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

1. 1 · Open with the idea. Bohr (1913) kept Rutherford's nucleus and added quantum rules.
2. 2 · Postulate 1: circular orbits. mv²/r = (1/4πε₀)(Ze²/r²): Coulomb attraction gives the centripetal force.
3. 3 · Postulate 2: quantum condition. Only orbits with mvr = nh/2π (n = 1, 2, 3, ...) are allowed.
4. 4 · Postulate 3: stationary states. In an allowed orbit the electron does not radiate; its energy is constant.
5. 5 · Postulate 4: frequency condition. A jump between levels gives or takes one photon: hν = E(higher) − E(lower).
6. 6 · Real life. Sodium street lamps glow orange because of such fixed jumps.
7. 7 · Close in one line. So, the atom is stable and gives a line spectrum.

Scoring tip: Number the four postulates and write the formula with each. One line of explanation per postulate is enough for 3 marks.