## Niels Bohr atomic model of the hydrogen atom

In 1913, Neils Bohr adopted the Rutherford model for the explanation of the atomic spectra of the hydrogen atom by Bohr’s theory. But according to classical mechanics, when a charged particle subjected to the acceleration it emits radiation and loses energy to hit the nucleus.

Thus according to Rutherford atomic theory, the energy of the electron loses continuously and the observed spectra should be continuous spectra.

But the actual electromagnetic spectrum consists of well-defined lines of definite frequencies. To resolve these anomalous Niels Bohr’s proposed a new atomic model of the hydrogen atom.

### Postulates of Bohr atomic model

- An atom possesses several stable circular orbits in which an electron can stay. Thus an electron stays in a particular orbit where no emission or absorption of energy occurs. These orbits are called energy levels of an atom.

The energy levels designated by numbers 1, 2, 3, …….. or capital letters, K, L, M, …….. starting from the nucleus of the atom. Thus the energy associated with a certain energy level increases with the increase of its distance from the nucleus.

If E_{1}, E_{2}, E_{3} …… denotes the number of energy levels of 1 (K – Shell), 2 (L – Shell), 3 (M – Shell) ……. then the order of energy levels are

E_{1}ㄑE_{2}ㄑE_{3}ㄑ……

- When an electron can jump from one orbit to another higher energy on the absorption of energy but one orbit to another lower energy with the emission of energy.

- The angular momentum of an electron moving in an orbit is an integral multiple of h/2π. This integral multiple is known as the principal quantum energy level of an atom.

where m = mass of an electron,

v = tangential velocity of an electron,

r = radius of an atom and

n = principal quantum number.

#### The attraction between nucleus and electrons

Let the nucleus has a mass m′ and the electron has mass m, the radius of the circular orbit = r, and the linear velocity of the electron = v.

Thus on the revolving electron, two types of forces acting,

- Centrifugal force
- The electric force of attraction

Centrifugal force = mv^{2}/r

The electric force of attraction between two opposite charges is given from Coulomb’s law of electrostatic attraction.

∴ Electrostatic force = e^{2}/r^{2}

Centrifugal force and electric force acted in the opposite direction. Thus the electron may keep on revolving in its orbit if these two forces act in the opposite direction must balance by each other.

#### Bohr radius of the hydrogen atom

Remarkable suggestion from Niels Bohr, the angular momentum of the electron is an integral multiple of h/2π.

where mvr = angular momentum and n = quantum number having the values 1,2,3, ……∞.

Thus putting the value of v on the force equation

#### The radius of the first orbit of the hydrogen atom

From the above equation, we can easily find out the radius of the permitted energy levels of the hydrogen in terms of the quantum number. Thus when n = 1, the radius of the first stationary orbit of hydrogen.

∴ r_{1} = 0.529 × 10^{-8} cm = 0.529 Å

The ratio of first Bohr orbit and nth orbit of the hydrogen atom

∴ r_{n} = n^{2} × r_{1}

Question

Calculate the radius of the second orbit of hydrogen if the radius of the first orbit of hydrogen = 0.529 Å.

Answer

The radius of the second orbit of the hydrogen atom

r_{2} = n^{2} × r_{1} = 2.12 Å

#### The velocity of an electron in Bohr model

From the principle of quantization to the revolving electron in the hydrogen atom

Putting the values of r in the above equation, we have

Thus the velocity of the second orbit will be one half of the first orbit and one-third of the first orbit and so on.

Question

Calculate the velocity of the hydrogen electron in the first and third energy levels of an atom. How to calculate the number of rotation of an electron per second in the third energy level?

Answer

The velocity of an electron nth level of the hydrogen atom

v_{n} = 2πe^{2}/nh

where n = 1, 2, 3, ……..

Thus the velocity of an electron in the first energy level

v_{1} = 2πe^{2}/1^{2} × h

= 2πe^{2}/h

= 2.188 × 10^{8} cm sec^{-1}

The velocity of the third energy level

v_{3} = v_{1}/3

∴ v_{3} = (2.188 × 10^{8} cm sec^{-1})/3

= 7.30 × 10^{7} cm sec^{-1}

The radius of the third energy level

= 3^{2} × 0.529 × 10^{-8} cm

Circumference of the third energy level

= 2πr = 2 × 3.14 × 0.529 × 10^{-8} cm

Thus the rotation of an electron per second in the third energy level of the hydrogen

= (7.30 × 10^{7})/(2 × 3.14 × 9 × 0.529 × 10^{-8} )

= 2.44 × 10^{14} sec^{-1}

### The energy of an electron in Bohr’s model

The energy of an electron moving in one particular energy level can be calculated by the total energy or the sum of the kinetic energy and the potential energy of an electron.

Thus the total energy for the nth level of the hydrogen atom

where mv^{2} = e^{2}/r

Putting the value of r in the above equation

When n = 1, the energy of the first orbit of the hydrogen

As n increases the energy becomes less negative and hence the system becomes less stable. Because with increasing n, r also increases and orbit makes less stable.

Let the energies associated with 1st, 2nd, 3rd,…., nth orbits are E_{1}, E_{2}, E_{3} … E_{n}.

∴ E_{1}ㄑE_{2}ㄑE_{3}ㄑ……..ㄑE_{n}

#### The energy of an electron in first Bohr orbit

The energy of the moving electron in the first energy level obtained by putting n=1 in the energy expression of the hydrogen.

= – 21.79 × 10^{-12} erg = – 13.6 eV

= – 21.79 × 10^{-19} Joule = – 313.6 Kcal

Question

H, H^{+}, He^{+} and Li^{+2} – for which of the species Bohr’s model of the hydrogen atom is not applicable?

Answer

From the above species H, He^{+}, Li^{+2} contain one electron but H^{+}-ion has no electron. But the **Bohr’s model** of hydrogen atom applicable for one electronic system. Thus for H^{+}-ion, this model is not applicable.