Class 11 Chemistry Structure of Atom
ATOMIC STRUCTURE :
The existence of atoms has been proposed since the time of early Indian and Greek philosophers (400 B.C.) who were of the view that atoms are the fundamental building blocks of matter. According to them, the continued subdivisions of matter would ultimately yield atoms which would not be further divisible. The word ‘atom’ has been derived from the Greek word ‘a-Tomio’ which means ‘uncutable’ or ‘non-divisible’. These earlier ideas were mere speculations and there was no way to test them experimentally. These ideas remained dormant for a very long time and were revived again by scientists in the nineteenth century.
In this unit, we start with the experimental observations made by scientists towards the end of nineteenth and beginning of the twentieth century.
Dalton’s Atomic Theory :
All the objects around you, this book, your pen or pencil and things of nature such as rocks, water and plant constitute the matter of the universe. The matter is any substance which occupies space and has mass.
Dalton, in 1808, proposed that matter was made up of extremely small, indivisible particles called atoms. (In Greek atom means which cannot be cut). This concept was accepted for a number of years.
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 means that a chemical reaction is just a simple rearrangement of atoms and 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.
Drawbacks :
1. The discovery of isotopes and isobars showed that atom of the same element may have different atomic mass (isotopes) and atom of different kinds may have same atomic masses (isobars).
2. Atoms can be split into more fundamental particles: electrons, protons and neutrons.
Discovery of Subatomic Particles :
Dalton’s atomic theory was able to explain the law of conservation of mass, law of constant composition and law of multiple proportions very successfully. However, it failed to explain the results of many experiments; for example, it was known that substances like glass or ebonite, when rubbed with silk or fur, generate electricity. Many different kinds of sub-atomic particles were discovered in the twentieth century. However, in this section, we will talk about only two particles, namely electron and proton.
Cathode rays and Discovery of Electron :
Michael Faraday showed that chemical changes occur when electricity is passed through an electrolyte. He stated that electricity is made up of particles called atoms of electricity. G.J Stoney suggested the name of the electron for the atoms of electricity. But the real credit for the discovery of electrons goes to J.J. Thomson. In mid-1850s many scientists mainly Faraday began to study electrical discharge in partially evacuated tubes, known as cathode ray discharge tubes. 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 fluorescent 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)
The results of these experiments are summarised below.
(i) The cathode rays start from the cathode and move towards the anode.
(ii) 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 material like zinc sulphide (ZnS).
(iii) In the absence of an electrical or magnetic field, these rays travel in straight lines
(iv) In the presence of the 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.
(v) 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 a basic or fundamental constituent of all the atoms.
(vi) Cathode rays produce a heating effect. When these rays are made to strike on a metal foil, the latter gets heated.
(vii) Cathode rays produce X-rays when they strike on the surface of hard metals such as tungsten, copper molybdenum etc.
(viii) Cathode rays can pass through thin foils of metals like aluminium. However, these are stopped if the foil is quite thick.
(ix) Cathode rays ionize the gas through which they pass.
(x) Cathode rays affect the photographic plate. This is called fogging.