Question

Write the important postulates of Bohr's atomic model and explain how spectral lines are formed in the hydrogen spectrum with a neat diagram.

Answer 1

Niels Bohr quantitatively gave the general features of hydrogen atom structure and it's spectrum. His theory is used to evaluate several points in the atomic structure and spectras.

The postulates of Bohr atomic model for hydrogen as follows.

Postulates : – 

→ The electron in the hydrogen atom can revolve around the nucleus in a circular path of fixed radius and energy. These paths are called orbits (or) stationary states. These circular orbits are concentric (having same center) around the nucleus.

→ The energy of an electron in the orbit does not change with time.

→ When an electron moves from lower stationary state to higher stationary state absorption of energy takes place.

→ When an electron moves from higher stationary state to lower stationary state emission of energy takes place.

→ When an electronic transition takes place between two stationary states that differ in energy by

∆E is given by ∆E = E₂ – E₁ = hυ

∴ The frequency of radiation absorbed (or) emitted.

υ = (E₂ - E₁)/h; E₁ and E₂ are energies of lower, higher energy states respectively.

→ The angular momentum of an electron is given by mvr = nh/2π.

An electron revolve only in the orbits for which it's angular momentum is integral multiple of h/2π.

Line spectra of Hydrogen – Bohr's Theory :

→ In case of hydrogen atom line spectrum is observed and this can be explained by using Bohr's Theory.

→ According to Bohr's postulate when an electronic transition takes place between two stationary states that differ in energy is given by

∆E = E_f – E_i E_f = final orbit energy E_i = initial orbit energy

→ In case of absorption spectrum n_f > n_i

→ energy is absorbed (+Ve).

→ In case of emission spectrum n_i > n_f

→ energy is emitted (– Ve).

→ Each spectral line in absorption (or) emission spectrum associated to the particular transition in hydrogen atom.

→ In case of large no.of hydrogen atoms large no.of transitions possible they results in large no.of spectral lines.

The series of lines observed in hydrogen spectra are

Transitions of the electron in the hydrogen atom (The diagram shows the Lyman, Balmer and Paschen series of transitions)

'n' value	Series	Region
1	Lyman series	UV region
2	Balmer series	Visible
3	Pachen series	Near I.R
4	Brackette series	I.R.
5	pfund series	Far I.R.