Mendeleef's Periodic Law and Mendeleef's Periodic Table

Attempts were made to classify the elements in a number of ways. Most noteworthy of these is that made by Russian chemist, Mendeleef' (men-de-lay-eff) who gave a law known as Mendeleef's Periodic Law which state as:

      The properties of elements are a periodic function of their atomic weight, i.e. if the elements are arranged in the increasing order of their atomic weights; the properties of the elements (i.e. similar elements) are repeated after definite regular intervals of periods.

Working on this law, Mendeleef' arranged the elements in the increasing order of their atomic weights in the form of a table which is known as  Mendeleef' Periodic Table after his name.

In this table the elements are arranged in groups (or columns) and periods (or rows).

Quantum Numbers

The term quantum number is used to identify the various energy levels available with the atom in which the electron of the atom can reside. The word quantum is used to signify that all the energy levels which are avail ables to an electron are governed by the laws of quantum mechanics.

Just as college student is identified by (1) his name (2) the name of his college (3) class in he is studying and (4) the locality in which his college is situated, an electron in an atom is completely described by its four quantum numbers. Thus the quantum numbers serve as identification numbers or labels which completely define the electron in an atom i.e. these numbers-

1. Specify the position (or)location of an electron in an atom.
2. Predict the direction of spin or self rotation of the electron.
3. Determine the energy and angular momentum of the electron.

Four quantum numbers which are required to completely specify the character of an electron in an atom.

1. Principle quantum number (n)
2. Azimuthal quantum number (l)
3. Magnetic quantum number (m)
4. Spin quantum number (s)

Each quantum number refers to a particular character of an electron in an atom.

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.