Chemistry/Chapter 3: Chemical Bonding
3.10.4 Molecular Orbital Diagram of Homo Nuclear Diatomic Molecules
Homonuclear Diatomic Molecules:
Explanation level
This topic explains the molecular orbital diagrams of homonuclear diatomic molecules — molecules made of two identical atoms. It covers H₂, He₂, N₂, O₂, and F₂.
Definition
Homonuclear Diatomic Molecules: Molecules composed of two identical atoms (e.g., H₂, N₂, O₂, F₂).
Molecular Orbital Diagram: A diagram showing the relative energy levels of molecular orbitals and the filling of electrons in them.
Main Idea
Molecular orbital diagrams show how atomic orbitals combine to form molecular orbitals. Electrons fill these orbitals according to the Aufbau principle, Pauli's exclusion principle, and Hund's rule. The bond order and magnetic properties of the molecule can be determined from the diagram.
Core Concept
- Atomic orbitals of the two atoms combine to form bonding and antibonding molecular orbitals.
- Electrons fill the molecular orbitals from lowest to highest energy.
- Bond order = Nb - Na2
- Unpaired electrons → paramagnetic; all paired → diamagnetic.
Detailed Explanation
1. Hydrogen Molecule (H₂)
- Hydrogen molecule is formed from the overlap of 1s atomic orbitals of two hydrogen atoms.
- They give rise to two molecular orbitals: σ 1s and σ* 1s.
- Total number of electrons = 2
- Electronic configuration of hydrogen atom is 1s1
- H2 = σ 1s2, σ* 1s0
Bond Order: Bond order = 2 - 02 = 1
Magnetic Character: - Diamagnetic (having no unpaired electron)
2. Helium Molecule (Hypothetical) He₂
Formation: - The energy level diagram for He₂ is similar to that of H₂ except that it has two more electrons occupying the antibonding σ* 1s orbital.
- Total number of electrons = 4
- Electronic configuration of helium is 1s2
- He2 = σ 1s2, σ* 1s2
Bond Order: Bond order = 2 - 22 = 0
Magnetic Character: - As there is no unpaired electron in the molecular orbitals of He₂ molecule, it is diamagnetic.
- As bond order of He₂ is zero, the molecule is unstable, and it does not exist.
- Helium exists only as mono-atomic molecules.
3. Nitrogen Molecule (N₂)
- Nitrogen contains three unpaired electrons in 2p orbitals.
- Electronic configuration of nitrogen is 1s2 2s2 2px1 2py1 2pz1
- Total number of electrons in N₂ molecule = 14
Electron Configuration: N2 = (σ 1s)2 (σ* 1s)2 (σ 2s)2 (σ* 2s)2 (π 2py)2 = (π 2pz)2 (σ 2px)2
Bond Order: Bond order = 10 - 42 = 3
Magnetic Character: - As there is no unpaired electron in the molecular orbitals of N₂ molecule, it is diamagnetic.
- Bond order of N₂ is three.
- Two N atoms are bonded through a triple bond.
- N₂ molecule is very stable and has a very high bond energy (946 kJ mol⁻¹).
- It is diamagnetic in nature and possesses a very short bond length.
4. Oxygen Molecule (O₂)
- Electronic configuration of oxygen is 1s2 2s2 2px2 2py1 2pz1
- Oxygen contains two unpaired electrons in 2p orbitals (2py and 2pz).
- Total number of electrons in O₂ molecule = 16
Electron Configuration: O2 = (σ 1s)2 (σ* 1s)2 (σ 2s)2 (σ* 2s)2 (σ 2px)2 (π 2py)2 = (π 2pz)2 (π* 2py)1 = (π* 2pz)1
Bond Order: Bond order = 10 - 62 = 2
- It is found from the molecular orbital diagram of oxygen molecule that the electrons present in the two π* orbitals are unpaired.
- The two unpaired electrons reside in the degenerate antibonding orbitals π* 2py and π* 2pz.
- These unpaired electrons not only go around the atom in their orbitals, but they also spin, which creates a magnetic field.
- Unpaired electrons spin in the same direction as each other, which increases the magnetic field effect.
- Due to the presence of these unpaired electrons, O₂ is found to be paramagnetic in nature.
- Paramagnetic behavior of O₂ molecule is not explained by Valence Bond Theory.
- It is obvious that the two oxygen atoms are bonded through a double bond.
- So, we conclude that the molecule should be very stable as it possesses high bond energy (498 kJ mol⁻¹) with bond length 1.21 Å.
5. Fluorine Molecule (F₂)
- Electronic configuration of fluorine is 1s2 2s2 2px2 2py2 2pz1
- Total number of electrons in F₂ molecule = 18
Electron Configuration: F2 = (σ 1s)2 (σ* 1s)2 (σ 2s)2 (σ* 2s)2 (σ 2px)2 (π 2py)2 = (π 2pz)2 (π* 2py)2 = (π* 2pz)2
Bond Order: Bond order = 10 - 82 = 1
Magnetic Character: - As there is paired electrons in the molecular orbitals of F₂ molecule, it is diamagnetic.
