DALTON's ATOMIC THEORY & SUB ATOMIC PARTICLES - Kailasha Online Learning LLP

Any object around you, your pen or pencil, your books, electrical equipment and things of nature such as rocks, water and plant and the list is so long, constitute the matter of the universe. Any substance which occupies space and has mass is Matter.

Hence, for anything to be matter, two things are mandatory viz.

Space occupied or we can say Volume
Mass

Dalton, in 1808, proposed a theory, which says that matter is made up of extremely small, indivisible particles called atoms. (In Greek atom means which cannot be cut).

The main postulates of Dalton’s atomic theory are

The matter is made up of small indivisible particles, called atoms.
Atoms can neither be created nor destroyed. This implies that during a chemical reaction, atoms simply change their arrangement. No, any new atom is created or atoms taking part in the reaction is destroyed during the chemical reaction. Hence, the same number of atoms must be present before and after the reaction.
Atom is the smallest particle of an element which takes part in a chemical reaction.
Atoms of the same element are identical in all respects especially, size, shape and mass.
Atoms of different elements have different mass, shape and size.
Atoms of different elements combine in a fixed ratio of small whole numbers to form compound atoms, called molecules.

However, the discovery of radioactivity have established, that atom is not the smallest indivisible particle but had a complex structure of its own and was made up of still smaller particles like electrons, protons, neutrons etc. At present, about 35 different subatomic particles are known but the three particles namely electron, proton and neutron are regarded as the fundamental particles.

Let us take a look at these three fundamental particles.

The existence of electrons in atoms was first suggested, by J.J. Thomson, as a result of experimental work on the conduction of electricity through gases at low pressures and high voltage, which produces cathode rays consisting of negatively charged particles, named as electrons.

Discovery of Electron

A cathode ray tube is made of glass containing two thin pieces of metal, called electrodes, sealed in it. The electrical discharge through the gases could be observed only at very low pressures and at very high voltages. The pressure of different gases could be adjusted by evacuation. When a sufficiently high voltage is applied across the electrodes, current starts flowing through a stream of particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). These were called cathode rays or cathode ray particles. The flow of current from cathode to anode was further checked by making a hole in the anode and coating the tube behind anode with phosphorescent material zinc sulphide. When these rays, after passing through anode, strike the zinc sulphide coating, a bright spot on the coating is developed (the same thing happens in a television set)

cathode ray discharge tube — Pic Credit – NCERT Textbook for Chemistry

The results of CRT experiments are as follows

The cathode rays start from the 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 a certain fluorescent or phosphorescent materials.
In the absence of the electrical or magnetic field, these rays travel in straight lines
In the presence of an electrical or magnetic field, the behaviour of cathode rays is 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. Thus, we can conclude that electrons are the basic constituent of all the atoms.

Charge to Mass Ratio of Electron

British physicist J. J. Thomson measured the ratio of electrical charge (e) to the mass of the electron (m_e ) by using cathode ray tube and applying electrical and magnetic field perpendicular to each other as well as to the path of electrons.

Thomson argued that the amount of deviation of the particles from their path in the presence of electrical or magnetic field depends upon:

the magnitude of the negative charge on the particle, greater the magnitude of the charge on the particle, greater is the interaction with the electric or magnetic field and thus greater is the deflection.
the mass of the particle – lighter the particle, greater the deflection.
the strength of the electrical or magnetic field – the deflection of electrons from its original path increases with the increase in the voltage across the electrodes, or the strength of the magnetic field.

When only the electric field is applied, the electrons deviate from their path and hit the cathode ray tube at point A. Similarly, when only the magnetic field is applied, electron strikes the cathode ray tube at point C. By carefully balancing the electrical and magnetic field strength, it is possible to bring back the electron to the path followed as in the absence of the electric or magnetic field and they hit the screen at point B. By carrying out accurate measurements on a number of deflections observed by the electrons on the electric field strength or magnetic field strength, Thomson was able to determine the value of e/m_e as:

e/m_e = 1.758820 C/Kg

Where m_eis the mass of the electron in kg and e is the magnitude of the charge on the electron in coulomb (C). Since electrons are negatively charged, the charge on an electron is -e.

The charge to mass (e/m) ratio for cathode rays is fixed whose values is 1.76 x 10⁸C/g.

Charge on electron

R.A. Millikan (1868-1953) devised a method known as oil drop experiment (1906-14), to determine the charge on the electrons. He found that the charge on the electron to be – 1.6 Χ 10-19 C. The presently accepted value of electrical charge is – 1.6022 Χ 10-19 C. The mass of the electron (me) was determined by combining these results with Thomson’s value of ratio.

= e/(e/m_e) = 1.6022 × 10^-19 C/1.758820 × 10^-11 kg-1

= 9.1094×10^-31 Kg

Since one is aware of the face that atom is electrically neutral, so if it contains negatively charged electrons it must also contain some positively charged particles. This was confirmed by Goldstein in his discharge tube experiment with perforated cathode.

Discovery of Proton and Neutron

Electrical discharge carried out in the modified cathode ray tube led to the discovery of particles carrying positive charge, also known as canal rays. The characteristics of these positively charged particles are listed below. On passing high voltage between the electrodes of a discharge tube it was found that some rays were coming from the side of the anode which passed through the holes in the cathode. These anode rays (canal rays) consisted of positively charged particles formed by ionization of gas molecules by the cathode rays.

unlike cathode rays, the positively charged particles depend upon the nature of gas present in the cathode ray tube. These are simply the positively charged gaseous ions.
The charge to mass ratio of the particles is found to depend on the gas from which these originate.
Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge.
The behaviour of these particles in the magnetic or electrical field is opposite to that observed for electron or cathode rays.

The charge to mass ratio ( e/m value) of positively charge particles was found to be maximum when the discharge tube was filled with hydrogen gas as hydrogen is the lightest element. These positively charged particles are called protons.

Later, a need was felt for the presence of electrically neutral particle as one of the constituent of atom. The neutral charge particle, neutron was discovered by James Chadwick by bombarding boron or beryllium with α–particles.

Properties of fundamental particles:

Name	Symbol	Absolute charge	Relative Charge	Mass/kg	Mass/u
Electron	e	-1.6022 × 10^-19	-1	9.1039 × 10^-31	0.00054
Proton	p	1.6022 × 10^-19	+1	1.67262 × 10^-27	1.00727
Neutron	n	0	0	1.67493 × 10^-27	1.00867

Mass of subatomic particles is usually represented in the atomic mass unit (amu). The atomic mass unit (amu) is 1/12 of the mass of an individual atom of ₆C¹², i.e.,
1.660 x 10^-27 kg.

i.e. 1 amu = 1/12 x mass of C¹²atom

The neutron and proton have approximately equal masses of 1 amu and the electron is about 1836 times lighter, its mass can sometimes be neglected as an approximation.

The electron and proton have equal, but opposite, electric charges and value of charge on the electron is 1.6×10^-19 C while the neutron is electrically neutral.