Page 70 - ICSE Chemistry 8

Around 20th century, through various experiments with electromagne sm and radioac vity, physicists
discovered that the so-called indivisible atom was actually a cluster of various subatomic par cles (mainly
electrons, protons and neutrons) which can exist separately from each other.
In this chapter, let’s learn how ancient scien sts and philosophers discovered the structure of an atom.

ANCIENT VIEWS ON ATOMIC STRUCTURE

Ancient scien sts and philosophers have me and again put forth diff erent theories based on their
thinking about the structure of an atom.

Maharishi Kanada’s View on Atom

Maharishi Kanada, a great Indian philosopher of 600 BCE,
Thirst for Knowledge
proposed that ma er consists of small indestruc ble
par cles called paramanu (param means ul mate and anu The whole universe is made up of fi ve
means par cle) which is an atom. A paramanu does not basic elements—earth, water, fi re, air
exist in free state, rather it combines with other paramanus and space. In India, it is believed that
to form a bigger par cle called the anu, which is nowadays our body is also made up of these fi ve
called a molecule. basic elements of nature, also known
as pancha mahabhutas. The pancha
Democritus and Atoms mahabhutas are related to our fi ve

senses of smell, taste, hearing, touch
Another ancient theory came from the Greek philosopher
and sight.
Democritus (460 BCE–370 BCE). Democritus said that
everything is made up of minute par cles called atoms.
Atomos is a Greek word meaning indivisible.

DALTON’S ATOMIC THEORY

Before John Dalton, the atomic theories put forth were mere specula ons. They
were not based on experimenta on. In 1808 John Dalton drew inferences from
his research and experimental observa ons and proposed an atomic theory. The
postulates of Dalton’s Atomic Theory are as follows.
• All ma er consists of ny par cles called atoms.

• Atoms are indivisible or indestruc ble. They can neither be created nor
John Dalton
destroyed.
• In chemical reac ons, atoms are rearranged, combined or separated.

• All atoms of a par cular element are iden cal in mass and other proper es.

• The atoms of one element are diff erent from the atoms of all other elements, i.e., atoms of diff erent
elements have diff erent mass, diff erent size and diff erent chemical proper es.
• Atoms of an element combine with atoms of another element (or elements) in fi xed whole numbers
to form compounds. For example, two atoms of hydrogen (H) combine with one atom of oxygen
(O) to form water (H O).
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