State the three postulates of Bohr's model of the hydrogen atom.

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Open with the idea. Bohr (1913) kept Rutherford's nucleus and added quantum conditions to the electron's motion.
2 · Postulate 1: stationary orbits. The electron revolves only in certain orbits, and in them it does not radiate energy.
3 · Postulate 2: quantum condition. In an allowed orbit, angular momentum mvr = nh/2π, n = 1, 2, 3, ...
4 · Postulate 3: frequency condition. A jump from a higher to a lower orbit emits a photon: hν = E(initial) − E(final).
5 · Draw the picture. [Diagram: nucleus with circular orbits n = 1, 2, 3; an arrow from n = 3 to n = 2 and a wavy arrow labelled hν]
6 · Real life. Firework colours at Diwali are photons from such jumps.
7 · Close in one line. So, orbits are stable and only fixed photon energies are possible, which gives line spectra.

Scoring tip: Write the formula with each postulate. 'Stationary orbit' and 'without radiating' are the key words for postulate 1.