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The Atom
What is an atom?

Philosophical question: Is matter
continuous and infinitely divisible, or is it
divisible only until a basic, invisible particle
that cannot be divided further is reached?
Who discovered the atom?
Democritus (400 BC) first suggests the idea of
the atom.
 Atom is a Greek word meaning “indivisible”.
 Aristotle, however, did not believe in atoms. He
thought all matter was continuous.
 Democritus does not get credit for discovering
the atom because he had no scientific evidence
to back it up.

Jump up to the 1700s
By the 1700s, all chemists accepted that
an element could not be broken down
further by ordinary chemical means.
 It was also clear that elements could join
to form compounds.

1790s
In the 1790s, with the help of improved
balances, scientists began to study
chemical reactions further.
 They came up with 3 laws.

Law of conservation of mass
States that matter can not be created nor
destroyed.
 During a chemical reaction, the mass of
the reactants will be the same as the mass
of the products.

Law of definite proportions
States that it does not matter the size of
the sample but elements always combine
in the same ratio by mass when forming a
compound
 Example: NaCl is always 39.34% sodium
and 60.66% Cl no matter the size of the
sample.

Law of multiple proportions
States that if two or more different compounds
are composed of the same elements, they
always combine in small whole number ratios.
 Example: The element carbon and oxygen can
combine to form two different compounds,
carbon monoxide and carbon dioxide. In carbon
monoxide (CO) the ratio is 1:1. In carbon
dioxide (CO2) the ratio is 1:2.

Dalton’s Atomic Theory
In 1808, John Dalton proposed an
explanation to the three laws by coming
up with the atomic theory of matter.
 Came up with the following Atomic Theory
of Matter:

– All matter is made up of atoms.
– Atoms can not be divided.
– Atoms of a given element are identical.
– Atoms can combine to form compounds.