NCERT Solutions for Class 11 Chemistry Chapter 2 Structure of Atom
NCERT Solutions for Class 11 Chemistry Chapter 2 Structure of Atom covers the discovery of subatomic particles, atomic models (Thomson, Rutherford, Bohr), electromagnetic spectrum, photoelectric effect, Planck's quantum theory, dual behavior of matter, Heisenberg's uncertainty principle, quantum mechanical model of atom, quantum numbers, orbital shapes, and electronic configurations (Aufbau principle, Pauli exclusion principle, Hund's rule).
Overview of Topics & Subtopics (Click to Jump)
1. Subatomic Particles & Early Atomic Models
Short Note:
• Subatomic Particles:
- Electron (e-): Discovered by J.J. Thomson (Cathode Ray Tube experiment). Charge = -1.602 × 10-19 C, Mass = 9.1 × 10-31 kg.
- Proton (p+): Discovered by E. Goldstein (Anode Ray experiment). Charge = +1.602 × 10-19 C, Mass = 1.672 × 10-27 kg.
- Neutron (n0): Discovered by James Chadwick. Neutral, Mass = 1.674 × 10-27 kg.
• Rutherford's α-Particle Scattering Experiment: Established that an atom consists of a tiny, dense, positively charged nucleus surrounded by electrons revolving in circular paths. Failed to explain atomic stability and line spectrum of hydrogen.
2. Electromagnetic Radiation, Planck's Quantum Theory & Bohr's Model
Short Note:
• Electromagnetic Radiation: Characterized by frequency (ν), wavelength (λ), and wave number (ν̄). Equation: c = ν λ and ν̄ = 1 / λ.
• Planck's Quantum Theory: Radiant energy is absorbed or emitted discontinuously in small packets of energy called quanta (or photons for light): E = h ν = h c / λ, where h = 6.626 × 10-34 J·s.
• Photoelectric Effect: Ejection of electrons from metal surface when light of suitable frequency (ν ≥ ν0) strikes it. Kinetic Energy K.E. = h ν - h ν0.
• Bohr's Atomic Model for Hydrogen:
- Electrons revolve in non-radiating stationary orbits where angular momentum L = m v r = n h / (2π).
- Radius of n-th orbit: rn = 0.529 × (n2 / Z) Å.
- Energy of electron in n-th orbit: En = -2.18 × 10-18 × (Z2 / n2) Joules/atom = -13.6 × (Z2 / n2) eV.
- Rydberg formula for hydrogen spectrum: ν̄ = 1/λ = RH [ (1/n12) - (1/n22) ], where RH = 109,677 cm-1.
3. Dual Nature of Matter & Heisenberg's Uncertainty Principle
Short Note:
• de Broglie Dual Behaviour of Matter: Every moving microscopic particle possesses wave property. De Broglie wavelength: λ = h / p = h / (m v).
• Heisenberg's Uncertainty Principle: It is impossible to determine simultaneously both the exact position and momentum (velocity) of a microscopic particle like an electron with absolute accuracy.
- Mathematical relation: Δx · Δp ≥ h / (4π) or Δx · (m Δv) ≥ h / (4π).
4. Quantum Mechanical Model, Quantum Numbers & Electronic Configuration
Short Note:
• Quantum Numbers:
1. Principal Quantum Number (n): Shell number / energy level (n = 1, 2, 3...). Determines orbital size and energy.
2. Azimuthal / Angular Momentum Quantum Number (l): Subshell / orbital shape (l = 0 to n-1). s (l=0, spherical), p (l=1, dumbbell), d (l=2, double dumbbell), f (l=3).
3. Magnetic Quantum Number (ml): Orbital orientation in space (ml = -l to +l). Total orbitals in subshell = 2l + 1.
4. Spin Quantum Number (ms): Direction of electron spin (+1/2 or -1/2).
• Rules for Filling Orbitals:
- Aufbau Principle: Orbitals are filled in order of increasing energy (determined by lowest (n + l) value).
- Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers. Maximum 2 electrons per orbital with opposite spins.
- Hund's Rule of Maximum Multiplicity: Electron pairing in degenerate orbitals (p, d, f) does not take place until each available orbital is singly occupied with parallel spins.
Intext & Conceptual Questions
Explanation:
1. Symmetrical Distribution of Electrons: Electrons in half-filled or fully-filled subshells are symmetrically distributed in space, which leads to lower energy and greater stability.
2. Maximum Exchange Energy: Electrons with parallel spins in degenerate orbitals can exchange their positions. Each exchange releases energy called exchange energy. Since half-filled and fully-filled configurations have the maximum number of possible exchanges, the total exchange energy released is maximum, making the system extra stable (e.g., Cr: [Ar] 3d5 4s1 and Cu: [Ar] 3d10 4s1).
Textbook Chapter End Exercise Questions
Given:
Frequency (ν) = 5 × 1014 Hz
Planck's constant (h) = 6.626 × 10-34 J·s
Avogadro's constant (NA) = 6.022 × 1023 mol-1
Step 1: Energy of one photon (E):
E = h ν
E = 6.626 × 10-34 J·s × 5 × 1014 s-1
E = 3.313 × 10-19 Joules
Step 2: Energy of one mole of photons (Etotal):
Etotal = E × NA
Etotal = 3.313 × 10-19 J × 6.022 × 1023 mol-1
Etotal = 1.995 × 105 J/mol = 199.51 kJ/mol.
The energy of one mole of photons is 199.51 kJ/mol.
Given:
Mass of electron (m) = 9.1 × 10-31 kg
Velocity (v) = 2.05 × 107 m/s
Planck's constant (h) = 6.626 × 10-34 kg·m2/s
De Broglie relation:
λ = h / (m v)
λ = (6.626 × 10-34) / (9.1 × 10-31 × 2.05 × 107)
λ = (6.626 × 10-34) / (1.8655 × 10-23)
λ = 3.55 × 10-11 m = 0.0355 nm (or 0.355 Å).
The de Broglie wavelength associated with the electron is 3.55 × 10-11 m.
Read More & Chapter Resources
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