Structure of an atomic

Introduction to Atomic Structure

  • Chemical behavior diversity is due to differences in atomic structure.

  • Objectives of the unit:

    • Understand discovery of subatomic particles (electrons, protons, neutrons) and their characteristics.

    • Describe atomic models by Thomson, Rutherford, and Bohr.

    • Comprehend quantum mechanical model of the atom.

    • Understand electromagnetic radiation nature and Planck’s quantum theory.

    • Explain the photoelectric effect and atomic spectra features.

    • Discuss de Broglie relation and Heisenberg uncertainty principle.

    • Define atomic orbitals using quantum numbers.

    • State Aufbau principle, Pauli exclusion principle, and Hund’s rule.

    • Write electronic configurations of atoms.

Historical Background

  • Early philosophers theorized that atoms are fundamental building blocks of matter, from Greek word "atomio" meaning "uncuttable."

  • John Dalton proposed the first atomic theory in 1808, establishing the atom as the ultimate particle of matter.

  • Dalton's atomic theory successfully explained laws of conservation of mass, constant composition, and multiple proportions, but could not explain electrical charge phenomena observed in experiments.

Discovery of Subatomic Particles

Cathode Rays and Electrons

  • Discovery of Electron:

    • In 1830, Faraday's experiments led to the consideration of the particulate nature of electricity.

    • Cathode ray discharge tubes were used to study electrical discharge in gases.

    • Cathode rays travel from cathode to anode; they are invisible but can be detected through phosphorescent materials.

    • The behavior of cathode rays in electric/magnetic fields indicated they consist of negatively charged particles called electrons.

Charge-to-Mass Ratio of Electrons

  • J.J. Thomson's Experiments (1897):

    • Investigated cathode rays using electric and magnetic fields to measure their charge-to-mass ratio.

    • Found e/me = 1.758820 × 10^11 C kg^–1.

Charge of the Electron

  • R.A. Millikan's Oil Drop Experiment (1906-14):

    • Determined charge of the electron to be approximately –1.6 × 10^–19 C.

    • Mass of the electron was subsequently calculated as 9.1094 × 10^–31 kg.

Discovery of Protons and Neutrons

  • Positively charged canal rays were discovered from modified cathode ray tubes.

  • Protons:

    • Initially characterized as positively charged particles originating from gases in cathode ray tubes.

  • Neutrons:

    • Discovered by Chadwick (1932) as neutral particles emitted when beryllium was bombarded with alpha particles; mass slightly greater than protons.