Problem 2 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of these ions is coloured as well as paramagnetic in nature? (a) \(\mathrm{Cu}^{2+}\) (b) \(\mathrm{Cu}^{+}\) (c) \(\mathrm{Nl}^{2+}\) (d) Both a and \(\mathrm{c}\)

Step-by-Step Solution

Answer

Both \(\mathrm{Cu}^{2+}\) and \(\mathrm{Ni}^{2+}\) are colored and paramagnetic.

Step 1: Review Electron Configurations

To determine if an ion is colored and paramagnetic, we start by reviewing the electron configurations of the ions. \(\mathrm{Cu}^{2+}\) has an electron configuration beginning with \([\mathrm{Ar}] 3d^9\), \(\mathrm{Cu}^{+}\) is \([\mathrm{Ar}] 3d^{10}\), and \(\mathrm{Ni}^{2+}\) is \([\mathrm{Ar}] 3d^8\). Ions with partially filled d-orbitals are typically colored and paramagnetic.

Step 2: Identify Paramagnetic Ions

Paramagnetism is due to unpaired electrons in an atom or ion. \(\mathrm{Cu}^{2+}\) with a \(3d^9\) configuration has one unpaired electron, making it paramagnetic. \(\mathrm{Cu}^{+}\), with a \(3d^{10}\) configuration, has no unpaired electron and is not paramagnetic. \(\mathrm{Ni}^{2+}\), with a \(3d^8\) configuration, has two unpaired electrons, making it paramagnetic.

Step 3: Identify Colored Ions

Ions with partially filled d-orbitals also tend to be colored due to d-d transitions. \(\mathrm{Cu}^{2+}\) and \(\mathrm{Ni}^{2+}\) both possess partially filled d-orbitals, making them colored, whereas \(\mathrm{Cu}^{+}\), with its filled \(3d\) orbitals, is not.

Step 4: Select the Ions that are Both Colored and Paramagnetic

From the analysis, \(\mathrm{Cu}^{2+}\) is both colored and paramagnetic due to its \(3d^9\) configuration, while \(\mathrm{Ni}^{2+}\) is also colored and paramagnetic with a \(3d^8\) configuration. \(\mathrm{Cu}^{+}\) is neither colored nor paramagnetic.

Key Concepts

Electron Configuration

Electron configuration is crucial in understanding the characteristics of transition metal ions. It refers to the distribution of electrons in an atom or ion's orbitals. For transition metals, this involves the filling of 3d, 4s, and sometimes 4p orbitals. Transition metal ions often have their outermost s electrons removed first during ionization.

In the context of Cu and Ni ions:

• ext{Cu}^{2+} possesses an electron configuration of ext{[Ar] 3d}^9 , meaning it has 9 electrons in its 3d orbitals.
• ext{Cu}^{+} has a complete 3d orbital configuration of ext{[Ar] 3d}^{10} .
• ext{Ni}^{2+} features an electron setup of ext{[Ar] 3d}^8 with 8 electrons in its 3d orbitals.

Therefore, ions with partially filled d-orbitals, like Cu^{2+} and Ni^{2+} , tend to exhibit unique properties, such as color and paramagnetism.

Paramagnetism

Paramagnetism results from the presence of unpaired electrons in an atom or ion. These unpaired electrons generate a net magnetic moment, allowing the substance to be weakly attracted to a magnetic field. When examining transition metal ions:

• ext{Cu}^{2+} has a 3d^9 configuration with one unpaired electron, making it paramagnetic.
• ext{Cu}^{+} , having a fully filled 3d^{10} , lacks unpaired electrons and is consequently not paramagnetic.
• ext{Ni}^{2+} possesses a 3d^8 configuration with two unpaired electrons, making it paramagnetic.

These unpaired electrons are also responsible for their colored nature when they facilitate d-d transitions.

Color in Transition Metal Ions

The color of transition metal ions arises from electronic transitions within their partially filled d-orbitals. This phenomenon is known as d-d transitions and occurs when electrons absorb visible light to jump from a lower energy d-orbital to a higher energy one. The specific wavelengths absorbed determine the color that is reflected, which is what we see.

• ext{Cu}^{2+} displays color due to its partially filled 3d^9 orbitals, which allow such transitions.
• ext{Cu}^{+} lacks color as its 3d^{10} setup is fully filled, prohibiting d-d transitions.
• ext{Ni}^{2+} exhibits color with its 3d^8 formation of electrons, which similarly enables d-d transitions.

This unique behavior of transition metals makes them heavily utilized in various applications, including pigments and dyes, due to their vibrant colors.