1. ATOMIC STRUCTURE

 

1.1 Introduction

The word “Atom” is a Greek word which means 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. Around 500 BC, an Indian philosopher Maharishi Kanad postulate that if we go on dividing matter (padarth), we shall get smaller and smaller particles. Ultimately a time will come when we shall come across the smallest particle beyond which further division will not possible. He named these particles Parmanu (or Kan after his name Kanad).

Around the same era, ancient Greek philosopher Democritus suggested that if we go on dividing matter, a stage will come when particles obtained will not be divided further. Democritus called these indivisible particles atoms. All these were based on philosophical considerations and not much experimental work was done.

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 Anode Rays.

              (iii) Discovery of X-Rays                      (iv) Discovery of Radio-activity.

              (v) Discovery of Isotopes and               (vi) Discovery of the new

                  Isobars                                              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.

Some sub-atomic particles are-

Neutrino:       It was discovered by Pauling.

It has zero charge and mass is less than electron.

 

Positron:      It was discovered by C.D. Anderson.

It has +1 charge and mass is very less than electron.

Meason:         It was discovered by Yukawa it may have +1, 0 ,-1 charge and mass is 200 times greater than electron .

Antiproton: It was discovered by Segre it has –1 charge and mass is equal to proton.

 

 1.2 Discovery of Fundamental Particles

1.2.1 Discovery of Electron (Cathode rays)

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. 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. Since the negative electrode is referred to as cathode, these rays were called cathode rays. Further investigations were made by W. Crookes, J. Perrin, J.J. Thomson and others. (Fig 1)

Cathode rays possess the following properties:

1. 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.
2. They produce a green glow when strike the glass wall beyond the anode. Light is emitted when they strike the zinc sulphide screen.
3. 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.
4. 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.
5. They possess kinetic energy. It is shown by the experiment that when a small pinwheel 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.
   The name `negatron’ was changed to ‘electron’ by Stoney.
6. Cathode rays produce X-rays. When Cathode rays fall on material having high atomic mass, new type of penetrating rays of very small wavelength are emitted called X-rays.
7. These rays affect the photographic plate.
8. These rays can penetrate through thin foils of solid materials and cause ionization in gases through which they pass.
9. The nature of the cathode rays is independent of:
   1. The nature of the cathode and
   2. The gas in the discharge tube.

Further experiments were carried out to determine the exact charge and mass of the electrons:

(a) Charge/mass ratio of an electron

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.

(b) Charge of an electron

The first precise measurement of the charge on the electron was made by Robert A. Millikan in 1917 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.

(c) 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 a moving electron, m_e = 

Where

m₀ is the rest mass of an electron =9.1096 × 10^-28 g or 9.1096 × 10^-31 kg

v is the velocity of the moving electron.

c is the speed of light=3×10⁸m/sec

When v becomes equal to c, mass of the moving electron becomes infinity and when the velocity of the electron becomes greater than c then 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

Note:

The device which is used by J.J. Thomson to do the first of all mass- separating experiments was Mass-spectrometry. Mass-spectrometry is an analytical technique to generate ions from either inorganic or organic compounds and to measure their mass-to-charge ratio, by any suitable method i.e. thermally, by electric fields or by impacting energetic electrons, ions or photons.

 

1.2.2 Discovery of Proton (Positive Rays)

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. 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 luminous rays were produced simultaneously passing through the holes or perforations of the cathode and moving in a direction opposite to the cathode rays. Thus, these rays consisted of positively charged particles moving away from the anode and were named as positive rays or anode rays or as canal rays.

Anode rays are not emitted from the anode but from a space between anode and cathode.

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.

                             Fig - Production of Anode rays

 The following characteristics of the positive rays were recognized:

1. The rays travel in straight lines and cast a shadow of the object placed in their path.
2. Like cathode rays, these rays also rotate the wheel placed in their path and also have heating effect. Thus, the rays possess kinetic energy, i.e., mass particles are present.
3. The rays produce flashes of light on zinc sulphide screen.
4. The rays are deflected by electric and magnetic fields in a direction opposite to that of cathode rays. These rays are attracted towards the negatively charged plate showing thereby that these rays carry positive charge.
5. These rays can pass through thin metal foils.
6. These rays can produce ionization in gases.
7. These rays are capable of producing physical and chemical changes.
8. Positive particles in these rays have e/m values much smaller than that of electron. This means either m is high or the value of charge is small in comparison to electron. Since positive particle is formed by the loss of electron or electrons, the charge on the positive particle must be an integral multiple of the charge present on the electron. Hence, for a smaller value of e/m, it is definite that positive particles possess high mass.
9. The e/m value is dependent on the nature of the gas taken in the discharge tube, i.e., positive particles are different in different gases.

Accurate measurements of the charge and the mass of the particles obtained in the discharge tube containing hydrogen, the lightest of all gases, were made by J.J. Thomson in 1906. These particles were found to have the e/m value as + 9.579 × 10⁴ coulomb/g. This was the maximum value of e/m observed for any positive particle. It was thus assumed that the positive particle given by hydrogen represents a fundamental particle of positive charge. This particle was named proton by Rutherford in 1911. Its charge was found to be equal in magnitude but opposite in sign to that of electron.

Thus, proton carries a charge + 1.602 × 10^-19 Coulomb, i.e., one unit positive charge.

The mass of the proton, thus, can be calculated.

Mass of the proton:

Hence, a proton is defined as a sub-atomic particle, which has a mass nearly 1 amu, and a charge of +1 unit (+1.602 × 10^-19 coulomb).

Protons are produced in a number of nuclear reactions. On the basis of such reactions, proton has been recognized as a fundamental building unit of the atom.

Radioactivity

After the discovery of electron and proton, it was well established that atom is divisible and is made up of charged particles. This was further confirmed by the phenomenon of radioactivity, discovered by Becquerel in 1896.

Radioactivity is the phenomenon of spontaneous emission of radiations by certain elements like uranium, radium etc. The elements emitting such radiations are called radioactive elements.

The phenomenon can be observed by placing the radioactive element in a cavity made in a block of lead and applying electric or magnetic field on the radiations being emitted and then allowing them to fall on the photographic plate.

Three types of radiations are emitted as explained below:

(i)           Those, which are deflected slightly towards the negative plate and hence carry   positive charge, are called a-rays. The particles present in them are called a-particles. Each a-particle has charge = + 2 units and mass = 4 u. Hence, they are same as helium nuclei and are represented as         .

(ii)         Those, which are deflected towards the positive plate to a larger extent and hence carry negative charge, are called b-rays. The particles present in them are called b-particles. Each b-particle has same charge and mass as that of electron. Hence, it is represented   as.

(iii)        Those that remain undeflected are called g-rays. They are simple electromagnetic radiations.

(iv)       The penetrating power of b-rays is nearly 100 times more than that of a–rays, while that of the g–rays is about 1000 times more than that of the b-rays. In other words, the a–rays have the least penetrating power while the g–rays have the maximum magnitude.

1.2.3 Discovery of Neutron

The discovery of neutron was actually made about 20 years after the structure of atom was elucidated by Rutherford.

Moseley, in 1913, performed experiments to determine the exact quantity of charge present on the nucleus.

1.2.3.1 Moseley Experiment–Atomic Number

Roentgen, in 1895, discovered that when high energy electrons in a discharge tube collide with the anode, penetrating radiations is produced called X-rays.

X-rays are electromagnetic radiations of very small wave-lengths (0.1–20 Å). X-rays are diffracted by diffraction gratings like ordinary light rays and X-ray spectra are, thus, produced. Each such spectrum is a characteristic property of the element used as anode.