Atomic structure

 

INTRODUCTION

In the fifth century (B.C.) the Greek Philosopher Democritus expressed the belief that all matter consists of very small indivisible particles, which he named ATOMOS (Greek =Atomos = not cutable = indivisible) nowadays called atoms. 

Daltons Atomic Theory:

In (1808) John Dalton, an English school teacher and Chemist suggested the fundamental ideas of atomic theory, which explains the chemical nature of matter and the existence of atoms. It is known as Dalton's atomic theory. The important postulates are as following:

1: All elements are made up of small indivisible, indestructible particles called atoms.

2: All atoms of a given element, are identical in all respects, having same size, mass and chemical properties. But the atoms of one element differ from the atoms of other element. 

3: Compounds are formed when atoms of more than one element combine in a simple whole number ratio.

4: A chemical reaction is a rearrangement of atoms, but atoms themselves are not changed, this means that atoms are neither created nor destroyed in chemical reactions.

Modern Atomic Theory:

Daltons atomic theory assumed that atoms of elements are indivisible and that no particles smaller than atoms existed. But as the time passed new experimental facts led to the modification and extension of Dalton's atomic theory. Atom is a complex organisation, composed of even smaller particles called sub-atomic particles (fundamental particles). These are electrons, protons and neutrons. Dalton's view that all atoms of an element have the same mass is modified in the light of discovery of isotopes. Even then, we can say that the Dalton's atomic theory was largely successful in explaining the laws of chemical combinations.

Fundamental Particles of an Atom:

The atom was generally identified as the smallest particle of an element, consisting of sub atomic particles, the electrons, protons and neutrons. The first hint about the sub-atomic particles, came with the discovery of electron by M. Faraday (1832), William Crooks (1879) and J.J. Thomson (1897). Later, the second sub-atomic particle, the proton was identified and isolated by Goldstein, German scientist (1886) and Ernest Rutherford (1919). Finally an English scientist James Chadwick revealed the third particle the neutron in 1932. The structure of atom as we know it today, is because of these findings.

DISCOVERY OF ELECTRONS

The fundamental particle carrying a negative charge was discovered in 1897, by the British physicist J.J. Thomson. The apparatus used this type of experiment is called the discharge tube (Neon sign and T.V. tube are examples of discharge tube) which consists of a glass tube, fitted with two metal electrodes connected to a high voltage source and a vaccum pump.

Fig a simple gas discharge tube

When the tube is evacuated and a current of high potential is passed between the electrodes, streaks of bluish light extending from negative electrode (cathode) towards positive electrode (anode). The rays appear to travel in straight lines, from the cathode to anode, cause the wall at the opposite end of the tube glow where they strike. These rays were called cathode rays. Thomson showed that these rays were deflected towards the positive plate in electric and magnetic field. This shows that these ravs consisted of negatively charged particles. The name electron was given to these units of negative charges. Electrons were obtained irrespective of the nature of cathode or the gas in the discharge tube. This proves that electrons are constituents of all matter.

Properties of Cathode Rays:

1. They cast shadows of objects placed in their path towards the anode, proving that they travel in straight lines.

2. They cause a light paddle wheel to rotate (revolve). Showing that they are material particles.

3. These rays although invisible cause some material to glow or produce fluorescence.

4. They are deflected towards the positive plate in an electric field, showing that they are negatively charged particles.

The (charge=mass) e/m ratio of cathode particles is 1.7588x108 c/g (coulomb per gram), i.e same for all electrons, regardless of any gas in the tube.

6. They can produce mechanical pressure indicating they possess kinetic energy (K.E.).

Discovery of Protons:

Since atoms are electrically neutral and electrons carry negative charge, it follows that for each electron, there must be one equivalent positive charge to neutrilize that electron. This particle is called a proton. It is one of the fundamental units of structure of all atoms. The simplest atom of hydrogen (H) is therefore made up of one electron and one proton.

e.g H→H+ + e-

(Proton) (Electron)

Protons were first observed in apparatus similar to cathode rays tube, with a perforated cathode by German Physicist Goldstein in 1886,their existence was verified and their properties were investigated in 1897 by J.J. Thomson.

Goldstein's apparatus for studying positive particles

Properties of positive rays:

1. These rays also travel in straight line from anode to cathode.

2. These are deflected towards the negative plate when passed through an electric field, showing that these carry a positive charge.

The (charge=mass) i.e., e/m ratio of positive particle is much smaller than that for electron and it varies with the nature of gas in the tube.

Discovery of Neutrons:

In 1932 the English Physicist James Chadwick discovered a third type of fundamental particle of atomic structure through artificial radioactivity, further it will be discussed in the definition of artificial radioactivity.