Question

An aqueous solution of iron(II) sulfate is paramagnetic. If \(\mathrm{NH}_{3}\) is added, the solution becomes diamagnetic. Why does the magnetism change?

Short answer

The addition of ammonia pairs up the electrons in the iron complex, eliminating paramagnetism and making the solution diamagnetic.

Step-by-step solution

Understand Paramagnetism and Diamagnetism

Paramagnetic substances have unpaired electrons that are attracted to a magnetic field. Diamagnetic substances have all electrons paired and are not attracted to a magnetic field.

Examine Iron(II) Sulfate

Iron(II) sulfate, \( ext{FeSO}_4\), contains the \( ext{Fe}^{2+}\) ion. In its electronic configuration, \( ext{Fe}^{2+}\) has 4 unpaired electrons in the \(3d\) subshell: \(3d^6\). This makes the solution paramagnetic due to these unpaired electrons.

Analyze the Effect of \\(\mathrm{NH}_{3}\\) Addition

When \(\mathrm{NH}_{3}\) is added, it acts as a ligand that forms a complex ion with \(\text{Fe}^{2+}\). \(\mathrm{NH}_{3}\) is a strong field ligand and causes electron pairing by shifting energy levels in the \(d\) orbitals through the crystal field stabilization process.

Transition to Diamagnetism

The interaction with \(\mathrm{NH}_{3}\) causes the \(\text{Fe}^{2+}\) ions to form a diamagnetic complex. In this complex, all the electrons are paired, eliminating any unpaired electrons, thus the solution becomes diamagnetic.

Key concepts

Paramagnetism

Paramagnetism is a fascinating property of some chemical compounds. It arises due to the presence of unpaired electrons in their atomic or molecular structure. These unpaired electrons are attracted to external magnetic fields, making substances with these properties paramagnetic.

Many transition metals, like iron in its various oxidation states, exhibit paramagnetism. In the case of iron(II) sulfate, the iron ion has unpaired electrons, leading to its paramagnetic nature. Such materials are typically weakly attracted to magnets, an interesting contrasting point to understand when comparing them with diamagnetic materials.

Diamagnetism

Diamagnetism is another type of magnetic behavior, but quite different from paramagnetism. In diamagnetic substances, all the electrons are paired. As a result, these substances are not attracted to magnetic fields.

In fact, diamagnetic materials can create a weak magnetic field in the opposite direction to an external magnet. This effect is why diamagnetic substances are weakly repelled by magnets. When a compound like iron(II) sulfate changes from paramagnetic to diamagnetic, it indicates that a chemical change has led to the pairing of all its electrons. This change can be brought about by various reactions or treatments involving the substance.

Electronic Configuration

The concept of electronic configuration is essential in understanding why certain substances like iron(II) sulfate demonstrate paramagnetic or diamagnetic properties. Electronic configuration refers to the arrangement of electrons around the nucleus of an atom.

For a paramagnetic iron(II) ion ( ext{Fe}^{2+}), the configuration can be written as ext{[Ar] 3d}^6. This configuration means that four out of six electrons in the ext{3d} subshell are unpaired. Such a structure is key to iron's magnetic properties.

Changes in electronic configuration, such as electron pairing or unpairing, directly impact whether a particular element or compound exhibits paramagnetism or diamagnetism.

Iron(II) Sulfate

Iron(II) sulfate, chemically expressed as ext{FeSO}_4, is a well-known compound in chemistry and is widely used in various applications. The iron ion present in this compound is in the +2 oxidation state, meaning it has lost two electrons from its neutral state.

The interesting property of iron(II) sulfate is its initial paramagnetic nature due to its unpaired electrons in the ext{Fe}^{2+} ion. This makes it notably interactive with magnetic fields. However, its interaction with other substances, like ammonia ( ext{NH}_3), can change its electronic configuration and, subsequently, its magnetic character.

Ligand Field Theory

Ligand Field Theory (LFT) helps us understand the change in magnetic properties of compounds like iron(II) sulfate when interacting with ligands such as ammonia ( ext{NH}_3). This theory explains how ligands affect the properties of the central metal ions in coordination complexes.

ext{NH}_3, as a strong field ligand, influences the d orbitals of the iron( ext{Fe}^{2+}) ion by causing electron pairing. This process, known as crystal field stabilization, alters the electronic configuration. The outcome is a reduction of unpaired electrons, transforming paramagnetic behavior to diamagnetic behavior.

Understanding Ligand Field Theory sheds light on why such changes occur in metal complexes, as it focuses on the nature and strength of the interaction between the metal ions and their respective ligands.