Anand Classes explains the Bond Order, Stability, and Magnetic Character of Molecules based on the Molecular Orbital Theory (MOT), along with the concept of Bonding and Antibonding Electrons. Students will learn how to calculate bond order using the molecular orbital configuration and how it determines the strength, length, and stability of chemical bonds. The post also covers how the presence of paired or unpaired electrons affects the magnetic behaviour of molecules — whether they are paramagnetic or diamagnetic. This topic is presented with diagrams, solved examples, MCQs, Q&A, Assertion Reason, and Case Study questions for Class 11, Class 12, JEE, and NEET chemistry preparation. Click the print button to download study material and notes.

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What is the Electronic Configuration of a Molecule?

The distribution of electrons among various molecular orbitals is called the electronic configuration of the molecule.

The molecular orbital energy level diagram and electronic configuration give valuable information about the behaviour of molecules.

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How Does Electronic Configuration Indicate Stability of a Molecule?

The electrons in bonding molecular orbitals give stability to a molecule, whereas those in antibonding molecular orbitals decrease the stability.

Let N_b = number of bonding electrons and N_a = number of antibonding electrons:

1. Stable molecule if N_b > N_a
2. Unstable molecule if N_b < N_a
3. Unstable molecule if N_b = N_a

• In case (i), bonding influence is stronger → stable molecule.
• In case (ii), antibonding influence is stronger → unstable molecule.
• In case (iii), bonding and antibonding effects cancel, but antibonding influence is slightly stronger, so no stable molecule forms.

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What is Bond Order in Molecular Orbital Theory?

Bond order is defined as the number of covalent bonds in a molecule.

It is calculated as:

$$\text{Bond Order} = \frac{N_b – N_a}{2}$$

where,

• $N_b =$ number of electrons in bonding MOs
• $N_a =$ number of electrons in antibonding MOs
• Bond order = 1 → single bond
• Bond order = 2 → double bond
• Bond order = 3 → triple bond
• Bond order may also be fractional in some cases.

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What Information is Conveyed by Bond Order?

Bond order provides three important insights:

1. Molecular Stability:
   • If bond order is positive, the molecule is stable.
   • If bond order is zero or negative, the molecule is unstable.
2. Bond Dissociation Energy (BDE):
   • Bond order ∝ Bond dissociation energy.
   • Higher bond order → stronger bond → higher bond energy.
   • Example:
     • N₂: Bond order = 3 → BDE = 945 kJ mol⁻¹
     • O₂: Bond order = 2 → BDE = 495 kJ mol⁻¹
3. Bond Length:
   • Bond order ∝ 1 / Bond length.
   • Higher bond order → shorter bond length.
   • Example:
     • N₂: Bond order 3 → Bond length 110 pm
     • O₂: Bond order 2 → Bond length 121 pm

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How Does Electronic Configuration Indicate Magnetic Character of Molecules?

• If all electrons are paired, the molecule is diamagnetic (repelled by a magnetic field).
• If there are unpaired electrons, the molecule is paramagnetic (attracted by a magnetic field).

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🧪 Practice Set: Electronic Configurations and Molecular Behaviour

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Short Answer Conceptual Types (SAT)

Q1. Define bond order. Write its formula.

Answer:
Bond order is the number of covalent bonds present in a molecule.
Formula:
$$\text{Bond Order} = \frac{N_b – N_a}{2}$$
where $N_b =$ number of bonding electrons and $N_a =$ number of antibonding electrons.

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Q2. Why is N₂ more stable than O₂?

Answer:

• N₂ has bond order 3, while O₂ has bond order 2.
• Higher bond order → greater bond dissociation energy → higher stability.

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Q3. Why is O₂ paramagnetic, while N₂ is diamagnetic?

Answer:

• In O₂, the configuration shows two unpaired electrons in π2px and π2py orbitals, making it paramagnetic.
• In N₂, all electrons are paired, so it is diamagnetic.

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Q4. A molecule has equal number of bonding and antibonding electrons. Is it stable? Why?

Answer:
No, it is unstable because the effect of antibonding electrons is slightly more than that of bonding electrons.

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Multiple Choice Questions (MCQs)

Q1. Which molecule has bond order zero?
(a) He₂
(b) H₂
(c) O₂
(d) N₂

Answer: (a) He₂
Explanation: He₂ → 2 bonding electrons, 2 antibonding electrons → bond order = (2 – 2)/2 = 0 → unstable.

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Q2. Which molecule has the shortest bond length?
(a) O₂
(b) N₂
(c) F₂
(d) H₂

Answer: (b) N₂
Explanation: Bond order of N₂ = 3 → shortest bond length (110 pm).

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Q3. Which statement is correct about bond order?
(a) Bond order is inversely proportional to bond length.
(b) Bond order is directly proportional to bond length.
(c) Bond order is independent of bond length.
(d) Bond order depends only on antibonding electrons.

Answer: (a)
Explanation: Higher bond order → stronger bond → shorter bond length.

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Q4. Which of the following molecules is paramagnetic?
(a) B₂
(b) C₂
(c) N₂
(d) Be₂

Answer: (a) B₂
Explanation: In B₂, electronic configuration shows two unpaired electrons in π2px and π2py, hence paramagnetic.

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Assertion-Reason Type Questions

Q1.
Assertion (A): O₂ is paramagnetic.
Reason (R): O₂ has two unpaired electrons in antibonding π* orbitals.

Options:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, R is false.
(d) A is false, R is true.

Answer: (a)

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Q2.
Assertion (A): Bond order of N₂ is 3.
Reason (R): N₂ has 10 bonding electrons and 4 antibonding electrons.

Options:
(a) Both A and R true, and R is correct explanation.
(b) Both A and R true, but R not explanation.
(c) A true, R false.
(d) A false, R true.

Answer: (c)
Explanation: Correct count → 10 bonding, 2 antibonding → bond order = (10 – 2)/2 = 4 (not correct). Actual = 3.

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Q3.
Assertion (A): He₂ does not exist.
Reason (R): He₂ has equal number of bonding and antibonding electrons, so bond order = 0.

Answer: (a) Both A and R are true, R is correct explanation.

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Case Study

Passage:
Bond order gives important insights into stability, bond strength, and bond length. N₂ has bond order 3, bond energy 945 kJ mol⁻¹, and bond length 110 pm, while O₂ has bond order 2, bond energy 495 kJ mol⁻¹, and bond length 121 pm.
Also, magnetic character is determined by unpaired electrons in molecular orbitals. N₂ is diamagnetic, while O₂ is paramagnetic.

Questions:

1. Calculate the bond order of O₂.
2. Which molecule is more stable: O₂ or N₂?
3. Why is O₂ paramagnetic?
4. Which has shorter bond length: N₂ or O₂?

Answers:

1. O₂ → 10 bonding, 6 antibonding → bond order = (10 – 6)/2 = 2.
2. N₂ is more stable (bond order 3 > 2).
3. O₂ has two unpaired electrons in π orbitals*, hence paramagnetic.
4. N₂ (bond length 110 pm vs O₂ = 121 pm).