Structure of Atom — The Complete Basics
A clear, exam-ready guide to atomic structure for JEE — subatomic particles, Thomson to Bohr models, the hydrogen spectrum formulas, quantum numbers, and the Aufbau, Pauli and Hund rules.
By the PadhoDost Team · 📖 8 min read · Updated 4 August 2026
Part of JEE (Main + Adv) prep🧠 A stadium with a marble at its centre
If an atom were blown up to the size of a huge cricket stadium, the nucleus would be a tiny marble at the very centre, and the electrons would be like specks buzzing around the outer stands. Almost the entire atom is empty space, yet nearly all its mass sits in that central marble — the nucleus.
The atom is the basic unit of matter, made of a dense positive nucleus (protons + neutrons) surrounded by negatively charged electrons. Our picture of it evolved in stages: Thomson's 'plum pudding' model, Rutherford's nuclear model (from the gold-foil experiment), Bohr's orbits, and finally the modern quantum-mechanical model of orbitals.
Meet the subatomic particles
| Particle | Charge | Relative mass | Discovered by |
|---|---|---|---|
| Electron | -1 | ~1/1836 u | J. J. Thomson |
| Proton | +1 | 1 u | E. Goldstein (canal rays) |
| Neutron | 0 | 1 u | James Chadwick |
Bohr's model of the atom
Niels Bohr proposed that electrons revolve only in certain fixed circular orbits (energy levels) without radiating energy. Each orbit has a definite energy, so these are called stationary states. An electron absorbs energy to jump to a higher orbit and releases a photon of light when it falls back — this is why atoms give sharp line spectra rather than a continuous rainbow.
📝 Wavelength of the first Balmer line of hydrogen (n=3 to n=2)
For hydrogen Z = 1. The electron falls from n2 = 3 to n1 = 2.
1/lambda = R (1/n1^2 - 1/n2^2) = 1.097 x 10^7 (1/2^2 - 1/3^2)
1/2^2 - 1/3^2 = 1/4 - 1/9 = (9 - 4)/36 = 5/36 = 0.1389
1/lambda = 1.097 x 10^7 x 0.1389 = 1.524 x 10^6 per metre
lambda = 1 / (1.524 x 10^6) = 6.56 x 10^-7 m = 656 nm
This is red light — the famous H-alpha line of the Balmer series.
Filling electrons: quantum numbers and rules
The four quantum numbers and three filling rules
- ✓n (principal): shell size and energy; n = 1, 2, 3... Maximum electrons in a shell = 2n^2.
- ✓l (azimuthal): subshell shape; l = 0 to (n-1), giving s, p, d, f.
- ✓m (magnetic): orientation of the orbital; m ranges from -l to +l.
- ✓s (spin): direction of electron spin; +1/2 or -1/2.
- ✓Aufbau principle: electrons fill the lowest-energy orbitals first (order set by the n + l rule).
- ✓Pauli exclusion principle: no two electrons in an atom can have all four quantum numbers identical (so an orbital holds at most 2 electrons, with opposite spins).
- ✓Hund's rule: electrons singly occupy each orbital of a subshell before any pairing begins.
⚡ Quick check
What is the maximum number of electrons that the third shell (n = 3) can hold?
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