Atomic Structure: From Dalton to Quantum Theory
Structure of the Atom
In 1808, English chemist J. Dalton formulated his atomic theory, postulating that matter consists of indivisible atoms. This concept prevailed until the discovery of natural radioactivity by Henri A. Becquerel in 1896, revealing alpha, beta, and gamma rays, suggesting atoms were divisible.
The Electron: Plum Pudding Model
Studies on gas conductivity at low pressure revealed cathode rays, streams of negatively charged particles. J.J. Thomson identified these as electrons. In 1911, R.A. Millikan determined the electron’s charge through his oil drop experiment.
The Proton: Rutherford Model
E. Goldstein observed positively charged particles in discharge tubes, later identified as protons. In 1919, E. Rutherford and J. Chadwick detected protons by bombarding atoms. Rutherford’s model proposed a nucleus with protons and a surrounding electron cloud.
- Most alpha particles passed undeflected through metal foil, indicating atoms are mostly empty space.
- Some particles were deflected, suggesting a dense, positively charged nucleus.
Isotopes
Atoms of an element can have the same mass but different chemical properties, called isotopes.
The Neutron
In 1932, J. Chadwick discovered neutrons, neutral particles in the nucleus with a mass similar to protons.
Atomic Parameters
- Atomic Number (Z): Number of protons, determining the element.
- Mass Number (A): Number of protons and neutrons, determining the isotope.
Isotope notation: AZX, where X is the element symbol.
Atomic mass is the average of isotopic masses in nature.
Origins of Quantum Theory
James C. Maxwell’s classical electromagnetic theory explained light as electromagnetic waves. However, early 20th-century experiments led to the development of quantum theory, initially applied to light energy and later to atomic theories.