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.

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.

Discovery of Neutron

Up to 1932 it was postulated that an atom is composed of only protons and electrons, and that the mass of an atom is due to the mass of protons present in the nucleus, since the mass of electrons is negligible. Mass of electrons is negligible. Mass of each protons is equal to 1.00376 atomic mass unit (a.m.u).

Rutherford noticed that the atomic masses of different atoms could not be explained if it is supposed that the atom is composed only of protons and electrons and therefore he predicted in 1920 that some kind of neutral particles having mass equal to protons must be present in an atom.

It was in 1932 that Chadwick discovered these neutral particles for which he was awarded the Nobel Prize in Physics in 1936. He obtained these neutral particles by bombarding light nuclei Li and Be with x-particles. He named these particles as neutrons on account of their neutral nature.

Definiton of A Neutron

Having known the mass and charge of a neutron, it can be defined as a sub-atomic particle which bears a mass equal to that of a proton (i.e. equal to 1 amu) and has no charge on it. For this reason a neutron is represented as
 1  n .
 0

Explanation of Production of Productive Rays

When a high voltage is applied across the metallic electrodes of a discharge tube filled with a gas under reduced pressure, the molecules of the gas are dissociated into atoms. Now when high speed electrons (i.e. cathode rays), originated from cathode strikes these atoms, the atoms are converted into cations which pass through the holes of the cathode and constitute positive rays. The electrons obtained in the conversion of the gaseous atoms into cations move towards anode as negative rays.

         M                 +          e-(originated from cathoode) =>                M+                    +               2e- 
Gaseous Atoms                                                                 Pass through the perforated           Move towards anode 
                                                                                                                          cathode as positive rays                  as negative rays

Now it is believed that the protons like electrons are also the common universal constituents of all atoms, excepting hydrogen which has no neutron.

Definition of Proton

Having known the mass and charge on a proton, it can be defined as a sub atomic particle which bears mass equal 1 amu and positive charge equal to +1 on it. In other words, a positive is a  sub atomic particle which bears one unit mass and one unit of positive charge on it.

Anode (or Positive) Rays-Discovery of Proton

Production of positive rays. We have seen that since an atom as a whole, is electricity neutral and it has negatively charged particles known as electrons, it should have some particles which should be positively charged. In order to show the presence of positively charged particles in an atom, attempts were made by many scientists. In 1886, German physicist, E. Goldstrein took a discharge tube provided with a cathode perforated with extremely fine holes and observed that when a large potential difference was applied between the electrodes, not only cathode rays of electrons were originated from the cathode, moving from cathode towards anode, but also new type of colored rays were rays were also simultaneously originated from the anode, moving from anode towards cathode through the fine holes or canals in the cathode and causing aglow on the wall of opposite to anode.

Initially these rays were called canal rays, since they pass through the canals or holes of the cathode. These are also called anode rays as they seem to originate from the anode, J.J Thomson has shown that these rays are positively charged and hence he has called them positive rays.

The origin of these rays can easily be understood. In a discharge tube when gas atom (say H atom) lose negatively charged electrons, they acquire a positive charge. These positively charged Particles then move from anode to cathode.

Electrons Are The Common Universal Constituents of All Atoms

It has been shown by Thomson that the electrons emitted in the cathode rays have the same e/m value (= -1.758*108 coulomb g.), irrespective of the nature of the material of the cathode and that of the gas taken in the discharge tube. Similarly it has been shown by Millikan that the value of e for the electrons is also the same (= -1.602*10-18 coulomb),irrespective of the gas used in the experiment. From these observations it can be concluded that electrons are present in all forms of matter and are common or universal constitutions of all atoms.

Definition of Electrons

Having known the mass and charge on an electron, it can be defined as a subatomic particle which bears mass equal to 0.0005493 amu (which is taken to be equal to zero) and negative charge equal to-1 on it. In other words an electron is a mass less particle which bears one unit of negative charge on it. An electron is indicated as -10e where zero indicates that it has no mass and -1 stands to show how the presence of one unit of negative charge on it.

Gathode (or Negative) Rays - Discovery of Electron

Production of Gathode Rays- The first indication of the complex nature of atom was obtained from a study of conduction of electricity through gases at reduced pressure by Julius Plucker (1850). He took a cylindrical glass tube, called discharge tube which was 30 cm long and 4cm in diameter, closed at both ends, provided with metallic electrodes pump and low pressure gauge. To obtain very low pressures, molecular pump and charcoal cooled by liquid air that has the property of absorbing most of the residual gas were used. The two electrodes were connected to a high voltage battery (voltage was the order of 10,000 volts and more). The electrode connected to the negative terminal of the battery is known as cathode (-) while that connected to the positive terminal is called anode (+) or anti-cathode. Plucker pumped out most of the gas of the tube by means of high vacuum pump and low pressure gauge and thus reduced the pressure of the gas in the tube to about 0.001 mm Hg. Now he observed that when he applied a high voltage of the order of 10,000 volts across the metallic electrodes , the walls of the tube began to glow with a bluish or greenish light depending on the composition of the glass of which the tube is made. It thus appeared that the invisible rays of light were emitted from the cathode and moved towards anode at right angles and with large velocity. These rays were called cathode rays by Goldstein (1876) since these originated from the cathode.