Why Iron Is Magnetic but Iron Sulfide Usually Is Not

A simple experiment with a deeper explanation

Mixing iron filings with sulfur creates a striking contrast:

• Iron filings are attracted to a magnet.
• After heating, the materials react to form iron sulfide.
• The resulting compound is not noticeably attracted to an ordinary magnet.

The key idea is that chemical bonding changes the way iron's electrons are arranged and interact. Magnetism is not determined simply by whether a material contains iron. It depends on the electronic structure of the entire substance.

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1. What happens before heating?

Iron filings contain metallic iron, written as Fe.

In solid iron, the atoms are arranged in a metallic lattice. The outer electrons are not attached to one individual atom. Instead, they occupy energy bands that extend throughout the metal.

Iron atoms have partially filled 3d electron states. Some of these electrons remain unpaired. Unpaired electrons behave like tiny magnetic moments.

In ordinary iron, neighboring atomic moments interact so that large groups of atoms align in the same direction. These regions are called magnetic domains.

When a magnet is brought near the iron:

1. Domains pointing in many directions become rearranged.
2. More domains align with the external magnetic field.
3. The iron filings are strongly attracted to the magnet.

This type of magnetism is called ferromagnetism.

occurs when neighboring magnetic moments tend to align parallel to one another over large regions of a material.

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2. What changes during the reaction?

When iron is heated with sulfur, a chemical reaction occurs:

Fe + S -> FeS

The iron atoms and sulfur atoms do not merely form a physical mixture. They become chemically bonded in a new substance, iron sulfide.

This distinction is important:

• A mixture contains separate iron and sulfur particles.
• A compound contains atoms joined in a new electronic arrangement.

The iron atoms in FeS no longer behave like metallic iron atoms. Their electrons are redistributed between iron and sulfur, and the iron atoms are placed in a different crystal structure.

The result is a new material with different:

• Electron arrangement
• Bonding
• Crystal structure
• Magnetic interactions
• Electrical properties
• Physical appearance

The original properties of the elements are not simply retained.

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3. How do the bonds affect the electrons?

Sulfur is more electronegative than iron, meaning sulfur attracts bonding electrons more strongly.

The bonding in iron sulfide is not perfectly ionic or perfectly covalent. It has a combination of ionic and covalent character. A simplified description is:

Fe gives some electron density to S:

Fe -> Fe with increased positive character

S gains electron density:

S -> S with increased negative character

A more realistic picture is that electrons are redistributed into iron-sulfur bonds and into the crystal's allowed energy states.

This changes the iron 3d electrons in several ways:

• Their energy levels shift.
• Their spatial arrangement changes.
• Their overlap with neighboring atoms changes.
• Their magnetic moments may become arranged antiparallel rather than parallel.
• The strong domain behavior found in metallic iron is lost.

The decisive factor is not simply that iron has become "charged." The crucial factor is how the crystal structure changes the interaction between neighboring magnetic moments.

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4. Why FeS is not strongly attracted to a magnet

Iron sulfide is not usually ferromagnetic in the same way as metallic iron.

In many iron sulfide structures, neighboring iron magnetic moments interact through the sulfur atoms. This indirect interaction can favor antiferromagnetism, in which adjacent moments point in opposite directions.

A simplified arrangement looks like this:

Moment A: ->
Moment B: <-
Moment C: ->
Moment D: <-

The opposing moments largely cancel:

(+ magnetic moment) + (- magnetic moment) -> approximately zero overall moment

Therefore, the solid does not produce the strong net magnetism associated with iron metal.

describes a state in which neighboring magnetic moments align in opposite directions, causing substantial cancellation of the overall magnetic moment.

This explains why a piece of iron sulfide may contain iron atoms with magnetic moments but still fail to show strong attraction to an ordinary magnet.

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5. The role of exchange interactions

The alignment of magnetic moments is controlled by quantum-mechanical interactions called exchange interactions.

These interactions arise from two principles:

1. Electrons are indistinguishable quantum particles.
2. The Pauli exclusion principle restricts how electrons can occupy the same states.

Depending on the atoms, distances, orbitals, and bonding pathways, exchange interactions can favor either:

• Parallel alignment, producing ferromagnetism
• Antiparallel alignment, producing antiferromagnetism
• Weak or disordered alignment, producing paramagnetism

In metallic iron, the electronic structure favors strong parallel alignment over large regions.

In iron sulfide, iron-sulfur bonding changes the orbital overlap and the pathways through which neighboring iron atoms interact. The preferred arrangement can therefore become antiparallel.

A useful simplified comparison is:

Iron metal:

Fe - Fe - Fe - Fe
-> -> -> ->

Iron sulfide:

Fe - S - Fe - S
-> <- -> <-

The sulfur atoms alter the interaction between neighboring iron atoms. They act as part of the magnetic exchange pathway rather than being passive spacers.

