Quantum chemistry
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Introduction: The classical mechanics is Formulated by Sir Isaac Newton which is obeyed by macroscopic particles such as planets or rigid bodies however in quantum mechanics which deals with the study of microscopic particles such as electrons, protons, atoms and molecules which shows wave particle nature of light.
Black body radiations :
An ideal black body which is perfect absorber and perfect emitter of radiations.When radial energy is allowed to form carbon black or a blackened metallic surface the energy is almost completely absorb. When such body is heated it becomes first of all red then orange then yellow and last of all white every high temperature such body is called black body and The Radiations that absorb and emitted by black body is called black body Radiations.
A black body is one which is perfect absorber and perfect emitter.
The co-ordinations between energy density and wavelength at different temperatures is as in figure these curves have been following characteristics.For each temperature there is a particular wavelength at which the energy radiated is maximum the position of the maximum shift towards the lower wavelength with increasing temperature. At the higher temperature energy density is maximum These curves are usually referred to as the black body radiation curves
Planck’s quantum theory of radiation:
The main postulates of these theory
1. The energy is absorbed or emitted by black body non continuously but
discontinuously in the form of tiny bundles or packets of energy called quantum. in case of light quantum is called photon
2. Each quantum is associated with a certain definite amount of energy E which is proportional to frequency of radiation
i.e 𝐸 ∝ 𝛾
𝐸 = ℎ𝛾
Where h is planks constant = 6.625×10-34 Js
3. A body can to emit or absorb energy only one photon or some whole number multiple of photon or quantum
𝐸 = 𝑛ℎ𝛾
Where n = 1, 2, 3, 4 ……….
n cannot be faction
This known as quantization of energy based on the amount of loss planks obtain and expression for energy density of black body radiation at wavelength λ
photoelectric effect:
the phenomenon of emission of electrons from the surface of metal when irradiated with light (electromagnetic radiation) of suitable frequency is known as photoelectric effect. the electrons liberated in the photoelectric effect are called photoelectrons.
The current due to photoelectrons is called photocurrent or photoelectric current.
In photoelectric effect light energy is converted into electrical energy. Experimental study of photoelectric effect To study photoelectric effect, an experimental setup is made which is as shown in figure.
❖ The apparatus consist of highly evacuated glass tube (G)
❖ A photo sensitive metal C and another plate A are enclosed in the glass tube
❖ The glass tube has a parts window, through which ultra violet light from source S is incident on the plate C
❖ The plate C is connected to the negative terminal of HT battery
❖ The positive terminal of the battery is connected to the plate A that is anode
❖ The micro ammeter is included in the circuit to measure photo electric current
❖ When the light of suitable frequency (threshold frequency) falls on metal plate C electrons are liberated. These electrons are moves towards the anode and current is recorded by micro ammeter
Laws of photo electric emission:
1. The kinetic energy of the ejected electrons is proportional to the frequency of incident light.
2. The kinetic energy of ejected electrons is independent of intensity of incident light.
3. Photo electric current and number of photo electrons are directly proportional to the intensity of incident light. Increasing intensity increases the number of ejected electrons.
4. Number of electrons are ejected from the metal unless the frequency of light use is greater than a certain minimum value known as threshold frequency (𝛾0).
5. The minimum negative potential of the anode for which all the photo electrons are prevented from the anode is called stopping potential (Vs).
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