Chapter 4: Atoms and Elements
 Recognize that all matter is composed
of atoms.
 Explain the early ideas that led to the
current understanding of the atom.

Atoms compose matter.

The properties of atoms determine the properties
of matter.

An atom is the smallest identifiable unit of an
element.

An element is a substance that cannot be broken
down into simpler substances.

There are about 91 different elements found in nature,
and consequently about 91 different kinds of atoms.
 Scientists have succeeded in making about 20 synthetic
elements (not found in nature).

The exact number of naturally occurring elements is
controversial because some elements previously
considered only synthetic may actually occur in nature in
very small quantities.
 Many ancient scholars believed matter
was composed of such things as earth,
water, air, and fire.
 Many believed matter could be
endlessly divided into smaller and
smaller pieces.

Democritus (460–370 B.C.E.)
suggested that if you divide
matter into smaller and
smaller pieces, you end up
with tiny, indestructible
particles.
 Everything else is empty space.
 Democritus called these particles
“atomos”
 meaning “indivisible”
 Democritus is the first person on record
to have postulated that matter is
composed of atoms.
 Aristotle (384–322 B.C.)
disagreed with Democritus
because he did not believe
empty space could exist.
 Believed everything was
composed of fire, air, earth,
and water.
 Law of Conservation of Mass:
 Antoine Lavoisier (1743-1794)
 In a chemical reaction, matter is neither
created nor destroyed.

Joseph Proust (1754-1826): a
French chemist who made
observations on the
composition of compounds.
 In 1797, found that the elements
composing a compound always
occurred in fixed proportions.
 Law of Definite Proportions:
 All samples of a given compound,
regardless of their source or how they were
prepared, have the same proportions of
their constituent elements.
2H:1O
 John Dalton (1766-1844):
an English chemist who
offered convincing
evidence in support of
Democritus.
 In 1804, published his law of
multiple proportions.
 Law of Multiple Proportions:
 When two elements (A and B) form two
different compounds, the masses of
element B that combine with 1 g of
element A can be expressed as a ratio of
small whole numbers.
1 g Carbon
1 g Carbon
2.66 g Oxygen
1.33 g Oxygen
2.66 g Oxygen
1.33 g Oxygen
2.00
= 1.00
Consistently getting small whole number ratios supports
the idea that there is are indivisible atoms that make up the
compounds.
 In 1808 (over 2000 years after
Democritus) John Dalton formalized a
theory of atoms that gained broad
acceptance.
 Dalton’s atomic theory has five parts:
1. Each element is composed of tiny,
indestructible particles called atoms.
2. All atoms of a given element have the same
mass and other properties that distinguish
them from the atoms of other elements.
3. Atoms combine in simple, whole-number
ratios to form compounds.
4. Atoms cannot be subdivided, created, or
destroyed
5. In chemical reactions, atoms are combined,
separated, or rearranged.
 Compare and contrast the atomic
theories of Democritus and Dalton.
Mark an X under each name if a
statement in the table applies to that
person’s theory.
Statement
All matter is made of tiny particles.
Matter is made of empty space
through which atoms move.
Atoms cannot be divided.
Atoms cannot be created.
Atoms cannot be destroyed.
Different atoms combine in wholenumber ratios to form compounds.
Matter is composed of fire, air,
water, and earth.
Democritus
Dalton