Atomic Structure Class 11 Chemistry Chapter 2 Notes

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1. Structure of an Atom

  1. Atom = Nucleus + Electrons
    • Nucleus contains protons and neutrons
    • Electrons revolve around the nucleus
    • Nuclear model proposed by Ernest Rutherford
    • Energy-level concept explained by Niels Bohr

2. Subatomic Particles

2.1 Protons (+)

  1. Discovered by Goldstein (1886)
  2. Positively charged particle
  3. Located in the nucleus
  4. Determines the atomic number of an element

2.2 Neutrons (0)

  1. Discovered by James Chadwick (1932)
  2. Electrically neutral
  3. Present in the nucleus
  4. Mass number = Protons + Neutrons

2.3 Electrons (–)

  1. Discovered by J.J. Thomson (1897)
  2. Negatively charged particle
  3. Move in orbitals (electron clouds)
  4. Responsible for:
    • Chemical bonding
    • Electrical conductivity

3. Important Atomic Concepts

  1. Most of the atom’s mass lies in the nucleus (Rutherford)
  2. Atomic number = Number of protons
    • In a neutral atom: protons = electrons
    • Established by Henry Moseley
  3. Isotopes
    • Same atomic number
    • Different mass number
    • Discovered by Frederick Soddy
  4. Isobars
    • Same mass number
    • Different atomic numbers
    • Studied by F.W. Aston
  5. Electrons do not move in fixed paths
    • Exist in probability clouds
    • Explained by Schrödinger & Heisenberg

4. Bohr’s Atomic Theory (1913)

4.1 Postulates

  1. Electrons revolve in fixed circular orbits (energy levels)
  2. Each orbit has definite energy
  3. Electrons do not radiate energy while in an orbit
  4. Energy is absorbed or emitted during transitions
  5. Angular momentum is quantized:
    mvr = nh / 2π

4.2 Applications

  1. Explains hydrogen emission spectrum
  2. Explains Balmer and Lyman series
  3. Calculates energy levels of hydrogen-like ions
  4. Foundation for quantum theory

5. Spectrum

5.1 Spectrum

  • Range of wavelengths of electromagnetic radiation

5.2 Types of Spectra

(a) Continuous Spectrum

  1. Contains all wavelengths without gaps
  2. Example: Sunlight (VIBGYOR)
  3. Wavelength range: 4000 Å – 7000 Å

(b) Line Spectrum

  1. Discrete lines at specific wavelengths
  2. Two types:
    • Absorption spectrum (dark lines on bright background)
    • Emission spectrum (bright lines on dark background)

6. Hydrogen Spectrum

  1. Hydrogen has one electron
  2. Electricity excites electron to higher energy levels
  3. Electron returns and emits light
  4. Emission occurs in UV, visible, and IR regions
  5. Five spectral series:
    • Lyman (UV): n₁ = 1
    • Balmer (Visible): n₁ = 2
    • Paschen (IR): n₁ = 3
    • Brackett (Mid-IR): n₁ = 4
    • Pfund (Far-IR): n₁ = 5

6.1 Rydberg Formula

[ \bar{v} = R_H \left( \frac{1}{n_1^2} – \frac{1}{n_2^2} \right) ]

7. Planck’s Quantum Theory (1900)

Postulates

  1. Energy is not absorbed or emitted continuously
  2. Energy is exchanged in packets called quanta
  3. Energy of quantum: E=hvE = hvE=hv
    • h=6.625×10−34 J⋅sh = 6.625 \times 10^{-34} \, J·sh=6.625×10−34J⋅s
    • vvv = frequency

8. X-Rays

8.1 Discovery

  • Discovered by W. Roentgen (1895)

8.2 Properties

  1. Very short wavelength (0.1–10 Å)
  2. High energy
  3. Travel at speed of light
  4. Not deflected by electric or magnetic fields

8.3 Types

  1. K-series: Heavy elements, high energy
  2. L-series: Light elements, low energy

8.4 Uses

  1. Medical diagnosis
  2. Cancer treatment
  3. Airport security
  4. Crystallography

9. Moseley’s Law

  1. Proposed in 1913
  2. X-ray frequency ∝ √(atomic number)
  3. Defined modern atomic number

10. Radioactivity

10.1 Discovery

  • Discovered by Henri Becquerel (1896)
  • Named by Marie Curie

10.2 Example

92U238→90Th234+2He4_{92}U^{238} \rightarrow _{90}Th^{234} + _2He^{4}92​U238→90​Th234+2​He4

10.3 Types of Radiation

  1. Alpha (α)
    • +2 charge
    • High ionization
    • Low penetration
  2. Beta (β)
    • –1 charge
    • Medium penetration
  3. Gamma (γ)
    • No charge or mass
    • Very high penetration

11. Uses of Nuclear Radiation

  1. Medicine
  2. Agriculture
  3. Power generation
  4. Industry
  5. Geology
  6. Archaeology (Carbon-14 dating)

12. Quantum Numbers

  1. Principal (n): Size & energy
  2. Azimuthal (ℓ): Shape
  3. Magnetic (m): Orientation
  4. Spin (s): Electron spin

13. Orbital Shapes

  • s: Spherical
  • p: Dumbbell (3 orientations)
  • d: Complex (5)
  • f: Very complex (7)

14. Rules for Filling Orbitals

  1. Pauli Exclusion Principle
  2. Aufbau Principle
  3. (n + ℓ) Rule
  4. Hund’s Rule

15. Electronic Configuration Examples

  • Carbon (6): 1s² 2s² 2p²
  • Oxygen (8): 1s² 2s² 2p⁴