About the Atom

About the Atom

Rakesh Mohan Hallen

Atomos is a Greek word, which means something that cannot be divided further. The familiar English word 'atom' indeed originated from this Greek word. It has been in vogue for at least past two centuries. A very large number of phenomena, which form the backbone of this column, can only be explained using this word. No wonder many of our readers are very curious to have some more details about atoms. Some of their queries are as follows:

What is an atom?

* Atoms are not visible even under the electron microscope, then how do scientists see them?

* Why is every orbit of an atom associated with some amount of energy?

* Do the atoms of a substance have any characteristic of colour?

* From where do electrons and protons get their charge?

* Isn't the Bohr's model of atoms adequate for most practical purposes?

Atom is a concept in science just like energy or gravity. It helps us to relate the scientific explanations to a large number of phenomena. Unlike a small dust particle, a virus or a bacteria the atom cannot be seen with our eyes either directly or through an optical microscope. We can at best infer the structure of a crystalline solid to be made up of discrete particles through images we develop from an instrument known as a field ion microscope. At best it is an image painted in the mind of a scientist from the results of constant interplay between experiments and theories through the past several centuries. As the technology developed, newer experiments were devised, the results of some of these experiments threw up new questions, which older theories about the constituents/structure of an atom could not explain. Thus the atom as we "see" it today is a result of many visions and revisions. To give our young readers an idea of the processes in science that led to our present conception of an atom a brief account of the developments is recounted below.

As long as the activity of curious human beings was limited essentially to speculation, as was the case in the time of the earliest Greek philosophers atoms were indeed the ultimate units, which made up matter. But as the early scientists studied the various chemical transformations in matter, it was soon evident that all atoms cannot be exactly alike. At best only atoms of a particular element are alike. The early chemists could even ascribe weight to the atoms of a particular substance, they called it the 'atomic weight'. Many experimental studies were also carried out regarding the effect of electric current on different chemicals and the analysis of light emitted/absorbed from different substances. These observations relating to electrical and optical phenomenon in nature made it necessary to revise the concept of an indivisible atom.

The experiments of Michael Faraday (1791- 1867) indicated that there is a definite relation between the electric current passed and the amount of a particular element discharged/deposited by it. A certain quantity of electricity, known as a coulomb, can liberate only an integral fraction of a particular quantity of a substance--known as a mole. A mole, as we all know, signifies the quantity of a substance containing a particular number (Avagadro's Number) of atoms/molecules. Thus the experiments of Faraday strongly implied the existence of "units of electric charge". In 1891, a physicist named Johnstone Stoney, postulated that this charge must be carried by a particle, he named this particle the electron. But another scientist gathered the experimental evidence for the existence of electrons: J. J. Thomson. It had long been known that if high electric voltages were applied to a rarefied

gas, by sealing wires into two ends of a glass tube, spectacular glows are seen. If an object was placed between the cathode and the anode its shadow could be observed near the anode. Thus it was thought that cathode rays were a kind of radiation just like light. In his experiments, Thomson applied external electric and magnetic fields to the cathode rays. He observed that these rays, unlike rays of light, were deflected as a result of the field applied. Thomson could explain these observations if he assumed that these rays were a stream of negatively charged particles (electrons). He could also calculate the velocity of these particles and the ratio of their electric charge to their mass (e/m) by applying the Newton's laws of mechanics and Coulomb's laws of electric forces. The e/m ratio of the cathode rays was also found to be independent of the gas used in his experiments. He calculated this ratio to be 1.8 x 10-11 coulombs/kg.

The e/m ratio calculated for the negatively charged particle in a cathode ray, by Thomson, was very large compared with that measured earlier for the hydrogen atom by means of certain experiments on electrolysis. Since it was known that an atom of hydrogen is electrically neutral and has only one electron. It was evident that electrons were much lighter than the positively charged particle: the protons. Also the electric charge on a proton is equal in magnitude to that carried by an electron. However, there was no experimental evidence pointing towards the relative or absolute size of electron or proton. In absence of such evidence Thomson naturally believed that the positively charged particle occupied space proportional to its mass and proposed a model for atoms, the so-called plum-pudding model. In this model electrons are embedded in the positively charged particles as plums in a pudding. But this model did not last very long.

The development of a model for an atom closer to the presently accepted model began with the experiments of Henri Becquerel. He discovered radioactivity in 1896. He had stored photographic plates wrapped in black paper in a drawer together with some uranium salts. When he developed the plates, he saw they had turned black. Many other scientists around that time might have had similar observation, but while they inferred that something was wrong with plates, Becquerel was alert to the possibility that uranium was emitting some kind of radiation, capable of penetrating structure of atoms using these particles. In one such experiment, he observed that when alpha particles were bombarded on a gold foil, most of them passed through the foil with only small deviations but some did bounce backwards. He could explain his observations by proposing a new model of the atom. In his model protons were located in a very small fraction of the space occupied by an atom (less than 10-14 of