Structure of Atom
Class 11 Chemistry · Chemistry Part I · Chapter 2
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Summary
This unit traces the experimental discovery of the atom's internal structure and the models built from it. Cathode-ray experiments revealed the electron, whose charge-to-mass ratio J. J. Thomson measured (\(1.758\times10^{11}\,\text{C kg}^{-1}\)) and whose charge Millikan's oil-drop experiment fixed (\(-1.602\times10^{-19}\,\text{C}\)). Canal rays revealed the proton, and Chadwick discovered the neutron in 1932. Thomson's plum-pudding model gave way to Rutherford's nuclear model after the gold-foil \(\alpha\)-scattering experiment showed a tiny, dense, positively charged nucleus. The atomic number \(Z\) equals the proton count, the mass number \(A\) equals protons plus neutrons, and isotopes and isobars are defined. Rutherford's model could not explain atomic stability or line spectra. Planck's quantum theory (\(E=h\nu\)) and Einstein's explanation of the photoelectric effect established the dual wave-particle nature of radiation. Bohr quantised electron orbits and energy, successfully explaining the hydrogen line spectrum (Lyman, Balmer, Paschen, Brackett and Pfund series), but failed for multi-electron atoms. de Broglie's matter-wave relation and Heisenberg's uncertainty principle led to the quantum-mechanical model based on Schrödinger's equation, in which orbitals are described by four quantum numbers and electrons are filled following the Aufbau principle, Pauli exclusion principle and Hund's rule.
Discovery of electron, proton and neutronThomson and Rutherford atomic modelsAtomic number, mass number, isotopes and isobarsPlanck's quantum theory and photoelectric effectBohr's model and hydrogen spectrumde Broglie relation and uncertainty principleQuantum-mechanical model and quantum numbersShapes of orbitals and electronic configuration
Key terms
Isotopes
Atoms of the same element with the same atomic number but different mass numbers, differing in number of neutrons.
Quantum
The smallest discrete packet of energy that can be emitted or absorbed as electromagnetic radiation, \(E=h\nu\).
Photoelectric effect
The ejection of electrons from a metal surface when radiation of frequency above a threshold frequency strikes it.
Atomic orbital
The wave function \(\psi\) of an electron in an atom; \(|\psi|^{2}\) gives the probability of finding the electron at a point.
Uncertainty principle
Heisenberg's rule that the position and momentum of an electron cannot both be determined exactly at the same instant.
Aufbau principle
Orbitals are filled in order of increasing energy, the lower \((n+l)\) orbital filling first.
Extra questions & answers
Try one from this chapter
The neutron was discovered by: