Application Of The Wave Nature Of Electron

• The wave nature of electrons has been utilized in the construction of electron microscope of high resolving power. This microscope is an important tool in research.
• Wave nature of electrons has been used in the study of thin surface films.

Thomson And Reid's Experiment To Verify The Wave-Nature Of Electrons

         In 1928, English physicists G.P. Thompson and A. Reid, working with high speed electrons, showed that when a narrow beam of high-speed electrons produced from heated tungsten filament was allowed to fall on the surface on of a thin metallic foil like Al Au, Pt etc. and the diffracted electrons are received on photographic plate kept in a short distance away, these electrons produce a different luminous spot composed of electronic bright and dark rings which are called electron beam diffraction rings or patterns.

        These diffraction patterns are obtained due to the fact that metal consists of many microscopic crystals arranged in random fashion so that some are always at the proper angles to give reflection rings given by electron beam are found to be similar to those given by X-rays (which are waves) when they are diffracted by the same meta foil. This similarity between the electron beam diffraction rings and those of X-rays rings confirms that electrons are also waves.

                         (a) X-rays beam diffraction rings                                   (b) Electron beam diffraction rings

Difference Between de Broglie's Matter Waves and Electromagnetic Wave

Main difference are given below:

• de Broglie's matter waves are not radiated into empty space or emitted by oscillating particles i.e. they are never dissociated from the particle but remain attached to the particle. On the other hand the electromagnetic waves (i.e. radiations) can be absorbed or emitted.
• de Broglie's matter waves travel with speeds which depend on the nature of the matter particles while electromagnetic waves proceed with the speed of light.
• de Broglie's matter waves are not electromagnetic in nature.

Dual Nature of Matter or Electron: Wave-Particle Nature of Matter-de Broglie's Concept of Matter Wave

         According to  Max Planck s quantum theory of radiations some phenomenons like photoelectric effect and black body radiations can be explained, if light radiation is supposed to behave as a material particle which is localized at a particular point in space, while according to Maxwell's wave theory of radiation certain other phenomenons like diffraction, interference and polarization can be explained, if light radiation is supposed to behave as a wave which is spread out over space. Thus we find that light shows dual character, i.e. it behaves both as a material particle and as wave.
        In 1924 a French physicist, Lonius de Brogile extended the idea of dual character of light to matter particles and suggested that all mater particles in motion (e.g. electrons, protons, neutrons, atoms or molecules etc) have a dual character, i.e. all matter particles posses characteristics of both a material particles and a wave. The wave associated with matter particles is called de Broglie's matter wave. The revulationary idea proposed by de Broglie that a material object posses both the particle and the wave properties known as the wave-particle duality in matter. The idea of wave-particle duality in matter led de Broglieto a very important conclusion that since an electron is also a kind of matter particle, it also, like matter particle, has a dual character i.e. an electron, like all other matter particles, bahave both as a material particle and as awave.

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.

Mosley's Law

Rogenton while making some experiments with a discharge tube in 1895, found that when cathode rays are allowed to fall on a metal target called anti-cathode or target material placed in their path, new type of radiation s are produced. These radiations are called X-rays. These rays are also electromagnetic in character and are comparable to light waves. The stream of cathode rays (i.e. electrons) produced from the cathode converges to a point through a concave surface on the anti-cathode and give rise to X-rays.

What Is Atomic Number?

Atom has a nucleus which can carries positive charge on it. The positive charge carries by the nucleus is called nuclear charge which is equal to the total number of positive charges on the nucleus of the atom of that element. This total number of unit positive charges on the nucleus is called the atomic number of the element. Now since each proton has +1 charge and neutron has no charge, the total number unit positive charge carried by the nucleus is equal to the number of protons present in the nucleus of the atom. Thus atomic number of element can also be defined as the number of protons present in the nucleus. Thus:

'Atomic number of an element is defined as the number of  unit positive charges on the nucleus (nucleus charge) of the atom of that element or as the number of protons present in the nucleus.'

Now since the atom as a whole is electricity neutral, the number of protons (positively charged particles) in the nucleus is equal to the number of electrons (negatively charged particles) revolving in orbits around the nucleus. Consequently atomic number of an element is also equal to the number of electrons revolving in orbits round the nucleus. Thus:

Atomic Number (Z) = No. of unit positive charges on the nucleus (Nuclear charge).
                               = Total number of unit positive charges carried by all protonsnt in the nucleus.
                               = No. of protons in the nuclens (p)
                               = No. of electrons revolving in orbits (e)
or,

Z = p = e

Atomic  number of an element is also defined as the serial number of its position in the periodic table, starting from hydrogen as the first element.