Notes On d-Block: Colour And Magnetic Properties - CBSE Class 12 Chemistry

Most d-block metal compounds are coloured in their solid or liquid states.

In the case of transition metal ions, under the influence of ligands, the degeneracy of the 5 d-orbitals is lost and they separate into two distinct energy levels.


eg set: d x2- y2 and dz2 orbitals

t2g set: dxy, dxz and dyz orbitals.

When white light is incident on a transition metal ion, the electron in the lower energy d-orbital set absorbs certain radiations and gets promoted to a d-orbital set of higher energy. The transmitted radiation devoid of the absorbed radiations is the complementary colour of the absorbed light. This complementary colour is the colour of the substance.

Magnetic properties:

Substances, depending on their behaviour in an external magnetic field, are classified into 2 types

Paramagnetic substances: These are weakly attracted to a magnetic field

Reason: presence of unpaired electrons in the atom

Diamagnetic: These are repelled to a magnetic field.

Reason: absence of unpaired electrons in the atom.

The magnetic moment of the electron is due to spin angular momentum as well as orbital angular momentum.

For 3d series of elements, the orbital angular momentum of the electrons is small, and hence, its contribution in these elements can be ignored. Therefore, the observed magnetic moment can be ascribed to the spin of the unpaired electrons only.

The magnetic moment for these elements is determined by the spin only formula

μ = √(n(n+2))

n = number of unpaired electrons

μ = magnetic moment in units of Bohr Magneton (BM)

When n=1, µ= 1.73 BM

When n =2, µ=2.83

In case of iron, cobalt and nickel, the unpaired electron spins are much pronounced. As a result, these elements are much more paramagnetic than the rest of the elements. These elements are called ferromagnetic substances.

Summary

Most d-block metal compounds are coloured in their solid or liquid states.

In the case of transition metal ions, under the influence of ligands, the degeneracy of the 5 d-orbitals is lost and they separate into two distinct energy levels.


eg set: d x2- y2 and dz2 orbitals

t2g set: dxy, dxz and dyz orbitals.

When white light is incident on a transition metal ion, the electron in the lower energy d-orbital set absorbs certain radiations and gets promoted to a d-orbital set of higher energy. The transmitted radiation devoid of the absorbed radiations is the complementary colour of the absorbed light. This complementary colour is the colour of the substance.

Magnetic properties:

Substances, depending on their behaviour in an external magnetic field, are classified into 2 types

Paramagnetic substances: These are weakly attracted to a magnetic field

Reason: presence of unpaired electrons in the atom

Diamagnetic: These are repelled to a magnetic field.

Reason: absence of unpaired electrons in the atom.

The magnetic moment of the electron is due to spin angular momentum as well as orbital angular momentum.

For 3d series of elements, the orbital angular momentum of the electrons is small, and hence, its contribution in these elements can be ignored. Therefore, the observed magnetic moment can be ascribed to the spin of the unpaired electrons only.

The magnetic moment for these elements is determined by the spin only formula

μ = √(n(n+2))

n = number of unpaired electrons

μ = magnetic moment in units of Bohr Magneton (BM)

When n=1, µ= 1.73 BM

When n =2, µ=2.83

In case of iron, cobalt and nickel, the unpaired electron spins are much pronounced. As a result, these elements are much more paramagnetic than the rest of the elements. These elements are called ferromagnetic substances.

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