Page 9 - Chemistry_Form_2

Chemistry
for Secondary Schools

Task 1.2

Use reliable resources to analyse various atomic theory discoveries and come up
with an idea about the structure of the atom.
FOR ONLINE READING ONLY
About the year 400 BC, a Greek philosopher known as Democritus was the
first to consider the idea that matter is made up of particles. Such an idea was
not accepted because there was no experimental evidence to support it. About
2000 years later, an English man called John Dalton
revived the discussion. He used experimental
evidence to convince people that matter is made up
of particles called atoms. Through that experiment,
he deduced Dalton’s spherical model of an atom
shown in Figure 1.1. Dalton’s discovery helped
scientists understand chemical reactions and how
substances combine.
Figure 1.1: Dalton’s model of
the atom
Dalton atomic theory

In 1803, Dalton developed the theory about the atom. The four main points
(assumptions) of Dalton’s Atomic Theory are summarised as follows:
1. Matter is made up of tiny particles called atoms. (The word atom means
‘unsplittable’ in Greek).
2. Atoms can neither be created nor destroyed.
3. Atoms of the same element are identical and have the same mass and
properties. Atoms of a given element are different from those of any other
element. Atoms of different elements can be distinguished from one another
by their respective relative weights.
4. Compounds are formed by a combination of two or more different kinds of
atoms. Atoms always combine in simple whole number ratios.

Dalton never imagined that anyone would ever be able to see an atom. However,

modern technology has provided direct evidence that shows the positions
and patterns of individual atoms. The use of modern technology has enabled
scientists to carry out experiments on the atom that Dalton could not. This has

led to slight modifications to Dalton’s Atomic Theory and thus formulated the so
called modern concepts of Dalton’s Atomic Theory.

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