Problem 142 - A Complete Resource Book in Chemistry for JEE Main

Question

Which ion can show different colours in different compounds? (a) \(\mathrm{Sr}^{2+}\) (b) \(\mathrm{Ni}^{2+}\) (c) \(\mathrm{Pb}^{2+}\) (d) \(\mathrm{Cd}^{2+}\)

Step-by-Step Solution

Answer

The ion that can show different colors is \\(\mathrm{Ni}^{2+}\\).

Step 1: Analyzing the Ion Property

Certain ions can show different colors in different compounds due to their ability to undergo d-d transitions. These transitions occur in transition metal ions where the electron configuration allows for the absorption of visible light, leading to different colors. Non-transition metal ions typically do not show such variations in color.

Step 2: Determining Transition Metal Ions

Among the given ions, identify which ions are transition metals. Transition metals are elements that have partially filled d orbitals. In the options given, \(\mathrm{Ni}^{2+}\) is the only transition metal ion. \(\mathrm{Sr}^{2+}\), \(\mathrm{Pb}^{2+}\), and \(\mathrm{Cd}^{2+}\) do not have the electronic configuration allowing for such transitions.

Step 3: Reasoning Based on Electron Configuration

The \(\mathrm{Ni}^{2+}\) ion comes from nickel, which is a transition metal. Its electron configuration allows for unpaired electrons in the d orbitals. This characteristic enables it to exhibit different colors when forming compounds, depending on the ligands attached to the metal ion.

Step 4: Conclusion

Since \(\mathrm{Ni}^{2+}\) has the properties of a transition metal and can have different colored compounds due to d-d transitions, it is the ion that can show different colors in different compounds. The other ions, being non-transition metals, lack these properties.

Key Concepts

d-d transitions

When you hear about "d-d transitions," it’s all about the behavior of electrons within transition metals. Transition metals are unique due to their partially filled d orbitals. This means electrons can jump between different energy levels within these d orbitals. When light hits a transition metal ion, electrons absorb part of this energy and jump to higher energy d orbitals. This process is called a d-d transition. What makes it interesting is that only specific wavelengths of light are absorbed, while the rest are reflected or transmitted, giving the compound its color.

Imagine shining a light on nickel (Ni) compounds; the electron configuration allows such d-d transitions to occur, causing color to appear. When the electron returns to its lower energy state, it releases light, often resulting in beautiful hues seen in transition metal compounds.

electron configuration

Electron configuration is like a map guiding us through the arrangement of electrons in an atom's orbitals. For transition metals, especially, it plays a crucial role. Transition metals have their electrons filling the d orbitals, which are not completely filled. For example, nickel (Ni), which becomes \( \text{Ni}^{2+} \) when it loses two electrons, has its electrons arranged in such a way that it enables d-d transitions.

The proper electron configuration for \( \text{Ni}^{2+} \) is [Ar] 3d\(^{8}\). This means it has eight electrons in its d orbital. The presence of partially filled d orbitals is essential for the ion to exhibit different colors in different compounds. This configuration allows the electrons to have unpaired states, which are crucial for d-d transitions.

color variation in compounds

Color variation in compounds primarily occurs because of the interactions between transition metals and their surrounding environment. For transition metal ions like \( \text{Ni}^{2+} \,\) the color we see is due to the specific wavelengths of light being absorbed while others are emitted or reflected. This is caused by d-d transitions within the metal ion and can differ significantly depending on the ligands (molecules or ions surrounding the metal).

Ligands can influence the energy difference between d orbitals, thus changing the wavelength of light absorbed. As a result, the color observed in a transition metal compound can change with different ligands. An example is nickel in different environments producing hues from green to blue, or even yellow, depending on its ligands and the resulting electron transitions. This interaction is a classic demonstration of how complex and captivating chemistry can be!

• Key points: Transition metal ions with partially filled d orbitals can show color variations.
• Ligands greatly influence the color by affecting the energy levels of d orbitals.