Atomic Structure IIT JEE Chemistry

Subatomic Particles

• Electrons have a relative mass of 1/1836, approximately 9.109×10–31 kg, and a charge of -1.602×10–19 C.
• Protons have a relative mass of 1, approximately 1.673×10–27 kg, and a charge of +1.602×10–19 C.
• Neutrons have a relative mass of 1, approximately 1.675×10–27 kg, and no charge.

Atomic Models

• Thomson's Model (Plum-Pudding Model):
  • Proposes atoms are spheres of positive charge with electrons embedded.
  • Limitations include inability to explain Rutherford's scattering experiment results.
• Rutherford's Model:
  • Suggests most mass and positive charge are concentrated in the nucleus, with electrons revolving around it.
  • Limitations include failure to explain atomic stability and electronic structure.

Atomic Terms

• Atomic Number (Z): Total number of protons (and electrons in neutral atoms).
• Mass Number (A): Sum of protons and neutrons in an atom.
• Nucleons: Collective term for protons and neutrons.
• Isotopes: Same atomic number, different mass number (e.g. ¹H¹, ¹H², ¹H³).
• Isobars: Different atomic numbers but the same mass number (e.g. ¹⁵P³² and ¹⁶S³²).
• Isotones: Same number of neutrons but different protons/mass numbers (e.g. ⁶C¹⁴, ⁸O¹⁶, ⁷N¹⁵).
• Isoelectronic Species: Same electron count (e.g. N₂, CO, CN⁻).
• Nuclear Isomers: Same atomic number and mass number, different radioactive properties (e.g., Uranium-X and Uranium-Z).
• Isosters: Same number of atoms and electrons (e.g., N₂ and CO).

Wave Properties

• Wavelength (λ): Distance between consecutive crests or troughs.
• Frequency (ν): Number of waves passing a point per second; related to wavelength by ν = c/λ.
• Velocity (c): Speed of the wave; calculated as c = νλ.
• Wave Number: Number of wavelengths per cm.
• Amplitude (a): Height of the wave crest or depth of the trough, indicating intensity.

Electromagnetic Waves

• Radio Waves: Wavelength range of 3×10¹⁴ to 3×10⁷ Å.
• Microwaves: Wavelength range of 3×10⁹ to 3×10⁶ Å.
• Infrared (IR): Wavelength range of 6×10⁶ to 7600 Å.
• Visible Light: Wavelength range of 7600 to 3800 Å.
• Ultraviolet (UV): Wavelength range of 3800 to 150 Å.
• X-rays: Wavelength range of 150 to 0.1 Å.
• Gamma Rays: Wavelength range of 0.1 to 0.01 Å.

Atomic Spectrum of Hydrogen Atom

• Rydberg constant (RH) is 108978 cm⁻¹.
• Spectral Series:
  • Lyman: n₁=1, n₂=2,3,4... (UV region).
  • Balmer: n₁=2, n₂=3,4,5... (Visible region).
  • Paschen: n₁=3, n₂=4,5,6... (IR region).
  • Brackett: n₁=4, n₂=5,6,7... (IR region).
  • Pfund: n₁=5, n₂=6,7,8... (IR region).

Photoelectric Effect

• Ejection of electrons from metal surfaces occurs when light of suitable frequency strikes.
• The minimum frequency required for ejection is the threshold frequency (vo).
• Ejected electron energy is proportional to light frequency; number of ejected electrons depends on light intensity.

Planck’s Quantum Theory

• Energy is emitted or absorbed in packets called quanta (photons for light).
• The energy of each quantum is E = hν; Planck’s constant (h) is 6.626 × 10⁻³⁴ J sec.
• Energy absorption is quantized in integer multiples of hν.

Bohr’s Atomic Model

• Electrons orbit the nucleus in paths with fixed energy levels.
• Angular momentum (mvr) is quantized, being an integer multiple of h/2π.
• Energy jumps between levels involve electromagnetic radiation (E = hν).
• Hydrogen atom energy levels are expressed as En = -RH (1/n²).
• For hydrogen-like species, radius (rn) and energy (En) for nth orbit are calculated by rn = 52.9 × n²/Z pm and En = -2.18×10⁻¹⁸(Z²/n²) eV, respectively.