Cathode Rays – Discovery of Electron
· The cathode rays start from cathode and move towards the anode.
· These rays themselves are not visible but their behaviour can be observed with the help of certain kind of materials (fluorescent or phosphorescent) which glow when hit by them. Television picture tubes are cathode ray tubes and television pictures result due to fluorescence on the television screen coated with certain fluorescent or phosphorescent materials.
· In the absence of electrical or magnetic field, these rays travel in straight lines
· In the presence of electrical or magnetic field, the behaviour of cathode rays are similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons.
· The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube.
Important characteristics of cathode rays are as follows:
· They travel at a speed of about to m/s.
· They produce a blackening on a photographic plate when they are incident on it.
· They consist of negatively charged particles.
· They generate heat when they strike a target.
· They are deflected by electric and magnetic fields and the direction of deflection shows that they are negatively charged particles.
· They can ionise the gases through which they pass
· Cathode rays produce fluorescence when they fall on certain substances like. The color of fluorescence varies with the chemical nature of the substance.
Effect of Electric Field on Cathode Rays:
· When an electric field is applied to the path of cathode rays, they are deflected towards the positive plate of the electric field. It shows that cathode rays are made up of negatively charged particles.
Effect of Magnetic Field on Cathode Rays:
· When a magnetic field is applied to the path of cathode rays, they are deflected. It shows that cathode rays are made up of negatively charged particles.
For determination of the ratio of charge/mass of electrons
· In 1897, J.J. Thomson determined the e/m value (charge/mass) of the electron by studying the deflection of cathode rays in electric and magnetic fields.
· The value of e/m has been found to be coulomb.
In 1909, the first precise measurement of the charge on an electron was made by Robert A. Millikan using his oil drop experiment. The charge on the electron was calculated to be -1.6022 X 10-19 coulomb. An electron has the smallest charge known; so it was, designated as a unit negative charge.
Mass of the electron: The mass of the electron is calculated from the values of m.
Discovery of Protons - Positive Rays
· Goldstein (1886) repeated the discharge tube experiment but he used a perforated cathode and noticed the emission of positive rays or canal rays.
Note: These rays do not originate from anode, and so it is wrong to call them anode rays.
· The specific charge (e/m) of canal rays particles varied with nature of gas and was found to be maximum if was used.
· The positive rays particles were thus, called positively charged gaseous atoms left after the removal of electron or ionized gaseous atoms. However, if gas is used in discharges, the positive rays particles are named as protons
· Thus, a subatomic particle, that is a fundamental constituent of all matter, is called a proton; it has a mass 1.673 and charge
Discovery of the Neutron
· The neutron particles were discovered by Chadwick (1932) by bombarding a thin sheet of beryllium by alpha-particles which in turn lead to emission of some kind of radiation.
· This emitted radiation was not deflected by electric field.
· Thus, it was electrically neutral particles having a mass slightly greater than that of the protons. He named these particles as neutrons.
ATOMIC MODELS OF ATOMS
J.J. Thomson's model of an atom-
- J.J. Thomson was the first one to propose a model for the structure of an atom.
- He was awarded the Nobel Prize in Physics in 1906 for his work on the discovery of electrons.
Thomson proposed that:
- An atom possesses a spherical shape in which positively is uniformly distributed and the electrons are embedded in it.
- The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
- An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom.
- Although this model was able to explain the overall neutrality of the atom, but was not consistent with the results of later experiments.
Rutherford’s Nuclear Model of Atom
Rutherford conducted a series of experiments using alpha particles. A beam of alpha-particles was directed against a thin foil of gold, platinum, silver, or copper. The foil was surrounded by a circular fluorescent zinc screen. Whenever an alpha-particle struck the screen, it produced a flash of light.
(a) Most of the -particles went straight without suffering any deflection.
(a) Many of the particles went straight through the metal foil undeflected, indicating
(b) Some of them were deflected through small angles.
(b) Some of the -particles were deflected from their original paths through moderate angles, indicating that whole of the positive charge is concentrated in a space called nucleus. It is proposed to be present at the center of the atom.
(c) A very small number (about 1 in 20,000) did not pass through the foil at all but suffered large deflections or even rebounds.
(c) A very small number of the -particles suffered strong deflections or even rebound on their path indicating that the nucleus is rigid and -particles recoil due to direct collision with the positively charged heavy mass.
According to this model :
· The positive charge and most of the mass of the atom was densely concentrated in extremely small region. This very small portion of the atom was called nucleus by Rutherford.
· The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths called orbits. Thus, Rutherford's model of atom resembles the solar system in which the nucleus plays the role of sun and the electrons that of revolving planets.
· Electrons and the nucleus are held together by electrostatic forces of attraction.
Limitations Of Rutherford's Atomic Model:
· An electron in the nuclear model describing planet like orbits is under acceleration. According to the electromagnetic theory of Maxwell, charged particles when accelerated should emit electromagnetic radiation. Therefore, an electron in an orbit will emit radiation, the energy carried by radiation comes from electronic motion. The orbit will thus continue to shrink. Thus, the Rutherford model cannot explain the stability of an atom.
· This model of an atom fails to explain the distribution of electrons in different orbits around the nucleus.
· According to Rutherford's model of an atom, the atomic spectrum should be continuous. But the atomic spectrum is found to be discontinuous. Rutherford's model fails to explain the discontinuity of the atomic spectrum.
· This model also fails to explain the line spectra of atoms, which show discrete lines, each line corresponds to a fixed frequency.
Electric Charge (C)
+1.6022 x 10-19
1.6726 x 10-24
1.6740 x 10-24
-1.6022 x 10-19
9.1094 x 10-28
· Atomic number of an element Total number of protons present in the nucleus Total number of electrons present in the neutral atom
· Atomic number is represented by Z.
· Mass number of an element No. of protons No. of neutrons
· Mass number is represented by A.
Isotopes, Isobars, and Isotones
· Atoms of the same element having same atomic number but different mass number are called isotopes.
· Isotopes are atoms of same element.
· They have same number of protons but different number of neutrons.
· They have same number of electrons.
· They occupy same position in the periodic table.
· They have similar chemical properties.
Applications of isotopes :
Example : 11H , 12H , 13H
· Atoms of different elements with different atomic number but same mass number are called isobars.
· Isobars are atoms of different elements.
· They have different number of protons and neutron.
· They have different number of electrons.
· They occupy different positions in the modern periodic table.
· They have different chemical properties.
1840Ar , 2040Ca
Atoms of different elements having the same number of neutrons are called isotones.
Example : 614C , 7N15