Summery of Anatomy of atom and subatomic particles for Neet.

 

ATOM'S

BY ASHU

"The rich diversity of chemical behavior of different elements can be traced to the differences in the internal structure of atoms of these elements."

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 that would not be further divisible. The word ‘atom’ has been derived from the Greek word ‘a-tomio’ which means ‘uncut-able’ 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. The atomic theory of matter was first proposed on a firm scientific basis by John Dalton, a British school teacher in 1808. His theory called Dalton’s atomic theory, regarded the atom as the ultimate particle of matter (Unit 1). 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 get electrically charged. In this unit we start with the experimental observations made by scientists towards the end of the nineteenth and beginning of the twentieth century. These established that atoms are made of sub-atomic particles, i.e., electrons, protons, and neutrons — a concept very different from that of Dalton.

2.1 DISCOVERY OF SUB-ATOMIC PARTICLES

2.1.1 Discovery of Electron.

In the mid-1850s many scientists mainly Faraday began to study electrical discharge in partially evacuated tubes, known as cathode ray discharge tubes. 

It is depicted in Fig. 2.1. 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 of the glass tubes. When 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 the anode with phosphorescent material zinc sulfide. When these rays, after passing through the anode, strike the zinc sulfide coating, a bright spot is developed on the coating [Fig. 2.1(b)]. Fig. 2.1(a) A cathode ray discharge tube Fig. 2.1(b) A cathode ray discharge tube with perforated anode 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 behavior can be observed with the help of certain kinds 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.

 (iii) In the absence of an electrical or magnetic field, these rays travel in straight lines (Fig. 2.2).

 (iv) In the presence of an electrical or magnetic field, the behavior of cathode rays is 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 the basic constituents of all atoms.

2.1.2 Charge to Mass Ratio of Electron

 In 1897, British physicist j.J. Thomson measured the ratio of electrical charge (e) to the mass of the electron (me ) by using cathode ray tube and applying electrical and magnetic fields perpendicular to each other as well as to the path of electrons (Fig. 2.2). When an only an electric field is applied, the electrons deviate from their path and hit the cathode ray tube at point A (Fig. 2.2). Similarly when only 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 which is followed in the absence of electric or magnetic field and they hit the screen at point B. Thomson argued that the amount of deviation of the particles from their path in the presence of electrical or magnetic field depends upon: (i) 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. (ii) the mass of the particle — lighter the particle, greater the deflection. (iii) 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. By carrying out accurate measurements on the amount of deflections observed by the electrons on the electric field strength or magnetic field strength, Thomson was able to determine the value of e/me as:

e/m = 1.758820 × 1011 C kg–1 

Where m/e is 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 electron is –e.

2.1.3 Charge on the 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 the charge on the electron to be – 1.6 × 10–19 C. The present accepted value of the electrical charge is – 1.602176 × 10–19 C. The mass of the electron (m/e ) was determined by combining these results with Thomson’s value of e/me ratio. = 9.1094×10–31 kg 

2.1.4 Discovery of Protons and Neutrons 

Electrical discharge carried out in the modified cathode ray tube led to the discovery of canal rays carrying positively charged particles. The characteristics of these positively charged particles are listed below. (i) Unlike cathode rays, the mass of positively charged particles depends upon the nature of gas present in the cathode ray tube. These are simply the positively charged gaseous ions. (ii) The charge to mass ratio of the particles depends on the gas from which these originate. (iii) Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge. (iv) The behavior of these particles in the magnetic or electrical field is opposite to that observed for electron or cathode rays. The smallest and lightest positive ion was obtained from hydrogen and was called a proton. This positively charged particle was characterized in 1919. Later, a need was felt for the presence of electrically neutral particles as one of the constituents of the atom. These particles were discovered by Chadwick (1932) by bombarding a thin sheet of beryllium with α-particles. When electrically neutral particles having a mass slightly greater than that of protons were emitted. He named these particles neutrons.