HISTORY OF THE ATOMIC THEORY
Democritus – Matter was “discontinuous”. He said that matter was made
up of tiny individual particles and consisted of spaces between the
particles. These particles are called atoms (derived from the Greek
word atomus meaning indivisible).
Aristotle – Matter was “continuous”, i.e. there were no spaces in
matter. He said that matter was made up of the four elements: fire,
air, water and earth.
John Dalton – He studied how atoms of different elements combined with
each other. He also began classifying the elements based on how they
behaved chemically and their characteristics.
The main points of Dalton’s theory published in 1805 are:
a) Matter was composed of tiny indivisible particles called atoms.
b) Atoms of the same element are identical and have a characteristic
c) Atoms of dissimilar elements are different.
d) Atoms of elements combine in fixed ratios to form new substances.
J.J. Thomson – He experimented with cathode ray tubes. It was Thomson
who discovered that cathode rays could be deflected by electric and
magnetic fields. He concluded that cathode rays were streams of
negatively charged particles called electrons.
He concluded that all matter consisted of these negative charges found
within the atoms themselves. It was known that two types of charges
existed in nature (positive and negative), but Thomson was unable to
figure out the exact location of the positive and negative charges.
His model of the atom described the atom as a positively charged sphere
with the electrons (negative charges) embedded inside the sphere. His
model is popularly known as the “plum pudding model”.
The cathode ray tube was first used in televisions.
Henri Becquerel – He observed that certain substances, e.g. uranium,
spontaneously emitted a type of radiation. The radiation was invisible
to the naked eye, but its effect could be observed on photographic
plates. This type of energy (radiation) was termed “Becquerel rays”.
Marie Curie – She proposed the term “radioactivity” t