Short Answer

The He₂ molecule is not stable because when two helium atoms combine, the number of electrons entering antibonding molecular orbitals equals the number entering bonding orbitals. This results in a bond order of zero, meaning no net bond is formed between the helium atoms.

Since helium already has a complete and stable electron shell, it has no tendency to share or gain electrons. As a result, He₂ cannot form a stable chemical bond and does not exist under normal conditions.

Detailed Explanation :

Why He₂ Molecule Is Not Stable

The stability of a molecule depends on whether the atoms can form a net stabilizing bond. According to molecular orbital (MO) theory, bonding occurs when more electrons occupy bonding molecular orbitals than antibonding molecular orbitals. This creates a positive bond order, showing that atoms are held together by attractive forces. However, in the case of the He₂ molecule, this condition is not fulfilled. The electronic structure of helium and the way electrons fill molecular orbitals explain why helium atoms do not combine to form a stable diatomic molecule.

Helium is a noble gas with a completely filled outer shell (1s²). Atoms with full shells do not require extra electrons and do not need to share electrons. This already suggests that helium would not easily form bonds. But MO theory gives a deeper and more accurate explanation of why He₂ is specifically not stable.

1. Electron Configuration of Helium Atoms

Each helium atom has:

• Atomic number = 2
• Electronic configuration = 1s²

This means every helium atom has a filled 1s orbital. When two helium atoms approach each other, their 1s orbitals combine to form:

• One bonding molecular orbital (σ1s)
• One antibonding molecular orbital (σ1s*)

The key question is how electrons fill these orbitals.

2. Filling of Molecular Orbitals in He₂

A He₂ molecule would contain 4 electrons total (2 from each helium atom). These electrons occupy molecular orbitals in order of increasing energy:

1. σ1s (bonding orbital) → lowest energy
2. σ1s* (antibonding orbital) → higher energy

The filling occurs as follows:

• Two electrons fill the bonding orbital (σ1s)
• Two electrons fill the antibonding orbital (σ1s*)

This results in equal occupancy of bonding and antibonding orbitals.

3. Bond Order Calculation for He₂

Bond order is calculated using:

For He₂:

• Bonding electrons = 2
• Antibonding electrons = 2

So:

A bond order of zero means:

• No net attraction exists
• No chemical bond forms
• The molecule is not stable

Therefore, He₂ cannot exist as a stable molecule.

4. Why a Bond Order of Zero Means Instability

Bond order shows the balance between attractive and repulsive forces:

• Bonding electrons pull atoms together.
• Antibonding electrons push atoms apart.

In He₂, these forces cancel each other completely because:

• Attraction (from bonding electrons) = Repulsion (from antibonding electrons)

With zero net bonding, there is no force to hold the two helium atoms together. As a result, if helium atoms collide, they immediately separate instead of forming a bond.

5. Noble Gas Stability and Lack of Reactivity

Helium is part of the noble gases, known for:

• Full valence shells
• Very low reactivity
• Minimal tendency to form bonds

Helium has the most stable electronic configuration possible for its size. Forming a molecule like He₂ would require electrons to occupy antibonding orbitals, increasing energy and reducing stability. Nature favors lower energy states, so helium atoms remain separate.

6. Comparison with Hydrogen (H₂)

Hydrogen atoms (H) have:

• 1 electron each

When they combine:

• Two electrons fill the bonding orbital
• Antibonding orbital remains empty

Bond order = 1 → stable molecule

This comparison shows why helium behaves differently:

• Hydrogen achieves greater stability by bonding
• Helium does not gain any stability by bonding

Since helium cannot lower its energy by forming He₂, the molecule does not form.

Conclusion

The He₂ molecule is not stable because the electrons from both helium atoms fill both the bonding and antibonding molecular orbitals equally, giving a bond order of zero. A bond order of zero means no net bond exists to hold the atoms together. Combined with helium’s naturally full and stable electron shell, there is no tendency for helium atoms to bond. Therefore, He₂ cannot exist under normal conditions and remains an important example demonstrating molecular orbital theory.