UNIT 1- SOME BASIC CONCEPTS OF CHEMISTRY

DAY 3
30.4.20
GOOD MORNING BOYS,
Today you will attend the MCQ FIRST then you will  learn about Hydrogen spectrum and Bohrs theory.

LEARNING OUTCOMES:
Students will be able to

• know different series in hydrogen spectrum
• learn Rydbergs formula
• understand Bohrs model for hydrogen atom.

Emission spectrum of Hydrogen atom

Spectral Lines for atomic hydrogen is given by

Rydberg equation

R = Rydberg’s constant = 109677 cm^-1

Bohr’s model for hydrogen atom:

Postulates 

1. Electron in hydrogen atom moves around the nucleus in circular path of fixed radius and energy. These paths are called orbits or energy levels.

2. As long as an electron remains in a particular orbit, it does not lose or gain energy and its energy remains constant.

3. However, when electron will move from a lower stationary state to a higher stationary state a certain amount of energy is absorbed by the electron or some energy is emitted when electron moves from higher stationary state to lower stationary state

4. Frequency of radiations emitted or absorbed when transition of an electron occurs, is given by

                                           Where, E₁ & E₂ represent the lower & higher energy states respectively.

5. An electron can move only in those orbits for which its angular momentum is an integral multiple of h/2π, i.e.,

[Where n =1,2,3.....]

 Assumptions of Bohar’s model are:

1. The radii of the stationary states are expressed as:

2.  Energy of an electron in nth orbit is given as:

Limitations of Bohr’s model of atom: 1. It failed to account for the finer details of the hydrogen spectrum.

2. It was unable to explain spectrum of atoms containing more than one electron.

3. It failed to explain splitting of the spectral lines in presence of electric (Stark effect) or magnetic field ( Zeeman effect).

4. It failed to explain formation of molecules from atoms by chemical bonding.

Dual behaviour of matter:

de Broglie proposed that matter exhibits dual behavior, i.e., matter shows both particle as well as wave nature and gave the following relation between wavelength (λ) and momentum (p) of a material particle.

 

[Where, m is the mass of the particle, v its velocity and p its momentum]

The above equation is named as de Broglie’s relation.
Heisenberg’s Uncertainty Principle:

It states that it is impossible to determine simultaneously, the exact position and exact momentum (or velocity) of an electron. The product of their uncertainties is always equal to or greater than h/4π. I.e.,

 

Where, Δx = uncertainty in position

            Δp = uncertainty in momentum
  Significance of Uncertainty Principle

• Heisenberg’s uncertainty principle rules out the existence of definite paths or trajectories of electrons and other similar particles
• The effect of Heisenberg Uncertainty Principle is significant only for motion of microscopic objects and is negligible for that of macroscopic objects.

Reasons for the Failure of the Bohr Model:

1. It ignores the dual behavior of matter.

2. It contradicts Heisenberg’s uncertainty principle.

Thats all for the day

Take care

stay safe

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