Classification of Elements and Periodicity in Properties

Ionization Enthalpy and Valency

Are you interested to know about some more properties of elements in the periodic table? How about Ionization Enthalpy of the elements? Do you want to know if elements possess energy just like us? This topic will give you the answer. So let’s study about Ionization Enthalpy and Valency of elements in the periodic table.

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Ionization Enthalpy

Ionization Enthalpy of elements is the amount of energy that an isolated gaseous atom requires to lose an electron in its ground state. Losing of electrons results in the formation of cations.


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

A (g) → A+ (g) + e

 In the same way, the second ionization energy is nothing but the energy that you need to remove the second electron from its valence shell. It is explained by the equation given below:

A(g)  →   A2+ (g) + e

You need to provide a specific amount of energy to remove an electron from an atom. Hence, the ionization enthalpies of chemical elements are always positive. The second most outer electron will be more attracted to the nucleus than the first outer electron. Therefore, the second ionization energy will always be more than the first ionization energy. In the same way, third ionization enthalpy will be greater than the second one.

Browse more Topics under Classification Of Elements And Periodicity In Properties

Factors for Ionization Enthalpies

The Ionization Energy depends on two factors, which are:

  • The force of attraction between electrons and the nucleus.
  • 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, shielding effect is more prominent when the inner orbitals are completely filled.

General Periodic Trends

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

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, 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.

Trends in Ionization Enthalpy Across a Period

  • As we move from left to right across a period, 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.

What is Valency?

Valence or Valency is one of the most fundamental properties of the elements. You can study it with the help of electronic configuration. You find valence electrons in the outermost shell. Valency is the number of valence electrons in the outermost shell.

The valency of atoms of s-block and p-block elements is the number of valence electron or eight minus the number of valence electrons. The general valencies of these d and f block elements are 2 and 3.


Variation along a Period

While moving left to right across a period, the number of valence electrons of elements increases and varies between 1 to 8. But the valency of elements, when combined with H or O first, increases from 1 to 4 and then it reduces to zero.

Nowadays, valency is also termed as the oxidation state. Consider two compounds that contain oxygen. For example, Na2O and F2O. In F2O, the electronegativity of F is more than oxygen.

Hence, each of F atoms will attract one electron from oxygen i.e. F will show -1 oxidation state and O will show +2 oxidation state. Whereas, in the case of Na2O, oxygen is highly electronegative than sodium atom.

So, oxygen will attract two electrons from each sodium atom showing -2 oxidation state and Na will have +1 oxidation state. Thus, we see that the oxidation state of the element represents the charge possessed by an atom due to loss or gain of electrons (due to the electronegativity difference between the combining atoms) in the molecule.

A Solved Question for You

Q: How does valency vary in a group?

Ans: As we move down in a group the number of the valence electron does not change. Hence, all the elements of one group have the same valency.

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One response to “Periodic Properties of Elements”

  1. bamukambwire james says:

    Its a wonderful information

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