Early Greek Theories of
Matter
Greek philosophers first proposed an
atomic theory of matter in the 5th
century B.C. They believed that all
substances were composed of small,
indivisible particles called atoms They
were thought to be different sizes, have
regular shapes and be in constant motion.
Empty space was through to exist
between atoms. Aristotle severely
criticized this theory arguing that atoms
in continuous motions in a void is illogical
(his opinion might have been colored by
the fact that belief in a void at the time
was considered to be atheistic)
Aristotle’s Theory
Aristotle developed a theory of
matter based on the idea that
all matter is made up of four
basic substances – earth, air,
fire and water. He believed
that each basic substance had
different combinations of four
specific qualities – dry, hot,
cold and moist. His theory of
the structure of matter was
the prevailing model for almost
2000 years, including the period
of alchemy in the middle ages.
Fire
Hot
Dry
Air
Earth
Wet
Cold
Water
Dalton’s Atomic Theory
Dalton introduced and atomic theory of
matter in 1803, which came to replace
Aristotle’s model. His theory states:
•
•
All matter is composed of tiny, indivisible
particles called atoms
• Atoms of an element have identical
properties
Atoms of different elements have different
properties
• Atoms of two or more elements can
combine in constant ratios to form new
substances
Dalton’s Atomic Theory
Dalton’s theory was successful in
explaining the laws of conservation of
mass, definite composition and
multiple proportions. Since atoms are
indivisible, and are rearranged only
when compounds are formed, you
must end up with the same number
and kinds of atoms after a chemical
reaction.
Development of Atomic Theory
In the late 1800’s, J.J. Thompson found
evidence for the existence of negatively
charged particles that could be removed
from atoms. In 1987, he postulated the
existence of electrons, subatomic particles
possessing a negative charge. With the new
idea, Thompson developed a model of the
atom that has electrons evenly distributed
inside the spherical positive part of the
atom. In 1904, H. Nagaoka represented the
atom as a large, positively charge sphere
surrounded by a ring of negative electrons.
Development of Atomic Theory
It was Ernest Rutherford who did work (at
McGill University) to test the current
atomic models. He shot alpha particles
through very thin pieces of gold foil. He
predicted that all the particles would travel
through the foil and although most did, a
small percentage were deflected. He
deduced that an atom must contain a tiny,
positively charge core (the nucleus) which is
surrounded yb a mostly empty space
containing negative electrons.
Development of Atomic Theory
In 1914, Rutherford coined the word “proton” for the
smallest unit of positive charge. It was one of his
students (H.G.J Moseley) who showed that the
positive charge in the nucleus of atoms increases
by one unit in progressing from each element to the
next in the periodic table. This discovery led
Moseley to the concept of atomic number, defined
as the number of protons in the nucleus of an
atom. Originally the elements were listed and
numbered in order of increasing atomic mass, this
sequence now had an explanation, the periodic
table is a list of elements in order of the number of
protons in the nucleus
Development of Atomic Theory
The estimated mass of a proton and the number of
protons could not account for all the atomic mass.
Rutherford predicted the existence of a neutral
particle similar in mass to a proton. In 1932 James
Chadwick demonstrated that atomic nuclei must
contain heavy neutral particles as well as positive
particle in order to account for the atom’s mass.
These were called neutrons. An atom, therefore is
composed of a nucleus containing protons and
neutrons, and a number of electrons equal to the
number of protons. An atom is electrically neutral
Isotopes
Frederick Soddy, a colleague of Rutherford’s
at McGill, was the first to propose that the
number of neutrons can vary from atom to
atom, even in atoms of the same element.
An isotope is a form of an element in which
the atoms have the same number of protons
as all other form of that element, but a
different number of neutrons.
Carbon
All carbon atoms (atomic number 6) have 6
protons in the nucleus. The most common
form of carbon, carbon-12, also has 6
neutrons
Carbon-14, however, is an isotope of carbon
because it has 6 protons and 8 neutrons in
the nucleus.
.
Isotopes
Different isotopes of the same element
have the same chemical properties but
different masses.
All elements exist naturally as a mixture
of isotopes. The mass number is based
on the most common isotopes.
Summary of the Rutherford
Model
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An atom contains equal numbers of
negatively charged electrons and positively
charged protons
Most of the mass of the atom and all of its
positive charge is contained in a tiny core
called the nucleus
The nucleus contains protons and neutrons
that have approximately the same mass each
The number of protons is the same as the
atomic number
The total number of protons and neutrons
is the same as the mass number