Short Answer

Bonding and antibonding orbitals are molecular orbitals formed when atomic orbitals combine. A bonding orbital is formed when atomic orbitals overlap constructively, increasing electron density between the nuclei and stabilizing the molecule. An antibonding orbital is formed when atomic orbitals overlap destructively, decreasing electron density between nuclei and destabilizing the molecule.

Electrons in bonding orbitals strengthen the bond, while electrons in antibonding orbitals weaken or cancel it. The balance between electrons in these orbitals determines bond order and molecular stability.

Detailed Explanation :

Bonding and Antibonding Orbitals

Bonding and antibonding orbitals are central ideas of molecular orbital theory. When two atoms come close, their atomic orbitals combine mathematically to form new orbitals that extend over the entire molecule. These new molecular orbitals can either stabilize the molecule (bonding orbitals) or destabilize it (antibonding orbitals). The type of orbital formed depends on how the waves (phases) of the atomic orbitals interact.

Bonding orbitals increase electron density between nuclei, pulling atoms together. Antibonding orbitals decrease electron density between nuclei, pushing atoms apart. The presence of electrons in these orbitals explains whether a molecule is stable, how many bonds it forms, and whether it is magnetic or reactive.

1. Formation of Bonding and Antibonding Orbitals

Atomic orbitals are wave functions. When they combine:

• Constructive interference (same phase) → bonding orbital
• Destructive interference (opposite phase) → antibonding orbital

These interactions determine how electron density is distributed in the new molecular orbital.

Bonding Molecular Orbital

• Forms when electron waves add together
• Increases electron density between nuclei
• Reduces repulsion and stabilizes the molecule
• Lower in energy than parent atomic orbitals

Antibonding Molecular Orbital

• Forms when electron waves cancel each other
• Produces a node (zero electron density) between nuclei
• Increases repulsion and destabilizes the molecule
• Higher in energy than parent atomic orbitals

Antibonding orbitals are labeled with an asterisk (), such as σ or π*.

2. Types of Bonding and Antibonding Orbitals

(a) Sigma (σ and σ) Orbitals*

Sigma bonding orbitals form from head-on overlap:

• s–s overlap
• s–p overlap
• p–p overlap (end-to-end)

The resulting σ orbital is strong and stable because electron density lies directly between the nuclei.
σ* (sigma antibonding) has a node between nuclei and is unstable.

(b) Pi (π and π) Orbitals*

Pi bonding orbitals form from sideways overlap of p orbitals.
Electron density lies above and below the internuclear axis.
π* orbitals have nodes and reduce bonding strength.

3. How Electrons Occupy These Orbitals

Electrons fill molecular orbitals in a predictable way:

• Lower energy orbitals (bonding) fill first
• Each orbital can hold two electrons
• Hund’s rule and Pauli principle apply

Electrons in bonding orbitals contribute to stability.
Electrons in antibonding orbitals decrease bond strength.

4. Bond Order and Stability

Bond order tells whether a molecule is stable:

• Bond order > 0 → molecule is stable
• Bond order = 0 → molecule does not exist

Examples:

• H₂: bond order = 1 → stable
• He₂: bond order = 0 → unstable
• O₂: bond order = 2 → stable and paramagnetic because of unpaired electrons in π* orbitals

This clearly shows how bonding and antibonding orbitals determine molecular existence.

5. Bonding vs. Antibonding: Energy Comparison

Bonding orbitals:

• Lower energy
• Greater electron density between nuclei
• Pull atoms together

Antibonding orbitals:

• Higher energy
• Node between nuclei
• Push atoms apart

If more electrons are placed in antibonding orbitals than bonding orbitals, no bond forms.

6. Role in Magnetic Properties

Antibonding orbitals explain magnetic behavior:

• If antibonding orbitals contain unpaired electrons, the molecule is paramagnetic.
• Example: O₂ has two unpaired electrons in π* orbitals.

Bonding orbitals alone cannot explain this; molecular orbital theory is needed.

7. Importance in Predicting Molecular Properties

Bonding and antibonding orbitals help explain:

• Bond strength
• Bond length
• Stability
• Magnetic nature
• Reactivity
• Electronic transitions

A molecule with more electrons in bonding orbitals has a shorter, stronger bond.
More electrons in antibonding orbitals weaken or break the bond.

Conclusion

Bonding and antibonding orbitals form when atomic orbitals combine to create molecular orbitals that stabilize or destabilize a molecule. Bonding orbitals increase electron density between nuclei and lower energy, while antibonding orbitals create nodes and raise energy. The balance of electrons in these orbitals determines bond order, stability, magnetic behavior, and overall molecular properties. Understanding these orbitals is essential for predicting how molecules form and behave.