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A. THE DEVELOPMENT OF THE ATOMIC

Dalton’s Atomic Theory

John Dalton (1776 – 1844) did not doubt the validality of thr concept proposed by
Democritus. His assurance was based on Laws of Conservation of Mass and Definite

Proportions, which concept corresponded to. Based on that think, in 1803 Dalton formulated his

atomic theory consisting of the points below :

a. Matter is made up of tiny particles called atoms.
b. An element is matter composed of identical atoms that

have different masses and properties.
c. Different elements have different masses and properties.

d. A coumpound is composed of at least two atoms from
different elements with a fixed and specific proportion.

Atoms in compound are combined together through atomic

bonding.

e. Atom cannot be created nor destroyed. Chemical reactions
Figure 3. John dalton
are merely a rearrangement of atoms that undergo the

reaction.
Figure 4. Dalton’s Atomic Theory

Thomson’s Atomic Theory

In 1898, J.J. Thomson provided an atomic model on

the assumption that an electron has a smaller mass than an

atom. Thus, it is considered as a particle composing an atom.

This was based on the e/m value of cathode rays which is

~1.800 times that of ion of the smallest atom, hydrogen.

Because an atom has a neutral charge, the negatively charged

Figure 5. J.J. Thomson

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