Anand Classes provides comprehensive Class 11 Chemistry notes on Bond Length in Covalent Bonds covering its definition, measurement, and the key factors that affect bond length. Understand how atomic size, bond order, hybridization, and electronegativity influence the distance between two bonded atoms. These easy-to-understand notes include examples, diagrams, and important MCQs to help you prepare for NEET, JEE, and CBSE board exams. Perfect for quick revision and exam success. Click the print button to download study material and notes.
What are Bond Parameters in Covalent Bonds?
Covalent bonds are characterised by certain parameters such as bond length, bond angle, and bond energy.
What is Bond Length?
Definition:
Bond length is defined as the equilibrium distance between the centres of the nuclei of two bonded atoms in a molecule.
It represents the equilibrium internuclear separation distance of the bonded atoms. Bond length is measured using spectroscopic, X-ray diffraction, and electron diffraction techniques.
Each atom in a bonded pair contributes to the bond length. In a covalent bond, the contribution from each atom is called the covalent radius of that atom.
For a covalent molecule AB, the bond length can be expressed as:
$$
R = r_A + r_B
$$
where $R$ is the bond length and $r_A$, $r_B$ are the covalent radii of atoms A and B, respectively.
Example:
The O–H bond length in ethanol is the sum of the covalent radii of H and O:
$$
R = 37 , \text{pm} + 74 , \text{pm} = 111 , \text{pm}
$$
Bond length is usually expressed in Angstroms (Å) or picometers (pm):
$$
1 , \text{Å} = 10^{-10} , \text{m}, \quad 1 , \text{pm} = 10^{-12} , \text{m}
$$
What is the Difference Between Covalent Radius and Van der Waals Radius?
- Covalent radius: Half the distance between two similar atoms joined by a covalent bond in a molecule.
- Van der Waals radius: Half the distance between two similar adjacent atoms in two neighbouring molecules in the solid state.
Example (Chlorine):
- Internuclear distance in Cl₂: 198 pm
$$
r_\text{covalent} = \frac{198}{2} = 99 , \text{pm}
$$
- Distance between two nonbonded nearest-neighbour Cl atoms: 360 pm
$$
r_\text{van der Waals} = \frac{360}{2} = 180 , \text{pm}
$$
The van der Waals radii are always larger than covalent radii.
What are the Average Bond Lengths of Some Common Bonds?
| Bond | Bond Length (pm) |
|---|---|
| O—H | 96 |
| C—H | 107 |
| C—N | 143 |
| C—C | 154 |
| Si—Si | 234 |
| Ge—Ge | 244 |
| N—O | 136 |
| C==C | 134 |
| C==N | 138 |
| N==O | 122 |
| C≡C | 120 |
| C≡N | 116 |
What are the Bond Lengths in Some Common Molecules?
| Molecule | Bond Length (pm) |
|---|---|
| H₂ (H—H) | 74 |
| F₂ (F—F) | 144 |
| Cl₂ (Cl—Cl) | 198 |
| Br₂ (Br—Br) | 228 |
| I₂ (I—I) | 267 |
| HF (H—F) | 92 |
| HCl (H—Cl) | 127 |
| HBr (H—Br) | 141 |
| HI (H—I) | 160 |
| N₂ (N≡N) | 109 |
| O₂ (O==O) | 121 |
What Factors Affect Bond Length?
(i) How Does Bond Multiplicity Affect Bond Length?
Bond length decreases with increase in bond multiplicity:
$$
\text{C≡C} < \text{C==C} < \text{C—C}
$$
Similarly:
$$
\text{N≡N} < \text{N==N} < \text{N—N}, \quad \text{O==O} < \text{O—O}
$$
(ii) How Does Atomic Size Affect Bond Length?
Bond length increases with increase in the size of the atoms. For atoms in the same family:
$$
\text{C—C} < \text{Si—Si} < \text{Ge—Ge}
$$
This is because as atomic size increases, valence electrons are farther from the nucleus, increasing the distance between the bonding nuclei (bond length).
Short Answer Conceptual Type Questions (SAT) on Bond length
Q1. What is bond length?
Answer:
Bond length is the equilibrium distance between the centres of the nuclei of two bonded atoms in a molecule.
Q2. What is the difference between covalent radius and van der Waals radius?
Answer:
- Covalent radius: Half the distance between two similar atoms joined by a covalent bond.
- Van der Waals radius: Half the distance between two similar adjacent atoms in two neighbouring molecules.
Van der Waals radius is always larger than covalent radius.
Q3. How does bond multiplicity affect bond length?
Answer:
Bond length decreases with increase in bond multiplicity:
$$
\text{C≡C} < \text{C==C} < \text{C—C}
$$
Q4. How does atomic size affect bond length?
Answer:
Bond length increases with the size of the atoms. For example:
$$
\text{C—C} < \text{Si—Si} < \text{Ge—Ge}
$$
Multiple Choice Questions (MCQs) With Answers and Explanation on Bond length
Q1. The bond length of H—H in H2 is:
(a) 74 pm
(b) 92 pm
(c) 127 pm
(d) 111 pm
Answer: Correct option (a)
Explanation:
From experimental data, the bond length of H2 (H—H) is 74 pm.
Q2. Which of the following statements is true?
(a) Covalent radius > van der Waals radius
(b) Van der Waals radius > covalent radius
(c) Covalent radius = van der Waals radius
(d) Covalent radius = 2 × van der Waals radius
Answer: Correct option (b)
Explanation:
The van der Waals radius measures the overall size including the valence shell in a nonbonded situation, making it larger than the covalent radius.
Q3. Which factor does not affect bond length?
(a) Bond multiplicity
(b) Atomic size
(c) Electronegativity difference
(d) Nuclear charge
Answer: Correct option (c)
Explanation:
Bond length depends mainly on bond multiplicity and atomic size, not directly on electronegativity difference.
Assertion-Reason Type Questions With Answers and Explanation on Bond length
Q1.
Assertion (A): C≡C bond length is shorter than C=C bond length.
Reason (R): Bond length decreases as bond multiplicity increases.
Answer: Both A and R are true, and R is the correct explanation of A.
Explanation:
A triple bond has more shared electrons, pulling the nuclei closer together, resulting in a shorter bond length.
Q2.
Assertion (A): Van der Waals radius of Cl is 180 pm.
Reason (R): It is half the distance between two nonbonded nearest neighbouring Cl atoms.
Answer: Both A and R are true, and R is the correct explanation of A.
Case Study Based Question on Bond length
Passage:
An H2 molecule has a bond length of 74 pm, while Cl2 has a bond length of 198 pm. The bond length increases with atomic size. Van der Waals radius of Cl is 180 pm.
Questions:
- Explain why Cl—Cl bond length is greater than H—H bond length.
- What is the covalent radius of Cl?
- Why is van der Waals radius larger than covalent radius?
Answers:
- Bond length increases with atomic size. Cl atoms are larger than H atoms.
- Covalent radius of Cl:
$$
r_\text{covalent} = \frac{198}{2} = 99 , \text{pm}
$$
- Van der Waals radius measures the size including the valence shell in a nonbonded situation, so it is larger.