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Joyce Wang
Atomic Theory
• The physical theory of the nature of the
structures, behavior, and properties of the
atom.
Greek Philosophers
• From the earliest times,
philosophers debated what the
world was made of
• Democritus was one of the
earliest of these philosophers
• Democritus thought that the
physical world was made of tiny
individisble particles called
atomos, “indivisible”
Greek Philosophers
• Plato and Aristotle
challenged his view by
saying there can be no
indivisible particles
• Because of their influence,
the atomic view of matter
faded for centuries
• The Aristotelean
philosophy dominated
Western culture for
centuries
Aristotle
Reemergence of Atomic Theory
• Concept of atoms reemerged in Europe during the 17th
century, when scientists wanted to explain properties of
gases. Ex: air in constant motion
• Isaac Newton favored the atomic theory- at this time he
was the most famous scientist.
• John Dalton’s chemical Atomic Theory:
– Each element made of small particles (atoms)
– All atoms of same element are identical; atoms of different
elements are different and have different properties
– Atoms of an element are not changed into different types of
atoms by chemical reactions; atoms are not created or
destroyed in chemical reactions
– Compounds are formed when atoms of more than one element
combine; given compound always has the same ratio of atoms
Dalton
• According to Dalton’s theory, atoms are the basic
building blocks of matter. They’re the smallest
particles of an element that retain the chemical
identity of the element.
• His theory explains several simple laws of
chemical combination that were known in his
time.
– Law of constant composition
– Law of conservation of mass
– Law of multiple proportions
Discovery of Subatomic Particles
• Until 19th century, everyone believed the atom was
only a round sphere
• Scientists used instruments called cathode rays to
study electrical discharge.
• 1897, J.J. Thompson challenged this view when he
discovered the electron.
• Thompson came up with the plum pudding model
J. J .
Thompson’s
Plum pudding
model
How did he discover it?
• The cathode ray tube
The Cathode Ray Tube
• If electric current passes through a vacuum,
a stream of glowing materials can be seen.
These were electrons.
• Thompson discovered that this glowing
material would bend towards a positively
charged electric plate.
• Therefore, he concluded that the particles
were negatively charged.
Robert Millikan
• Thompson measured the value of 1.76 x 10^8 coulombs
per gram for the ratio of electron’s electrical charge to
its mass.
• Once the charge-to-mass ratio of electrons were known,
Robert Millikan could figure out either the charge or the
mass of an electron from the other value.
• He performed the “Mikllikan oil-drop experiment” and
measured the charge of an electron.
• By using this value, he could find the mass of the
electron which he found to be 9.10 x 10^ -28 g
• The modern value is 9.10939 x 10^ -28 g
Rutherford
• Rutherford disproved the plum pudding model by
discovering a nucleus
• Rutherford scattered particles through a thin gold
foil and observed the angles they were deflected
in.
• Some of the particles were deflected right back in
the direction in which they had come.
• Rutherford explained this by saying that most of
the mass in the atom was in a positively charged
dense region in the very center of the atom.
Rutherford’s Experiment
James Chadwick
• 1932, the British scientist James Chadwick
discovered neutrons.
Henri Becquerel & Marie Curie
• Becquerel was studying pitchblende, and
discovered it emitted high-energy radiation
• This spontaneous emission of radiation is
called radioactivity
• Marie Curie began experiments to isolate
the radioactive components
Rutherford
• Revealed three types of
radiation: alpha, beta,
gamma radiation.
• Alpha particles have a
positive charge and are
fast moving particles.
• Beta particles are also
fast-moving but have a
negative charge.
• Gamma radiation is highenergy radiation and it
has no charge or particles.
Modern View of Atomic Structure
• Atoms are made of protons,
neutrons, and electrons.
• Atoms have an equal
number of electrons and
protons, so they have no net
electrical charge
• Protons and Neutrons reside
in the nucleus
• Nucleus is very tiny;
electrons take up the most
volume in the atom.
Isotopes, Atomic Numbers, Mass
Numbers
• All atoms of an element have the same number of protons
in the nucleus.
• The specific number of protons is different for different
elements. The number of electrons of an element should
match the number of protons.
• Isotopes: atoms of an element that differ in the number of
neutrons and therefore in mass.
• Atomic number: the number of protons. All atoms in an
element have the same atomic number.
• Mass Number: Total number of protons plus neutrons in
the atom.
• Nuclide: Atom of a specific isotope.