Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
31
The Structure of the Atom
The Structure of the Atom
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER
4
4.1 Early Theories of Matter
In ancient Greece, philosophers debated the nature of matter. The
philosopher Democritus held that matter was composed of elementary particles, called atoms, in otherwise empty space. Over two
thousand years would pass before the existence of the atoms was
proven.
The atomic theory In 1803, John Dalton proposed a theory to
explain the laws of conservation of matter, definite proportions, and
multiple proportions. Dalton’s atomic theory, based on experimental results, held that matter consists of atoms, that all atoms of an
element are alike, and that atoms of one element differ from atoms
of other elements. Further, the theory proposed that atoms were
indestructible and only were rearranged during chemical reactions.
4.2 Subatomic Particles and the Nuclear Atom
One of the most important results of Dalton’s theory was that it generally persuaded scientists that atoms exist. As a result, scientists
began research to discover the exact nature of the atom.
Discovery of electrons Experiments around the turn of the
twentieth century provided evidence that the atom consists of
smaller particles. When J. J. Thomson applied a high voltage to two
electrodes sealed inside an evacuated tube, an invisible beam, or ray,
was found to emanate from the negative electrode (cathode) of the
tube and travel toward the positive electrode (anode). The path of the
beam could be made visible by the glow it caused when it passed
across a plate coated with fluorescent material. The ray, called a
cathode ray, could be deflected by electric and magnetic fields.
Thomson concluded that the ray consisted of a stream of
negatively charged particles that had been dislodged from atoms.
These particles became known as electrons. Thomson went on to
determine that the mass of an electron is much less than that of a
hydrogen atom.
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▲
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
31
The Structure of the Atom
The Structure of the Atom
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER
4
4.1 Early Theories of Matter
In ancient Greece, philosophers debated the nature of matter. The
philosopher Democritus held that matter was composed of elementary particles, called atoms, in otherwise empty space. Over two
thousand years would pass before the existence of the atoms was
proven.
The atomic theory In 1803, John Dalton proposed a theory to
explain the laws of conservation of matter, definite proportions, and
multiple proportions. Dalton’s atomic theory, based on experimental results, held that matter consists of atoms, that all atoms of an
element are alike, and that atoms of one element differ from atoms
of other elements. Further, the theory proposed that atoms were
indestructible and only were rearranged during chemical reactions.
4.2 Subatomic Particles and the Nuclear Atom
One of the most important results of Dalton’s theory was that it generally persuaded scientists that atoms exist. As a result, scientists
began research to discover the exact nature of the atom.
Discovery of electrons Experiments around the turn of the
twentieth century provided evidence that the atom consists of
smaller particles. When J. J. Thomson applied a high voltage to two
electrodes sealed inside an evacuated tube, an invisible beam, or ray,
was found to emanate from the negative electrode (cathode) of the
tube and travel toward the positive electrode (anode). The path of the
beam could be made visible by the glow it caused when it passed
across a plate coated with fluorescent material. The ray, called a
cathode ray, could be deflected by electric and magnetic fields.
Thomson concluded that the ray consisted of a stream of
negatively charged particles that had been dislodged from atoms.
These particles became known as electrons. Thomson went on to
determine that the mass of an electron is much less than that of a
hydrogen atom.
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