Q.What are the postulates of Bohr theory of atom? OR What are ortho and para hydrogen? Explain.

Bohr's model postulates quantised circular orbits (stationary states) for electrons, a quantisation condition on angular momentum, and that energy exchange with the atom happens only during a jump between orbits.

This question offers an OR alternative (ortho/para hydrogen); the postulates of Bohr's theory (the primary question) are answered here.

Niels Bohr proposed the following postulates to explain the stability of the atom and the discrete (line) spectrum of hydrogen:

1. Electrons revolve around the nucleus only in certain fixed circular paths of definite energy, called stationary states or orbits (also called shells, labelled K, L, M, N... or n = 1, 2, 3...). As long as an electron stays in one of these stationary orbits, it does not radiate (lose) energy, despite being in constant circular motion - this postulate was needed because classical electromagnetic theory predicted an orbiting (accelerating) charge should continuously radiate energy and spiral into the nucleus, which does not happen in reality.

2. Only those orbits are permitted for which the angular momentum of the electron is an integral (whole-number) multiple of h/2pi (where h is Planck's constant):

   mvr = n.h/2pi, where n = 1, 2, 3, ... is a positive integer called the principal quantum number.

   This is the quantisation of angular momentum.

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