Photoelectric Effect

What is Photoelectric Effect?
        
        In some of their experiments Sir J.J Thomson and P.Lenard showed that when a beam of light of suitable wavelength or frequency is allowed to fall on the surface of a metal, the electrons are emitted (or ejected) from the surface of metal. This phenomenon of emission of electrons is known as photoelectric effect or photoelectric emission (photo means light). Although the electrons emitted from the surface are no different from all other electrons, it is customary to call these electrons as photoelectrons.

        It may be noted that only a few metal can eject electrons when radiations from visible (which has less energy and hence is less energetic) fall on them, but many more metals are very much capable of ejecting electrons when a radiations from ultraviolet light (which has more energy and hence is more energetic) fall on them.

Spectrum

What Is Spectrum?


       A spectrum is an array of waves or particles which is spread out according to the increasing or decreasing of some property such as wavelength or frequency.

Types of Spectrum


      Depending on the nature of the source emitting the radiation, there are two types of spectra.

1. Emission Spectra
2. Absorption Spectra

Emission spectra are further two types:-

• Continuous spectrum
• Discontinuous spectrum which may be a band spectrum or line spectrum. Line spectrum also called atomic spectrum.

Electromagnatic Radiations or Eectromegnetic Waves

Nature of Radiations or Emitted by Electron

When we object moves up and down or vibrates continuously, energy, in the form of a wave, is sent or transmitted (or propagated) by the vibrating object to a distant place. The wave travels at right angle to the vibrating vibratory motion of the vibrating object and away from it. Wave produced by a vibrating object can be represented by a wavy curve. The tops (i.e. maximum) of the wavy curve are called crests and the bottoms (i.e. minima) are called troughs. When a stone is thrown in a pond of water, water molecules start to vibrate up and down and transmit their energy as waves on water surface which are seen traveling towards the bank of pond.

In 1864 James Clark Maxwell showed by that when an electrics current is passed through a circuit (i.e. through a tungsten filament), it radiates energy in the form of waves. The energy radiated is called radiate energy and the waves produced are called electromagnetic waves or electromagnetic radiations. These waves are so called because similar sort of  waves can also be obtained by moving a charged body as well as a magnet to and fro in am genetic field. These waves do not require any medium for propagation or transmission; e.g. light and radio waves (which are electromagnetic waves) from the sun and stars reach the earth by traveling through non-material medium i.e. through empty space. All these waves travel with the speed of light.

The radiations (or wave) emitted by the vibratory motion of an electron are also electromagnetic radiations. These radiations also travel with the speed of light.

Bohr's Atomic Model

According To Rutherford's nuclear atomic model, an atom has a nucleus and negative electrons which are revolving round the nucleus  in the same way as the planets revolve round the sun in the solar system.This model could not say anything as to how and where these electrons were arranged.

This model could also not explain how the spectral lines are produced by the simplest atoms like hydrogen atom when an electron jumps from one orbit to another.

In order to explain why an electron revolving round the nucleus does not lose energy and consequently does not fall into the nucleus and how the spectral lines of the emission spectrum of H-atom are produced when aan electron jumps from one energy level to other, Niels Bohr, a Danish Physicist, in 1913, put forward a new atomic model which is based on Planck's quantum theory of radiation.

Before, we could deal with Bohr's atomic model we should have a clear understanding of the following topics:

• Electromagnetic radiations (or waves) and nature of  radiations emitted by electron.
• Emission spectrum, continuous spectrum and line spectrum of hydrogen atom.
• Plank's quantum theory of radiation, and its application in explaining photoelectric effect and Compton effect.

What Is Mass Number (A)?

We know that the nucleus of an atom contains neutrons and protons. The sum of the number of neutrons (n) and protons (p) present in the nucleus of an atom is called the mass number (A) of that atom, i.e.

Mass Number (A) = No. of protons (p) + No. of neutrons (n)

or, A = p+n

In cases where it is not necessary to differentiate between protons and neutrons, these particles are collectively called nucleons. Thus the mass number (A) of an atom is equal to the total number of nucleons in the nucleus of an atom. Obviously the mass number is a whole number.

Each different variety of atom as determined by the composition of its nucleus is called a nucleotide.

Importance Of Mosley's Law

Mosley's Law has been found to be very useful. The importance of this law is evident from the following points:

1. We have seen that this law has established that the atomic number is always numerically equal to the number of electrons revolving round the nucleus. According to this law it is the atomic number and not atomic weight on which the properties of the elements (both physical and chemical) depend. It thus says that the elements must be arranged in the periodic table in the increasing order of their atomic number and not atomic weight as originally enunciated by Mendeleef. Accordingly, Mosley's law has been used to place elements in their proper order in the periodic table in certain questionable cases, e.g. if the elements are placed according to their increasing atomic weight, K(Z=19) should come before Ar (Z=10) ; Ni (Z=28) should precede Co (Z=27) etc. But Mosley's law says that as per their atomic numbers their order should be reversed; i.e. K should be placed after Ar, Co should come before Ni etc. This fact is further supported by chemical properties.
2. Mosley's law has led to the discovery of many new elements. The elements like technetium (Z=43) have been discovered, promethium (Z=61), hafnium (Z=72), rhenium (Z=75) have been discovered on the basis of the gaps left by Mosley in Mosley's X-ray diagrams. These elements have been subsequently discovered.
3. This law has been helpful in determining the atomic number of rare-earths and has thus helped in fixing their position in the periodic table.