October 2, 2020

# Bohr Theory of Atomic Structure

#### Bohr Theory of Atomic Structure:

- Bohr’s model is valid only for
**mono-electronic systems**like**H**,**He**,^{+}**Li**.^{2+}

- It is basically an
**intuitive model**.

- It is given by
**Neil Bohr**,**1913**.

- This model explains the line spectrum of hydrogen atom.

#### Postulates of Bohr Theory of Atomic Structure:

- Atom consist of two parts-

- Very small, centralized and highly denser position called as
**Nucleus**. - Extra-nuclear part containing electrons, moving around the nucleus in well-defined circular path called as
**orbits**.

- Electrons revolve around the nucleus with high velocity, in those orbits for which angular momentum is quantized.

i.e **mvr** = **nh/2π**

where,** ‘n’** = 1, 2, 3,…………. but in friction; ** ‘v’** is the velocity; **‘m’ **is mass of the electron; **‘r’ **is the radius of the nth orbit

- Different energy levels are numbers as 1, 2, 3, 4, 5 or K, L ,M, N, O.

- The energy of the electron is constant in fixed orbit known as
**Stationary States**or**Energy Levels**.

- As the size of shell increases, energy increases but energy gap decreases (away from the nucleus energy increase from more negative to less negative and finally become zero).

- An electron can jump from any lower to any higher orbit, by absorbing energy is equal to the energy difference of two orbits.

- An electron can jump from any higher to any lower orbit, by emitting or releasing energy is equal to energy difference of two orbits.

- During absorption or emission of energy,
**Planck’s quantum equation**is obeyed i.e.**E = hν**, where**‘**‘ is the frequency of radiation absorbed or emitted and**ν****‘h’**is**Planck’s constant**.

**Ionization energy**is equal and opposite to the energy of electron in that orbit i.e.**I.E. = -E**._{n}

#### Limitations of Bohr’s Model of Atom:

- Inability to explain the
**line spectra**of multi-electron atoms.

- It fails to explain the multiple or fine structure of spectral lines.

- Splitting of lines under the influence of a strong magnetic field or electric field (
**Zeeman Effect**and**Stark Effect**).

- It violates
**de Broglie**concept of dual character of matter.

- It violates
**Heisenberg’s uncertainty principle**.

- Bohr theory never explains the union of atom to form molecules.

- Inability to explain the shape of molecule.

- It cannot explain the brightness of spectral lines.

- Bohr gives a flat model of the atom and the concept of orbit considering the electron as a particle but nowadays it is believed that atom possesses
**3D model**and concept of orbitals.

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