The word atom is a Greek word meaning indivisible, i.e., an ultimate particle which cannot be further subdivided. The idea that all matter ultimately consists of extremely small particles was conceived by ancient Indian and Greek philosophers. The old concept was put on firm footing by John Dalton in the form of atomic theory which he developed in the years 1803–1808. This theory was a landmark in the history of chemistry. According to this theory, atom is the smallest indivisible part of matter which takes part in chemical reactions. Atom is neither created nor destroyed. Atoms of the same element are similar in size, mass and characteristics; however, atoms of different elements have different size, mass and characteristics.
In 1833, Michael Faraday showed that there is a relationship between matter and electricity. This was the first major break-through to suggest that atom was not a simple indivisible particle of all matter but was made up of small particles. Discovery of electrons, protons and neutrons discarded the indivisible nature of the atom proposed by John Dalton.
The complexity of the atom was further revealed when the following discoveries were made in subsequent years.
|(i) Discovery of cathode rays.||(ii) Discovery of positive rays.|
|(iii) Discovery of X-rays. (iv) Discovery of radio-activity.||(iv) Discovery of radio-activity.|
|(v) Discovery of isotopes and isobars||(vi) Discovery of quarcks and the new atomic model.|
During the past 100 years, scientists have made contributions which helped in the development of modern theory of atomic structure. The works of J.J. Thomson and Ernst Rutherford actually laid the foundation of the modern picture of the atom. It is now believed that the atom consists of several particles called sub-atomic particles like electron, proton, neutron, positron, neutrino, meson, etc. Out of these particles, the electron, the proton and the neutron are called fundamental particles and are the building blocks of the atoms.
The nature and existence of electron was established by experiments on conduction of electricity through gases. In 1859, Julius Plucker started the study of conduction of electricity through gases at low pressure in a discharge tube. [A common discharge tube consists of a hard glass cylindrical tube (about 50 cm long) with two metal electrodes sealed on both the ends. It is connected to a side tube through which it can be evacuated to any desired pressure with the help of a vacuum pump.] Air was almost completely removed from the discharge tube (pressure about 10-4 atmosphere). When a high voltage of the order of 10,000 volts or more was impressed across the electrodes, some sort of invisible rays moved from the negative electrode to the positive electrode (Fig. 1). Since the negative electrode is referred to as cathode, these rays were called cathode rays. Further inverstigations were made by W. Crookes, J. Perrin, J.J. Thomson an others. Cathode rays possess the following properties:
- They travel in straight lines away from the cathode with very high velocities ranging from 109 _ 1011 cm per second. A shadow of metallic object placed in the path is cast on the wall opposite to the cathode.
- They produce a green glow when strike the glass wall beyond the anode. Light is emitted when they strike the zinc sulphide screen.
- They produce heat energy when they collide with the matter. It shows that cathode rays possess kinetic energy which is converted into heat energy when stopped by matter.
- They are deflected by the electric and magnetic fields. When the rays are passed between two electrically charged plates, these are deflected towards the positively charged plate. They discharge a positively charged gold leaf electroscope. It shows that cathode rays carry negative charge.
- They possess kinetic energy. It is shown by the experiment that when a small pin wheel is placed in their path, the blades of the wheel are set in motion. Thus, the cathode rays consist of material particles which have mass and velocity. These particles carrying negative charge were called negatrons by Thomson.
negatron' was changed toelectron’ by Stoney.
- Cathode rays produce X-rays. When these rays fall on material having high atomic mass, new type of penetrating rays of very small wavelength are emitted which are called X-rays.
- These rays affect the photographic plate.
- These rays can penetrate through thin foils of solid materials and cause ionisation in gases through which they pass.
- The nature of the cathode rays is independent of:
- the nature of the cathode and
- the gas in the discharge tube.
In 1897, J. J. Thomson determined the e/m value (charge/mass) of the electron by studying the deflections of cathode rays in electric and magnetic fields. The value of e/m has been found to be _1.7588 × 108 coulomb/g.
[The path of an electron in an electric field is parabolic, given as:
y = deflection in the path of electron in y-direction
e = charge on electron
E = intensity of applied electric field
m = mass of electron
u = velocity of electron
x = distance between two parallel electric plates within which electron is moving.
The path of an electron in a magnetic field is circular with radius `r’ given as:
m = mass of electron
= velocity of electron
e = charge on electron
B = intensity of applied magnetic field
The radius of the path is proportional to momentum.
By performing a series of experiments, Thomson proved that whatever gas be taken in the discharge tube and whatever be the material of the elecrodes, the value of e/m is always the same. Electrons are thus common universal constituents of all atoms.
J.J. Thomson gave following relation to calculate charge/mass ratio.
where the terms have usual significance given before = -1.7588 × 1011 C kg-1
Electrons are also produced by the action of ultraviolet light or X-rays on metal and from heated filaments. b-particles emitted by radioactive materials are also electrons.
The first precise measurement of the charge on the electron was made by Robert A. Millikan in 1909 by oil drop experiment. The charge on the electron was found to be -1.6022 × 10-19 coulomb. Since an electron has the smallest charge known, it was, thus, designated as unit negative charge.
Mass of the electron: The mass of the electron can be calculated from the value of e/m and the value of e.
= 9.1096 × 10-28 g or 9.1096 × 10-31 kg
This is termed as the rest mass of the electron, i.e., mass of the electron when moving with low speed. The mass of a moving electron may be calculated by applying the following formula:
Mass of moving electron = IMG
where is the velocity of the electron and c is the velocity of light. When becomes equal to c, mass of the moving electron becomes infinity and when the velocity of the electron becomes greater than c mass of the electron becomes imaginary.
Mass of the electron relative to that of hydrogen atom:
Mass of hydrogen atom = 1.008 amu
= 1.008 × 1.66 × 10-24 g (since 1 amu = 1.66 × 10-24 g)
= 1.673 × 10-24 g
Thus, Mass of an electron = × mass of hydrogen atom
= = 0.000549 amu
An electron can, thus, be defined as a sub-atomic particle which carries charge -1.60 × 10-19 coulomb, i.e., one unit negative charge and has mass 9.1 × 10-28 g, i.e., mass of the hydrogen atom (0.000549 amu).
[Millikan’s oil drop method is used to determine the charge on an electron by measuring the terminal velocity of a charged spherical oil-drop which is made stationary between two electrods on which a very high potential is applied.
where = coefficient of viscosity of the gas medium
E = field strength
(f = density of oil;= density of gas; g = gravitational force)]
With the discovery of electrons, scientists started looking for positively charged particles which were naturally expected because matter is electrically neutral under ordinary conditions. The first experiment that led to the discovery of the positive particle was conducted by Goldstein in 1886. He used a perforated cathode in the modified cathode ray tube (Fig. 2). It was observed that when a high potential difference was applied between the electrodes, not only cathode rays were produced but also a new type of rays were produced simultaneously from anode moving towards cathode and passed through the holes or canals of the cathode. These rays were termed canal rays since these passed through the canals of the cathode. These were also named anode rays as these originated from anode. When the properties of these rays were studied by Thomson, he observed that these rays consisted of positively charged particles and named them as positive rays.