Chapter 2 — Structure of Atom
What is inside an atom, and why its electrons may have only certain energies — from cathode rays and the nucleus to photons, Bohr orbits, matter waves, quantum numbers and the electronic configurations that run all of chemistry.
Heat a little common salt in a gas flame and the flame turns a brilliant yellow. Pass that light through a prism and you do not see a rainbow: you see two thin yellow lines, at $589.0$ and $589.6\ \mathrm{nm}$, and almost nothing else. Every element has its own set of lines — a fingerprint so reliable that helium was discovered in the light of the Sun (in 1868) before anyone found it on Earth, and astronomers today read the composition of stars billions of kilometres away from their spectra. Why should an atom emit only a few sharp colours and not all of them?
Answering that question took about thirty years and overturned classical physics. It led first to the discovery that atoms contain electrons, protons and neutrons, then to a tiny dense nucleus, then to the idea that energy comes in packets (quanta), and finally to the quantum-mechanical atom, in which electrons occupy orbitals labelled by four quantum numbers. That model explains the periodic table (Chapter 3), the shapes of molecules and the bonds between atoms (Chapter 4) — in short, the whole of chemistry. In JEE and NEET this chapter supplies some of the most reliable marks: Bohr-model and Rydberg calculations, the photoelectric equation, de Broglie wavelengths, quantum numbers and electronic configurations appear in almost every paper.
- describe the experiments that revealed the electron, proton, neutron and nucleus, and explain the Thomson and Rutherford models and why each failed;
- count protons, neutrons and electrons in any atom or ion, and recognise isotopes and isobars;
- move between wavelength, frequency, wavenumber and photon energy, and use $h\nu=h\nu_{0}+\tfrac12m_{e}v^{2}$ for the photoelectric effect;
- calculate the wavelengths of lines in the hydrogen spectrum and the energies and radii of Bohr orbits for hydrogen-like species;
- apply the de Broglie relation and the uncertainty principle, and explain why they rule out fixed orbits;
- assign the four quantum numbers, sketch s, p and d orbitals and count their radial and angular nodes;
- write the electronic configuration of any atom or ion up to $Z=36$ using the Aufbau principle, the $(n+l)$ rule, Pauli's principle and Hund's rule — including the exceptions Cr and Cu.
Sections in this chapter
Subatomic Particles and Early Atomic Models
Cathode rays and $e/m$, Millikan's oil drops, the proton and neutron, the Thomson and Rutherford models, atomic and mass numbers, isotopes and isobars.
Light, Quantisation and Bohr's Model
Waves and photons, black-body radiation, the photoelectric effect, the hydrogen spectrum and Bohr's model of the hydrogen atom.
The Quantum Mechanical Model: Quantum Numbers and Orbitals
Matter waves, the uncertainty principle, orbitals versus orbits, the four quantum numbers, orbital shapes, nodes and orbital energies.
Electronic Configuration of Atoms
The Aufbau principle and the $(n+l)$ rule, Pauli's exclusion principle, Hund's rule, configurations of atoms and ions, and the extra stability of half-filled and filled subshells.