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6. Why the compound does not simply behave like "iron plus sulfur"

A common misconception is:

If iron is magnetic, any substance containing iron should also be magnetic.

That is not correct.

The properties of a compound are not an average of the properties of its elements. For example:

• Sodium is a reactive metal, and chlorine is a poisonous gas.
• Sodium chloride is a stable crystalline solid used as table salt.
• Hydrogen and oxygen are gases.
• Water is a liquid with entirely different properties.

Likewise:

• Iron is a strongly ferromagnetic metal.
• Sulfur is nonmagnetic in the everyday sense.
• Iron sulfide is a new solid with a new electronic and magnetic structure.

The chemical identity of the material has changed.

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7. Is iron sulfide completely nonmagnetic?

Strictly speaking, "not magnetic" usually means "not strongly attracted to a normal magnet."

Iron sulfide may still have magnetic behavior that is too weak or too subtle to observe with a simple classroom magnet. Depending on its exact composition, crystal structure, temperature, and preparation conditions, iron sulfide can exhibit:

• Antiferromagnetism
• Paramagnetism
• Weak magnetic responses
• Different magnetic transitions

Iron sulfides are not all identical. Possible products can include compounds with different iron-to-sulfur ratios and different structures.

For example, iron sulfide may form as FeS under suitable conditions, but other iron-sulfur compounds can also exist. The reaction conditions, sulfur supply, temperature, heating time, and cooling process can influence the final product.

Therefore, the most accurate statement is:

The iron sulfide product is not strongly ferromagnetic like metallic iron because its bonding and crystal structure alter the electron arrangement and often cause magnetic moments to cancel.

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8. Why a magnet cannot easily separate the product

Before heating, a magnet can attract iron filings from a mixture of iron and sulfur.

After heating, the iron has reacted chemically with sulfur. The iron atoms are no longer present as separate metallic iron particles. They are incorporated into iron sulfide.

The magnet is therefore no longer pulling on metallic iron domains. Instead, it is interacting with a compound whose magnetic moments are largely cancelled or weakly ordered.

This is a practical demonstration of the difference between:

• A physical change, in which substances retain their chemical identities
• A chemical change, in which new substances form

Magnetic separation works before the reaction because iron metal is still present. It generally does not work effectively afterward because the iron has become part of a different compound.

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9. A practical demonstration

Materials

• Iron filings
• Sulfur powder
• Heat-resistant test tube or crucible
• Heat source
• Magnet enclosed in a plastic bag
• Safety goggles
• Heat-resistant gloves
• Fume extraction or a well-ventilated laboratory

Procedure

1. Test the iron filings with the covered magnet.
2. Observe the strong attraction.
3. Mix a small amount of iron filings with sulfur.
4. Test the mixture with the magnet before heating.
5. Heat the mixture until it reacts.
6. Allow the product to cool completely.
7. Test the cooled product with the magnet.

Expected observations

Before heating:

• The mixture is attracted to the magnet because it contains metallic iron.

After heating:

• A new dark solid forms.
• The material is not strongly attracted to the magnet.
• The original iron filings are no longer present as separate metallic iron.

Safety note

This reaction should be performed only under appropriate laboratory supervision. Heating sulfur can produce irritating sulfur-containing fumes, and hot materials can cause severe burns. The product should not be handled or tested until it has cooled completely.

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10. A concise electron-level summary

The difference can be summarized as follows.

Metallic iron

• Iron atoms are arranged in a metallic lattice.
• Partially filled 3d states provide unpaired electrons.
• Exchange interactions favor parallel alignment.
• Magnetic domains form.
• The material is strongly ferromagnetic.

Iron sulfide

• Iron atoms are chemically bonded to sulfur.
• Electron density is redistributed.
• The iron 3d energy levels and orbital overlap change.
• Exchange interactions may favor antiparallel alignment.
• Magnetic moments can cancel.
• The compound is not strongly ferromagnetic.

The most important principle is:

Magnetism depends on electron arrangement and interactions, not merely on the presence of a particular element.

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Personal reflection

What makes this experiment especially valuable is that it turns an abstract idea about electrons into a visible result.

At the beginning, a magnet can pick up the iron filings. After the reaction, it cannot produce the same effect. The iron has not disappeared, but its identity within the material has changed.

This is a powerful reminder that chemical reactions reorganize matter at the atomic level. A compound is not simply a loose combination of the properties of its elements. New bonds create new structures, and new structures create new properties.

In this case, the bonds involving sulfur change the way iron's electrons interact. That change is enough to transform a strongly magnetic metal into a compound with little observable attraction to an ordinary magnet.