Development of the Atomic Theory
Atom – The smallest particle into which an
element can be divided and still be the
Element – A pure substance that cannot be
separated into simpler substances by
physical or chemical means.
Atoms make up elements.
Elements are made of only one kind of
Elements combine to form compounds.
All matter is made of elements or
compounds, so all matter is made of
Atoms are so small that, until recently, no
one had ever seen one. But ideas, or
theories, about atoms have been around
for over 2,000 years.
Theory – A unifying explanation for a broad
range of hypotheses and observations
that have been supported by testing.
Democritus (440 B.C.)
Democritus proposed that if you kept
cutting a substance in half forever,
eventually you would end up with an
He called these particles atoms,
meaning “indivisible” in Greek.
Democritus thought that atoms were
small, hard particles of a single
material and in different shapes and
He thought that atoms were always
moving and formed different materials
by combining with each other.
Aristotle disagreed with Democritus’s
idea that you would end up with an
indivisible particle. Because Aristotle
had greater public influence,
Democritus’s ideas were ignored for
John Dalton (1803)
Scientists knew that elements
combined with each other in specific
proportions to form compounds.
Dalton claimed that the reason for this
was because elements are made of
He published his own three-part atomic
1) All substances are made of atoms. Atoms are
small particles that cannot be created, divided,
2) Atoms of the same element are exactly alike,
and atoms of different elements are different.
3) Atoms join with other atoms to make new
Much of Dalton’s theory was correct,
but some of it was later proven
incorrect and revised as scientists
learned more about atoms.