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ChemistryEdu Logo d-block Elements | Ionization Enthalpy#

Ionization Enthalpy#

  • The energy required to remove an electron from an isolated gaseous atom in its ground state is known as ionization enthalpy. It is depicted by ΔiH. It is expressed in Kilojoules per mole (KJ mol-1).

  • It is the quantitative measure of the tendency of an atom to lose electron.

  • Energy is always required to remove electron(s) from an atom. Hence, it is always positive.

  • In general, ionization enthalpy increases along each series from left to right due to increase in nuclear charge.

Irregular trend in first ionization enthalpy of 3d elements#

  • Removal of one electron alters the relative energies of 4s and 3d-orbitals. Due to reorganization of energy, the irregular trend is observed in first ionization enthalpy of 3-d metals.

The first ionization enthalpy of Cr is lower while that of Zn is higher. Explain.

The first ionization enthalpy of Cr is lower because of absence of any change in the d-configuration.

Electronic configuration of Cr: 1s22s22p63s23p63d54s1. The last electron in 4s can be easily removed.

The first ionization enthalpy of Zn is higher as electron has to be removed from completely filled 4s-orbital. (Electronic configuration of Zn: 1s22s22p63s23p63d104s2)

Second ionization enthalpy of 3d elements#

  • Second ionization enthalpy is the energy required to remove second outermost electron from a cation (with +1 charge) in gaseous state.
  • The second ionization enthalpy also shows very irregular trend in 3d metals.

The second ionization enthalpy of Cr and Cu are very high. Explain.

In case of Cr, second electron has to be removed from stable 3d5 configuration and in case of Cu, second electron has to be removed from stable 3d10 configuration.

The second ionization enthalpy of Zn is low. Why?

Electronic configuration of Zn+: 1s22s22p63s23p63d104s1

Second electron has to be removed from 4s shell, which is much easier and it will allow the formation of stable completely filled d10 configuration.

Third ionization enthalpy of 3d elements#

  • The third ionization enthalpy of these elements is very high because it is very difficult to remove electron from d-orbitals.

Why is it difficult to obtain oxidation state greater than 2 for transition elements?

It is difficult to obtain oxidation state greater than 2 for transition elements due to high value of third ionization enthalpy.

Generally, ionization enthalpy increases across a series. But third ionization enthalpy of Fe is less than that of Mn. Explain.

Electronic configuration of Mn2+: 1s2222p63s23p63d5

Electronic configuration of Fe2+: 1s22s22p63s23p63d6

Fe2+ readily loses electron to achieve stable d5 configuration while Mn2+ does not give electron easily due to presence of stable d5 configuration.