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How did the atomic theory develop and
change into the modern model of the atom?
Here come the atoms!
video
Science Standard 8.3.a: Students know the
structure of the atom and know it is
composed of protons, neutrons, and
electrons.
Science Standard 8.7.b: Students know each
element has a specific number of protons in
the nucleus (the atomic number) and each
isotope of the element has a different but
specific number of neutrons in the nucleus.
atom: smallest particle of an element.
electron: negatively charged particles in an
atom.
nucleus: the central core of an atom
proton: positively charged particles in an
atom’s nucleus.
energy level: the specific amount of energy
an electron has.
neutron: small particle in the nucleus of an
atom with no electrical charge
atomic number: the number of protons in
the nucleus of an atom.
isotope: atoms of the same element that
have different numbers of neutrons.
mass number: the sum of the protons and
neutrons in the nucleus of an atom.
Development of Atomic Theory
 Atomic theory has changed many times over
the years. As more evidence was collected, the
theory and models were revised.
 Around 430 B.C.,
 a Greek philosopher, Democritus,
 Idea= matter is formed of small pieces that could
not be cut into smaller parts.
 atomos which means “uncuttable”
Dalton’s Atomic Theory
thought that atoms were like smooth,
hard balls
that could not be
broken into smaller
pieces.
Dalton’s Atomic Theory
All elements are composed of atoms that cannot
be divided.
2. All atoms of the same element are exactly alike
and have the same mass. Atoms of different
elements are different and have different masses.
3. An atom of one element cannot be changed into
an atom of a different element. Atoms cannot be
created or destroyed in any chemical change,
only rearranged.
4. Every compound is composed of atoms of
different elements, combined in a specific ratio.
1.
Thomson’s Atomic Theory
In 1897, English scientist
J.J. Thompson found that
atoms contain negatively
charged particles.
The negatively charged
particles later became known as electrons.
Thomson suggested that atoms had
negatively charged electrons embedded
in a positive sphere.
Rutherford’s Atomic Theory
 In 1911, Ernest Rutherford found evidence that
countered Thompson’s model.
 His team discovered the
positive charge (proton) +
in an atom is clustered in
the center (nucleus).
According to Rutherford’s model, an atom was
mostly empty space.
Bohr’s Atomic Theory
In 1913, Niels Bohr, a Danish scientist,
suggested that electrons
move in specific orbits
around the nucleus
of an atom.
 Cloud Model
 In the 1920’s, scientists determined that electrons do not
orbit the nucleus like planets, but rather could be anywhere
in a cloudlike region.
 (SO WAS BOHR RIGHT?)
 An electron’s movement is
related to its energy level, or
the specific amount of energy
it has.
 Electrons of different energy
levels are likely to be found in
different places.
Bohr
Democritus
Thomas
• Atoms=
small and
uncuttable
• Electrons
move in
orbits
• Electrons
(-)
Dalton
Rutherford
Cloud Model
• Small
hard balls
• 4 rules of
elements
/atoms
• Protons
• Nucleus
• Mainly
empty
space
• Electrons are
in cloud like
formation
• Electrons have
different
energy levels
The Modern Atomic Model
 In 1932, English scientist James Chadwick
discovered another particle, the neutron, in the
nucleus of atoms.
The Modern Atomic Model
At the center of the atom is a tiny, massive
nucleus containing protons and neutrons.
Surrounding the nucleus is a cloudlike
region of moving electrons.
The particle was difficult to detect because
it has no charge.
Particles in an Atom
 An atom is composed of positively charged protons,
neutral neutrons, and negatively charged electrons.
Protons and neutrons are about equal in mass. An
electron has about 1/2,000 the mass of a proton or
neutron.
Isotopes
 Atoms of all isotopes of carbon contain six protons and six
electrons, but they differ in their number of neutrons.
Carbon-12 is the most common isotope.
 What is located in a
cloudlike region
surrounding the nucleus in
the modern atomic model?
electrons
 According to Dalton, all
elements are composed of
atoms that cannot be ___.
divided
 Answer #1-4
 Finish the
worksheet.
 Complete the extension.
EXTENSION
 Write a detailed SUMMARY of the section and
complete the UNANSWERED QUESTIONS section of
your notes.
 Choose two of the remaining Depth & Complexity
ICONS in your notes and explain how they relate to
this section.
 Finish the Development of Atomic Theory Timeline
Booklet.