Problem 103 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is/are characteristics of d-block elements? (a) They are generally diamagnetic (b) They form coloured complexes (c) They show variable oxidation states (d) Their ionization energies are very high

Step-by-Step Solution

Answer

The correct characteristics are (b) and (c).

Step 1: Analyze the Options

Let's look at each statement to understand d-block elements. Option (a) states they are 'generally diamagnetic.' D-block elements, also known as transition metals, often have unpaired electrons, making them paramagnetic rather than diamagnetic. Moving on to option (b), 'they form coloured complexes.' This is generally true because d-block elements' partially filled d-orbitals can absorb and emit visible light, leading to coloured complexes.

Step 2: Variable Oxidation States

Consider option (c), 'they show variable oxidation states.' D-block elements can exhibit a range of oxidation states due to the involvement of similarly low energy d and s orbitals. Hence, this statement is typically true for most d-block elements.

Step 3: Ionization Energies of d-block Elements

Examine option (d), 'their ionization energies are very high.' While d-block elements often have moderate to high ionization energies, this is not a defining characteristic. The presence of partially filled d levels can lower ionization energies compared to s-block elements in the same period.

Step 4: Conclusion

Summarize the analysis: - Option (a) is incorrect as d-block elements are usually paramagnetic. - Option (b) is correct because they form coloured complexes. - Option (c) is correct since they exhibit variable oxidation states. - Option (d) is misleading, as their ionization energy is not 'very high' compared to their trends across the periodic table.

Key Concepts

Paramagnetism in d-block Elements

D-block elements, commonly recognized as transition metals, have unique properties due to their electron configurations. One notable feature is paramagnetism. This property arises from the presence of unpaired electrons in the d-orbitals. Electrons have a property called 'spin', and when these spins are not paired with another electron of opposite spin, they create a magnetic field. This makes the element attracted to external magnetic fields—hence, they are paramagnetic.
Understanding paramagnetism involves considering the transition metals' electron configurations. Transition metals are characterized by incompletely filled d orbitals. Most d-block elements have variable oxidation states, which means they can gain or lose different numbers of electrons. As a result, altering the number of unpaired electrons changes their magnetic properties.
For instance, iron ( ext{Fe} ext{ }) can exist in several oxidation states, such as ext{Fe}^{2+}(d^6) with four unpaired electrons, exhibiting strong paramagnetism. This attribute contrasts with diamagnetism found in elements where all electrons are paired, resulting in negligible magnetic attraction.

Coloured Complexes of d-block Elements

Transition metals are renowned for forming coloured compounds, especially in the form of complexes. When d-block elements combine with other atoms or molecules, they create structures called complexes. The vivid colours arise from electronic transitions between d-orbitals.
The d-orbitals in transition metals can split into different energy levels when they form complexes. The energy gap between these split levels falls within the range of visible light wavelengths. When light strikes the complex, electrons may absorb specific colours and shift from a lower to a higher energy d-orbital state. The colours not absorbed are reflected or transmitted, contributing to the observed colour of the complex.
For example, copper(II) sulfate, a well-known blue solution, results from the absorption of all but the blue component of white light, creating the characteristic hue. Factors like the metal, oxidation state, and ligands (molecules bound to the metal) can influence the exact colour observed. This property aids in identifying and analyzing unknown substances in various fields.

Variable Oxidation States in d-block Elements

A fascinating characteristic of d-block elements is their ability to exhibit variable oxidation states. This flexibility stems from the potential to utilize both d and s-electrons for bonding.
The oxidation state is determined by the number of electrons an element can gain, lose, or share while forming compounds. In d-block elements, the similar energy levels of the 4s and 3d orbitals allow electrons to participate in different combinations during chemical reactions. The transition metals can thus adopt multiple oxidation states, which is not typical in other elements such as s-block elements.
For example, manganese in permanganate ion ( ext{MnO}_4^{-}) has an oxidation state of +7, while in manganese(IV) oxide ( ext{MnO}_2), it is +4. Such variability allows d-block elements to form a wide range of compounds with diverse properties and reactions. This versatility is crucial in fields like catalysis, where transition metals' ability to switch oxidation states makes them excellent catalysts for accelerating chemical reactions.