Short Answer

Paramagnetism and diamagnetism in MO theory describe how molecules behave in a magnetic field based on the presence of unpaired or paired electrons in molecular orbitals. A molecule is paramagnetic if it contains one or more unpaired electrons, and it is attracted to a magnetic field.

A molecule is diamagnetic if all electrons are paired in its molecular orbitals. Diamagnetic molecules are weakly repelled by a magnetic field. MO theory helps identify these properties by showing electron arrangement in bonding and antibonding orbitals.

Detailed Explanation :

Paramagnetism and Diamagnetism in MO Theory

Molecular Orbital (MO) theory not only explains how chemical bonds form but also helps predict the magnetic behavior of molecules. Magnetic properties depend on whether electrons in the molecule are paired or unpaired. Paramagnetism and diamagnetism arise from the arrangement of electrons in molecular orbitals, which MO theory clearly describes. This makes MO theory a powerful tool for identifying whether a molecule will be attracted or repelled by a magnetic field.

Paramagnetism is associated with unpaired electrons, while diamagnetism is associated with paired electrons. Since MO diagrams show exactly how electrons fill the bonding and antibonding orbitals, they allow us to determine the presence of unpaired electrons. This explanation is particularly useful for molecules such as O₂, whose magnetic behavior cannot be explained by Lewis structures but is easily explained by MO theory.

1. Paramagnetism in MO Theory

Paramagnetism occurs when a molecule contains unpaired electrons in its molecular orbitals. These unpaired electrons have magnetic moments that align with an external magnetic field, causing the molecule to be attracted toward the field.

Key Features of Paramagnetic Molecules

• Have one or more unpaired electrons
• Are attracted to magnetic fields
• Show strong magnetic effects compared to diamagnetic molecules

How MO Theory Explains Paramagnetism

MO theory provides an electron configuration for the entire molecule. If the molecular orbital diagram shows unpaired electrons in orbitals such as π*, σ*, or others, the molecule is paramagnetic.

Example: Oxygen (O₂)

O₂ is the most famous example of paramagnetism explained by MO theory.

MO configuration of O₂ includes:

• Two unpaired electrons in the π* antibonding orbitals

Because of these unpaired electrons, O₂ is paramagnetic and is strongly attracted to a magnetic field. This behavior cannot be explained by Lewis structures, which incorrectly suggest all electrons are paired.

Thus, MO theory gives the correct explanation of O₂’s magnetic nature.

2. Diamagnetism in MO Theory

Diamagnetism occurs when all electrons are paired in the molecular orbitals. Paired electrons have opposite spins, and their magnetic effects cancel each other out. As a result, diamagnetic molecules are slightly repelled by magnetic fields.

Key Features of Diamagnetic Molecules

• No unpaired electrons
• Weakly repelled by magnetic fields
• Show very small magnetic effects

How MO Theory Explains Diamagnetism

Inspection of the MO diagram reveals whether electrons are paired. If every molecular orbital contains electrons in pairs, the molecule is diamagnetic.

Example: Nitrogen (N₂)

N₂ has the following features according to MO theory:

• All electrons paired
• No electrons in antibonding orbitals that remain unpaired

This makes N₂ diamagnetic, which matches experimental behavior.

3. Comparison of Paramagnetism and Diamagnetism in MO Theory

The magnetic nature of a molecule depends entirely on the electron filling of molecular orbitals:

Property	Paramagnetism	Diamagnetism
Electron pairing	Has unpaired electrons	All electrons paired
Magnetic behavior	Attracted to a magnetic field	Repelled slightly
Example	O₂	N₂

(Explanation given in words; not in table format.)

The presence or absence of unpaired electrons is the deciding factor.

4. Importance of MO Theory in Predicting Magnetism

MO theory is very important because:

It explains cases that Lewis structures cannot.

For example:

• Lewis model predicts O₂ is diamagnetic → incorrect
• MO theory shows unpaired electrons → correct paramagnetism

It helps predict magnetic properties of ions.

Examples:

• O₂⁺ → one unpaired electron → paramagnetic
• O₂²⁻ → all electrons paired → diamagnetic

It improves understanding of bonding and electron distribution.

MO theory shows how electrons fill bonding and antibonding orbitals. These fillings affect:

• Bond order
• Stability
• Magnetism

Thus, magnetism is directly connected to molecular orbital arrangement.

5. Real-Life Applications

Understanding paramagnetism and diamagnetism has important uses:

• Oxygen cylinders: O₂ attraction to magnets shows paramagnetism.
• MRI scans: Paramagnetic contrast agents improve imaging.
• Catalysis: Magnetic behavior influences catalyst design.
• Material science: Determines magnetic properties of metals and alloys.

MO theory provides a foundation for predicting these behaviors accurately.

Conclusion

Paramagnetism and diamagnetism in MO theory depend on whether molecular orbitals contain unpaired or paired electrons. A molecule is paramagnetic if it has unpaired electrons and is attracted to magnetic fields. It is diamagnetic if all electrons are paired and it is weakly repelled from magnets. MO theory clearly shows electron arrangement in bonding and antibonding orbitals, making it a powerful method for predicting magnetic behavior. Examples such as O₂ and N₂ demonstrate how MO theory correctly explains real molecular properties.