- Bond order of F₂ is one.
- Two F atoms are bonded through a single bond.
- The F−F bond distance is longer (1.43 Å) than the bond distance for O₂ (1.21 Å) and N₂ (1.09 Å).
- The bond energy of F₂ molecule is quite low (159 kJ mol⁻¹).
- It is diamagnetic in nature because it has no unpaired electrons.
Summary Table
| Molecule | Total Electrons | Bond Order | Bond Type | Magnetic Character |
|---|---|---|---|---|
| H₂ | 2 | 1 | Single | Diamagnetic |
| He₂ | 4 | 0 | None (unstable) | Diamagnetic |
| N₂ | 14 | 3 | Triple | Diamagnetic |
| O₂ | 16 | 2 | Double | Paramagnetic |
| F₂ | 18 | 1 | Single | Diamagnetic |
Intermediate Level
Molecular Orbital Diagrams
| Molecule | Configuration | Bond Order | Magnetic |
|---|---|---|---|
| H₂ | σ 1s2 | 1 | Diamagnetic |
| He₂ | σ 1s2 σ* 1s2 | 0 | Diamagnetic |
| N₂ | (σ 1s)2 (σ* 1s)2 (σ 2s)2 (σ* 2s)2 (π 2py)2 = (π 2pz)2 (σ 2px)2 | 3 | Diamagnetic |
| O₂ | (σ 1s)2 (σ* 1s)2 (σ 2s)2 (σ* 2s)2 (σ 2px)2 (π 2py)2 = (π 2pz)2 (π* 2py)1 = (π* 2pz)1 | 2 | Paramagnetic |
| F₂ | (σ 1s)2 (σ* 1s)2 (σ 2s)2 (σ* 2s)2 (σ 2px)2 (π 2py)2 = (π 2pz)2 (π* 2py)2 = (π* 2pz)2 | 1 | Diamagnetic |
Key Points
- H₂: Stable, single bond, diamagnetic.
- He₂: Unstable, bond order 0, does not exist.
- N₂: Very stable, triple bond, diamagnetic.
- O₂: Stable, double bond, paramagnetic (unpaired electrons in π*).
- F₂: Stable, single bond, diamagnetic.
Important Points
- 1. H₂: Bond order 1; diamagnetic.
- 2. He₂: Bond order 0; unstable; does not exist.
- 3. N₂: Bond order 3; triple bond; diamagnetic; very stable.
- 4. O₂: Bond order 2; double bond; paramagnetic.
- 5. F₂: Bond order 1; single bond; diamagnetic.
- 6. Bond order = Nb - Na2.
- 7. Paramagnetic: Unpaired electrons; Diamagnetic: No unpaired electrons.
Remember
- - H₂: Bond order 1, diamagnetic
- - He₂: Bond order 0, unstable
- - N₂: Bond order 3, diamagnetic
- - O₂: Bond order 2, paramagnetic
- - F₂: Bond order 1, diamagnetic
- - Bond order: Nb - Na2
Common Mistakes
- 1. Forgetting that He₂ does not exist: Bond order = 0.
- 2. Thinking O₂ is diamagnetic: O₂ is paramagnetic due to unpaired electrons.
- 3. Mixing up bond orders: N₂ = 3, O₂ = 2, F₂ = 1.
- 4. Ignoring the paramagnetism of O₂: This is a key prediction of MOT.
- 5. Confusing bond order with bond length: Higher bond order → shorter bond length.
Quick Revision
- H₂: Bond order 1; diamagnetic.
- He₂: Bond order 0; unstable; does not exist.
- N₂: Bond order 3; triple bond; diamagnetic; very stable.
- O₂: Bond order 2; double bond; paramagnetic.
- F₂: Bond order 1; single bond; diamagnetic.
- Bond order = Nb - Na2.
- Paramagnetic: Unpaired electrons; Diamagnetic: No unpaired electrons.
60-Second Revision
- H₂: Bond order 1, diamagnetic
- He₂: Bond order 0, unstable
- N₂: Bond order 3, diamagnetic
- O₂: Bond order 2, paramagnetic
- F₂: Bond order 1, diamagnetic
- Bond order: Nb - Na2
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3.10.4 Molecular Orbital Diagram of Homo Nuclear Diatomic Molecules
Sticky Notes · Quick Revision
Key Point 1
- H₂: Bond order 1, diamagnetic
Key Point 2
- He₂: Bond order 0, unstable
Key Point 3
- N₂: Bond order 3, diamagnetic
Key Point 4
- O₂: Bond order 2, paramagnetic
Key Point 5
- F₂: Bond order 1, diamagnetic
Key Point 6
- Bond order: Nb - Na2
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