the enthalpies of chemical elements for ionisation are always optimistic.

2. Is Ionisation Energy and Ionisation Enthalpy the Same?

Ans. Ionization enthalpy is the minimum amount of energy required to detach from an isolated gaseous atom the most loosely bound electron and turn it into a gaseous cation. Whereas ionization energy reflects the amount of energy needed to expel the electron from the ground state to infinity.

3. Which Element has the Highest Ionisation Enthalpy?

Ans. Helium, i.e. 2372.3 kJ / mole or 24.5874 eV, is the maximum enthalpy for ionization in periodic table. This is because helium has two electrons in 1s orbit, i.e. electrons are very near to the nucleus and therefore needs very high energy to be released from the nucleus unscreened.