Which one of the following is a diamagnetic ion?

Sc³⁺

The correct answer is option 3. \(Sc^{3+}\).

Diamagnetic ions or substances are those in which all electrons are paired. They are not attracted to a magnetic field and may even be weakly repelled by it. All electrons are paired, resulting in no net magnetic moment.

Paramagnetic ions or substances contain one or more unpaired electrons. They are attracted to a magnetic field due to the presence of these unpaired electrons. Presence of unpaired electrons, resulting in a net magnetic moment.

Analyzing Each Ion:

1. \(Cu^{2+}\) (Copper(II) ion):

The atomic number of copper is 29 and its electron configuration is \([Ar] 3d^{10} 4s^1\).

The configuration of \(Cu^{2+}\) is \([Ar] 3d^9\).

In the \(3d^9\) configuration, there is one unpaired electron. Thus it is paramagnetic due to the presence of unpaired electrons.

2. \(Mn^{2+}\) (Manganese(II) ion):

The atomic number of copper is 25 and its electron configuration is \([Ar] 3d^5 4s^2\).

The configuration of \(Mn^{2+}\) is \([Ar] 3d^5\).

In the \(3d^5\) configuration, there are five unpaired electrons. Thus it is paramagnetic due to the presence of unpaired electrons.

3. \(Sc^{3+}\) (Scandium(III) ion):

The atomic number of copper is 21 and its electron configuration is \([Ar] 3d^1 4s^2\).

The configuration of \(Sc^{3+}\) is [Ar].

In the \([Ar]\) configuration, there are no unpaired electrons. Thus it is diamagnetic due to the absence of unpaired electrons.

4. \(Co^{2+}\) (Cobalt(II) ion):

The atomic number of copper is 27 and its electron configuration is \([Ar] 3d^7 4s^2\).

The configuration of \(Co^{2+}\) is \([Ar] 3d^7\).

In the \(3d^7\) configuration, there are three unpaired electrons. Thus it is paramagnetic due to the presence of unpaired electrons.

Summary:

Diamagnetic Ion: \(Sc^{3+}\) (Scandium(III) ion) is diamagnetic because it has a full \([Ar]\) configuration with no unpaired electrons.

Paramagnetic Ions: \(Cu^{2+}\), \(Mn^{2+}\), and \(Co^{2+}\) are paramagnetic due to the presence of unpaired electrons in their configurations.