Structure of Atom — Previous Year Questions (Class 11 Chemistry)
Explore the structure of atoms through the evolution from Dalton to quantum mechanical models. Understand orbitals, electron configuration, and the dual na
TL;DR: Explore the structure of atoms through the evolution from Dalton to quantum mechanical models. Understand orbitals, electron configuration, and the du…
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Explore the structure of atoms through the evolution from Dalton to quantum mechanical models. Understand orbitals, electron configuration, and the dual na
Structure of Atom — Previous Year Questions with Solutions
Q (2023, 3 marks): Calculate the wavelength of an electron moving with a velocity of 2.19 × 10^6 m/s. (h = 6.626 × 10^-34 J⋅s, m_e = 9.109 × 10^-31 kg)
Answer: Given:
Velocity of electron (v) = 2.19 × 10^6 m/s
Planck's constant (h) = 6.626 × 10^-34 J⋅s
Mass of electron (m_e) = 9.109 × 10^-31 kg
De Broglie wavelength:
λ = h/(m × v)
λ = (6.626 × 10^-34)/(9.109 × 10^-31 × 2.19 × 10^6)
λ = (6.626 × 10^-34)/(1.995 × 10^-24)
λ = 3.32 × 10^-10 m = 0.332 nm = 3.32 Å
Q (2022, 2 marks): An electron is in the n = 3 shell. How many orbitals are possible in this shell?
Answer: For a shell with principal quantum number n:
Number of orbitals = n^2
For n = 3:
Number of orbitals = 3^2 = 9 orbitals
Distribution:
s orbital (l=0): 1 orbital
p orbitals (l=1): 3 orbitals
d orbitals (l=2): 5 orbitals
Total = 1 + 3 + 5 = 9 orbitals
Q (2024, 3 marks): Write the electron configuration of iron (Fe, atomic number 26) and indicate the number of unpaired electrons.
Answer: Iron (Fe), Atomic number = 26
Electron configuration:
Fe: 1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2
Or in short form:
Fe: [Ar] 3d^6 4s^2
Ordering electrons in 3d orbital using Hund's rule:
3d: ↑ ↑ ↑ ↑ ↑ ↑ (each box gets one electron first)
Number of unpaired electrons in 3d = 4 electrons
Total unpaired electrons = 4
Q (2023, 4 marks): Calculate the energy required to ionize a hydrogen atom in its ground state. (Rydberg constant R_H = 1.097 × 10^7 m^-1, h = 6.626 × 10^-34 J⋅s, c = 3 × 10^8 m/s)
Answer: Given:
Rydberg constant (R_H) = 1.097 × 10^7 m^-1
Planck's constant (h) = 6.626 × 10^-34 J⋅s
Speed of light (c) = 3 × 10^8 m/s
For ionization from n = 1 to n = infinity:
1/λ = R_H(1/n_f^2 - 1/n_i^2)
1/λ = R_H(1/∞^2 - 1/1^2)
1/λ = R_H(0 - 1) = -R_H
Energy = hc × R_H
E = (6.626 × 10^-34) × (3 × 10^8) × (1.097 × 10^7)
E = (6.626 × 3 × 1.097) × 10^-19
E = 21.8 × 10^-19 J = 2.18 × 10^-18 J = 13.6 eV
Q (2022, 2 marks): Identify the orbitals described by the following quantum numbers: (a) n=1, l=0 (b) n=2, l=1 (c) n=3, l=2
Answer: (a) n=1, l=0:
l=0 means s orbital
n=1 means first shell
Orbital = 1s
(b) n=2, l=1:
l=1 means p orbital
n=2 means second shell
Orbital = 2p
(c) n=3, l=2:
l=2 means d orbital
n=3 means third shell
Orbital = 3d
Q (2024, 1 mark): How many electrons can be accommodated in an orbital with l = 2?
Answer: l = 2 represents a d orbital
For any orbital:
Maximum number of electrons = 2(2l + 1)
For l = 2:
Maximum electrons = 2(2 × 2 + 1) = 2(5) = 10 electrons
Alternatively:
m_l values range from -l to +l
For l = 2: m_l = -2, -1, 0, 1, 2 (5 values)
Each orbital can hold 2 electrons
Total = 5 × 2 = 10 electrons
Frequently Asked Questions
What is the Aufbau principle?
The Aufbau principle states that electrons fill atomic orbitals in order of increasing energy. Electrons first occupy orbitals with lower energy before moving to higher energy orbitals. The order follows the (n+l) rule.
Why are electrons not found at a fixed position in atoms?
According to quantum mechanics (Heisenberg's uncertainty principle), the exact position and momentum of an electron cannot be simultaneously determined. Instead, electrons exist in orbitals—regions of space where they are likely to be found.
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