Atoms and Molecules: An Introduction to Modern Atomic Theory

Atoms and Molecules - Introduction

INTRODUCTION

You have learnt, in your lower classes that matter is around us everywhere. Matter is made of atoms. Curiously the idea of atom was first proposed by the Greek philosophers in the fifth century BC (BCE). But, their theory was more philosophical than scientific.

The first scientific theory of the atom was proposed by John Dalton. Few of the postulates of Dalton’s theory about an atom were found incorrect by the later on studies made by J.J. Thomson, Rutherford, Neils Bohr and Schrodinger. In the light of the result of the researches most of the limitations of the Dalton’s theory were removed and a new theory known as the modern atomic theory was put forward.

‘The main postulates of modern atomic theory’ are as follows:

• An atom is no longer indivisible (after the discovery of the electron, proton, and neutron).
• Atoms of the same element may have different atomic mass. (discovery of isotopes ³⁵₁₇Cl, ³⁷₁₇Cl).
• Atoms of different elements may have same atomic masses (discovery of Isobars ⁴⁰₁₈Ar, ⁴⁰₂₀Ca).
• Atoms of one element can be transmuted into atoms of other elements. In other words, atom is no longer indestructible (discovery of artificial transmutation).
• Atoms may not always combine in a simple whole number ratio (E.g. Glucose C₆H₁₂O₆ C:H:O = 6:12:6 or 1:2:1 and Sucrose C₁₂H₂₂O₁₁ C:H:O = 12:22:11).
• Atom is the smallest particle that takes part in a chemical reaction.
• The mass of an atom can be converted into energy (E = mc²).

The modern atomic theory is the basis for all the studies of chemical and physical processes that involve atoms. You have studied the most fundamental ideas about an atom in your lower classes. Let us discuss some more concepts about atoms in this lesson.

Concept Map: Atoms and Molecules

An Introduction to Nuclear Physics: From Atoms to the Nucleus

Nuclear Physics - Introduction

Humans are very much interested in knowing about atoms. Things around us are made up of atoms. A Greek Philosopher 'Democritus' in 400 BC believed that matter is made up of tiny indestructible units called atoms. Later, in 1803, John Dalton considered that elements consist of atoms, which are identical in nature. J J Thomson discovered cathode rays, known as electrons, experimentally and Goldstein discovered positive rays, which were named as protons by Rutherford. In 1932, James Chadwick discovered the chargeless particles called neutrons. Presently, a large number of elementary particles like photon, meson, positron and nutrino have been discovered. In 1911, the British scientist, Ernest Rutherford explained that the mass of an atom is concentrated in its central part called Nucleus. You have already learnt about the atomic structure in the earlier classes.

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Topic: 10th Science : Chapter 6 : Nuclear Physics : Nuclear Physics | Introduction