Statement I: Most of the compounds of the transition elements are coloured in the solid and solution state

Statement II: During d–d transition, the electrons absorb certain radiation from the visible region of the spectrum

Statement I and statement II are true and statement II is correct explanation of statement I

The correct answer is option 1. Statement I and statement II are true and statement II is the correct explanation of statement I.

Let us delve into the explanation of each statement and their relationship with one another regarding the color of transition metal compounds.

Statement I: Most of the compounds of the transition elements are colored in the solid and solution state.

Transition metals are elements that have partially filled d orbitals. This means that their d orbitals are neither completely empty nor fully filled, which allows for a variety of electronic configurations.

Color in Compounds: Many compounds of transition metals exhibit colors because of the electronic transitions that can occur within the d orbitals. These transitions can absorb visible light, leading to the display of specific colors.

Statement II: During d–d transition, the electrons absorb certain radiation from the visible region of the spectrum.

d–d Transitions: In transition metal compounds, particularly those that are complexed with ligands, the energy levels of the d orbitals can split into different energies (this is called crystal field splitting). Electrons in these d orbitals can be excited from a lower energy d orbital to a higher energy d orbital by absorbing a photon of light.

Visible Light Absorption: The energy difference between these split d orbitals often corresponds to the energy of photons in the visible light spectrum. When light of a specific wavelength (color) is absorbed to promote an electron from a lower to a higher d orbital, the complementary color of the absorbed light is observed.

Relationship between the Statements

Transition Metal Compounds are Colored:

Absorption of Light: The color observed in transition metal compounds is due to the absorption of specific wavelengths of visible light. When a photon with energy matching the energy gap between the split d orbitals is absorbed, an electron transitions from a lower energy d orbital to a higher one.

Complementary Colors: The light that is not absorbed (i.e., the remaining light) is transmitted or reflected, and this is what gives the compound its characteristic color. For instance, if a compound absorbs light in the red region of the spectrum, it may appear green, which is the complementary color.

Example:

Chromium(III) Complexes: Chromium(III) ion, \(Cr^{3+}\), typically forms octahedral complexes where the d orbitals split into two sets with different energies (\(e_g\) and \(t_{2g}\)).

Absorption Spectrum: When light passes through such a complex, certain wavelengths corresponding to the energy gap between these sets of orbitals are absorbed, causing \(d-d\) transitions.

Color Observation: The light that is not absorbed determines the color of the complex. For example, \([Cr(H_2O)_6]^{3+}\) absorbs light in the red region, giving the complex a blue-green appearance.

Iron(III) Complexes: Iron(III) ion, \(Fe^{3+}\), in an octahedral field can absorb light leading to transitions within the d orbitals.

Energy Splitting: The splitting energy (Δ) in this case allows for specific d-d transitions upon absorption of visible light, and the observed color is due to the complementary light not absorbed.

Conclusion:

Statement I is True: Transition metal compounds are colored due to the d-d transitions of electrons within their d orbitals.

Statement II is True: These d-d transitions involve the absorption of light from the visible region of the spectrum.

Correct Explanation: The reason transition metal compounds are colored (Statement I) is indeed because of the d-d transitions (Statement II) where electrons absorb visible light. Thus, Statement II is the correct explanation for Statement I.

Hence, statement I and statement II are true and statement II is correct explanation of statement I.