Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

The aqueous solution containing which one of the following ions will be colourless? (a) \(\mathrm{S} \mathrm{c}^{3+}\) (b) \(\mathrm{Fe}^{3+}\) (c) \(\mathrm{Fe}^{2+}\) (d) \(\mathrm{Mn}^{2+}\)

Step-by-Step Solution

Answer

The aqueous solution containing ext{Sc}^{3+} will be colorless.

Step 1: Understand Color and Transition Metals

Transition metals often produce colored solutions due to d-d electron transitions. When light hits these compounds, electrons in lower-energy d-orbitals absorb certain wavelengths and jump to higher-energy d-orbitals, the color we perceive is the complementary color of the light absorbed.

Step 2: Assess Each Ion for d-Electron Configuration

Scandium ( ext{Sc}^{3+}) has a d-electron configuration of ext{d}^0 because it loses 3 electrons, leaving no d-electrons to undergo electronic transitions. Iron ( ext{Fe}^{3+} and ext{Fe}^{2+}) and Manganese ( ext{Mn}^{2+}) have partially filled d-orbitals, leading to potential color due to d-d transitions.

Step 3: Identify the Colorless Ion

Since ext{Sc}^{3+} has no d-electrons, there are no possible d-d transitions, making its aqueous solution colorless. In contrast, ext{Fe}^{2+}, ext{Fe}^{3+}, and ext{Mn}^{2+} have partially filled d-orbitals and generally form colored solutions.

Key Concepts

d-d electron transitions

In transition metal compounds, color arises from a fascinating process called d-d electron transitions. It all begins with the d-orbitals. These orbitals can be imagined as layers, where electrons can sit. In a transition metal, these d-orbitals are not always at the same energy level. When light shines on a transition metal compound, something exciting happens. Electrons sitting in one of these lower-energy d-orbitals absorb some of that light energy. This absorption allows them to "jump" to a higher-energy d-orbital. Now, the key part is that whenever an electron jumps from a lower to a higher state, specific wavelengths of light are absorbed. The wavelengths that are not absorbed give us color. So, the color we see is the complementary color of the light absorbed. With transition metals, this is why many of their compounds are vibrantly colored. This amazing phenomenon is central in chemistry when dealing with the wonder of transition metals.

Transition metals

Transition metals are a fascinating group of elements found in the center of the periodic table. They include metals like iron, copper, and manganese. These elements are unique due to their ability to exhibit multiple oxidation states. This means they can lose different numbers of electrons, which affects their chemical behavior in various reactions.
Characteristics of Transition Metals:

• They often form colored compounds, a result of d-d electron transitions.
• They can exhibit paramagnetism, which is the ability to be slightly magnetic when an external magnetic field is applied. This comes from unpaired electrons in their d-orbitals.
• Transition metals are typically very good conductors of electricity and heat.

These metals are important in many biological systems. For example, iron, a transition metal, is crucial for oxygen transport in blood. Their versatile chemistry also allows them to be used widely in industry, from catalysis to the creation of alloys.

Scandium ions colorless solution

Scandium ( Sc^{3^+} ) stands out among transition metals for its unique electron configuration. In chemical terms, scandium is a bit of an exception in its group. This is because when it loses three electrons to form the Sc^{3+} ion, it loses all of its d-electrons. With no d-electrons left, d-d transitions cannot occur. Lack of these transitions results in Sc^{3+} forming a colorless aqueous solution. When we compare scandium to other transition metals like iron or manganese, the difference is clear. Iron ( Fe^{2+} or Fe^{3+} ) and manganese ( Mn^{2+} ) often display colors in solutions due to their partially filled d-orbitals still engaging in d-d transitions. Meanwhile, scandium’s absence of these d-electrons explains its exceptionality of forming colorless solutions. This insight reflects how small differences in electronic configuration can lead to significant differences in chemical behavior and appearances.