Lecture Notes on Atomic Models and Preparation Strategies

Introduction

• Welcome and greetings to the class.
• Encouragement for students to stay calm and prepare for exams.
• Importance of following previous year question series (PYQ) with guidance.
• Mention of EduCart Physics Simple Paper for exam preparation with well-balanced questions.

Atom Basics

• An atom is the basic unit of matter and the smallest unit of a chemical element.
• Subatomic Particles:
  • Proton (positive charge)
  • Neutron (no charge)
  • Electron (negative charge)
• Nucleus: Center of the atom containing protons and neutrons.
• Atoms are electrically neutral; the number of protons equals the number of electrons.

Historical Atomic Models

Thomson's Plum Pudding Model

• Proposed the atom as a mixture (like a watermelon):
  • Positive charge in the center (pudding).
  • Negative charges (electrons) spread throughout (seeds).
• Limitations of Thomson's model: Could not explain certain experimental observations.

Rutherford's Experiment

• Conducted experiments with alpha particles and a gold foil.
• Key observations:
  • Most alpha particles passed through; some were deflected.
  • A small number of particles were deflected at large angles (indicating a dense center).
• Identified the nucleus as a small, dense center with positive charge.

Rutherford's Conclusions

• Most of an atom's volume is empty space.
• The nucleus contains most of the atom's mass and all positive charge.
• This model was a significant advancement, despite having limitations.

Limitations of Rutherford's Model

• Electron stability: If electrons revolved around the nucleus, they would lose energy and spiral inward.
• Inability to explain line spectra of atoms.

Bohr's Model of the Atom

• Addressed Rutherford's limitations:
  • Electrons move in defined orbits (stationary states).
  • Electrons do not radiate energy while in these orbits.
• Introduced quantization of angular momentum.

Important Formulas in Bohr's Theory

• Energy levels:
  • Energy of electron in hydrogen atom:
    • E = -13.6 eV / n² (where n = principal quantum number)
• Radius of n-th orbit:
  • Radius of hydrogen atom:
    • r_n = n² * (0.53 Å)

Spectral Series of Hydrogen

• Emission when an electron transitions from a high to a low energy state releases energy as light:
  • Lyman Series: n=1 (ultraviolet)
  • Balmer Series: n=2 (visible)
  • Paschen Series: n=3 (infrared)
• Wavelength calculations:
  • Derived from Rydberg formula:
    • 1/λ = R * (1/n1² - 1/n2²)

Key Concepts

• Impact Parameter: Distance determining the angle at which scattering occurs.
• Excitation Energy: Energy required to move an electron from one energy level to another.
• Ionization Energy: Energy required to remove an electron completely.
• Excitation Potential: Voltage required to excite an electron to a higher energy state.

Conclusion

• Encouragement to practice consistently for exams.
• Reminder about importance of mastering foundational concepts in atomic structure and behavior.
• Thank you for attending the lecture and best wishes for your studies!