Structure of Atom — Previous Year Questions (Class 11 Chemistry)
Atomic structure including nucleus, electrons, and electron configuration. Learn about quantum numbers, orbitals, and the periodic table basis.
TL;DR: Atomic structure including nucleus, electrons, and electron configuration. Learn about quantum numbers, orbitals, and the periodic table basis.
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Atomic structure including nucleus, electrons, and electron configuration. Learn about quantum numbers, orbitals, and the periodic table basis.
Structure of Atom — Previous Year Questions with Solutions
Q (2023, 3 marks): State Heisenberg's uncertainty principle. What does it mean physically?
Answer: Heisenberg's Uncertainty Principle: It is impossible to determine both position and momentum of an electron simultaneously with absolute precision.
Mathematically: Δx × Δp ≥ h / (4π)
Where Δx = uncertainty in position, Δp = uncertainty in momentum
Physical meaning:
1. Electrons don't have fixed orbits; they exist in regions of probability
2. Cannot know exact position and exact velocity simultaneously
3. If position is known precisely, momentum is very uncertain (velocity is uncertain)
4. If momentum is known precisely, position is very uncertain
5. This leads to concept of orbitals instead of orbits
Q (2022, 3 marks): What are quantum numbers? Write the four quantum numbers and their significance.
Answer: Quantum numbers describe the electronic state of an electron and are solutions to Schrodinger's wave equation.
1. Principal quantum number (n):
- Values: 1, 2, 3, 4, ... ∞
- Determines energy level and orbital size
- Shells: K(n=1), L(n=2), M(n=3), N(n=4)
2. Azimuthal quantum number (l):
- Values: 0, 1, 2, ... (n-1)
- Determines orbital shape
- l=0 (s), l=1 (p), l=2 (d), l=3 (f)
3. Magnetic quantum number (ml):
- Values: -l to +l
- Determines orbital orientation in space
- Number of orbitals = 2l + 1
4. Spin quantum number (ms):
- Values: +1/2 or -1/2
- Determines electron spin direction
- Only two electrons per orbital (Pauli exclusion principle)
Q (2023, 3 marks): Write the electron configuration of Ni (28) and Cr (24) using (a) notation and (b) orbital diagram.
Answer: Nickel (Ni, Z=28):
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ 4s²
Or: [Ar] 3d⁸ 4s²
Chromium (Cr, Z=24):
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹
Or: [Ar] 3d⁵ 4s¹
Note: Cr is exception (half-filled d-orbital is more stable)
Orbital diagram for Cr:
4s: ↑
3d: ↑ ↑ ↑ ↑ ↑
Q (2021, 3 marks): Explain Pauli's exclusion principle. How many electrons can occupy 3d orbitals?
Answer: Pauli Exclusion Principle: No two electrons in an atom can have identical set of quantum numbers.
Corollary: Maximum 2 electrons per orbital (with opposite spins)
For 3d orbitals:
l = 2 (d orbital)
ml values: -2, -1, 0, +1, +2 (5 values)
Number of d orbitals = 5
Maximum electrons in d orbital = 5 × 2 = 10 electrons
For 3d specifically:
3d¹⁰ configuration is possible
Example: Zn (30): [Ar] 3d¹⁰ 4s²
Q (2022, 3 marks): Compare Bohr's model and quantum mechanical model of atom. Why was Bohr's model rejected?
Answer: Bohr's Model:
1. Electrons move in definite circular orbits
2. Specific energy levels
3. Angular momentum = nh/2π
4. Explains hydrogen spectrum well
Quantum Mechanical Model:
1. Electrons in orbitals (region of probability)
2. Continuous energy levels
3. Derived from wave equation
4. Explains polyelectronic atoms
Why Bohr's model was rejected:
1. Failed for multielectron atoms (predictions didn't match)
2. Couldn't explain fine structure of spectral lines
3. Violated Heisenberg uncertainty principle
4. Electrons accelerating in orbits would radiate energy and collapse
5. No explanation for chemical bonding
6. Couldn't explain line spectra of polyelectronic atoms
Q (2023, 3 marks): Calculate the wavelength of photon emitted when electron transitions from n=3 to n=2 in hydrogen atom. (R = 1.09 × 10^7 m^-1)
Answer: Using Rydberg formula: 1/λ = R(1/n1² - 1/n2²)
n1 = 2, n2 = 3
1/λ = 1.09 × 10^7 (1/4 - 1/9)
= 1.09 × 10^7 × (9 - 4) / 36
= 1.09 × 10^7 × 5 / 36
= 5.45 × 10^7 / 36
= 1.514 × 10^6 m^-1
λ = 1 / (1.514 × 10^6) = 6.61 × 10^-7 m = 661 nm (red light)
Frequently Asked Questions
What is an orbital? How is it different from an orbit?
Orbit (Bohr model): Definite circular path where electron moves. Orbital (Quantum model): Region in space around nucleus where electron is likely to be found (90% probability). Orbits are deterministic; orbitals are probabilistic.
What is the aufbau principle and how is it used?
Aufbau principle: Electrons fill orbitals in order of increasing energy. Order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p... (following diagonal rule). This helps write electron configurations.
More Class 11 Chemistry PYQs
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- Aldehydes Ketones and Carboxylic Acids
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