Development of the Atomic Theory
What Do You Think?
Imagine that you have cut a
penny in half. Then, you take
one piece and half it again. Will
this continue forever, or will you
come to a point where no more
cutting is possible?
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
Democritus was a
Greek philosopher
who theorized that
all matter was made
of invisible particles
called atoms.
Democritus of Abdera,
about 460-370 BCE
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
John Dalton 1766-1844
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Dalton, a British
chemist and teacher,
noticed that elements
combine in specific
proportions to form
compounds, and
theorized that their
atoms combine at the
same proportions.
Development of the Atomic Theory
Thomson’s
experiments using
a cathode-ray tube
showed that
smaller particles
make up atoms.
Joseph John
“J.J.” Thomson
1856-1940
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
Rutherford, a former
student of Thomson’s
from New Zealand, tested
his teacher’s theories in
his Gold Foil Experiment.
Ernest Rutherford
1871- 1937
© Fall 2005, Pflugerville ISD, 8th Grade
•He expected his alpha
particles to go straight
through the foil, and
most of them did.
Development of the Atomic Theory
•But some of the
particles were
deflected or bounced
straight back!
Rutherford’s Gold Foil
Experiment
© Fall 2005, Pflugerville ISD, 8th Grade
•This showed that a
nucleus with a
positive charge
makes up the center
of an atom.
Development of the Atomic Theory
Bohr, a Danish
scientist who worked
with Rutherford,
described the motion
of electrons around
the nucleus.
Niels Bohr 1885-1962
© Fall 2005, Pflugerville ISD, 8th Grade
Development of the Atomic Theory
Bohr’s Atomic Model
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Bohr said that
electrons orbit
the nucleus at
specific energy
levels, and can
move from one
level to another.
Development of the Atomic Theory
The current atomic
theory states that
there are regions
inside an atom
where electrons are
likely to be found.
Electron Cloud Model
© Fall 2005, Pflugerville ISD, 8th Grade
These regions are
called electron
clouds.
The Atom
What Do You Think?
What is the smallest thing
you have ever seen? How
does it compare to the size
of an atom?
© Fall 2005, Pflugerville ISD, 8th Grade
Parts of the Atom
The nucleus is the small,
dense, positively charged
center of the atom.
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Parts of the Atom
Protons are positively
charged particles in
the nucleus.
Each proton is equal
to 1 atomic mass unit.
© Fall 2005, Pflugerville ISD, 8th Grade
Parts of the Atom
Neutrons are particles
in the nucleus that
have no charge.
Protons and Neutrons
are the most massive
particles in the atom.
© Fall 2005, Pflugerville ISD, 8th Grade
Parts of the Atom
Electrons are negatively
charged particles found
in electron clouds
outside the nucleus.
Super small mass
compared to a proton.
1800 electrons = 1 proton
© Fall 2005, Pflugerville ISD, 8th Grade
Protons and Electrons
cancel each other out
making atoms neutral.
Atomic Number and Isotopes
• The number of
protons is equal to
the number of
electrons in atoms.
• All atoms of an
element have the
same number of
protons.
• The atomic number
of an atom is equal
to the number of
protons in the
nucleus.
• If atoms have the
same number or
protons but a
different number of
neutrons, it is called
an isotope.
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Isotopes
• Elements that are
isotopes are always
the same element.
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• A different number
of neutrons gives it
a different mass.
Isotope Practice
• Helium has 2 protons
and 2 neutrons. Its
atomic mass is 4.
• Carbon has 6 protons
and 6 neutrons. Its
atomic mass of 12.
• Helium-3 (isotope)
has an atomic mass
of 3.
• Carbon-14 (isotope)
has an atomic mass
of 14.
• How many neutrons
does it have?
• How many neutrons
does it have?
© Fall 2005, Pflugerville ISD, 8th Grade
Atomic Mass
• Every element has
an atomic mass.
• The atomic mass is
the sum of the
number of protons
and neutrons in the
nucleus of an atom.
© Fall 2005, Pflugerville ISD, 8th Grade
• The mass is the
average of the
masses of all the
isotopes of the
element.
Forces in Atoms
• The forces that are always pushing and
pulling on atoms are:
–
–
–
–
Gravitational force
Electromagnetic Force
Strong Force
Weak Force
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Forces in Atoms
• Gravitational Force
– Acts between atoms.
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• Electromagnetic Force
– Same charges repel
each other.
Forces in Atoms
• Strong Force
– Holds the nucleus
together.
© Fall 2005, Pflugerville ISD, 8th Grade
• Weak Force
– In some radioactive
elements, this allow
some neutrons can
turn into protons.