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Developing a Model of an Atom:
Ernest Rutherford
 In the early 1900s, Ernest Rutherford designed an
experiment to test the Thomson model.
Developing a Model of an Atom:
Ernest Rutherford
 In the early 1900s, Ernest Rutherford designed an
experiment to test the Thomson model.
 To test the model, Rutherford aimed a beam of
positively charged particles at a thin sheet of gold
foil.
Developing a Model of an Atom:
Ernest Rutherford
 In the early 1900s, Ernest Rutherford designed an
experiment to test the Thomson model.
 To test the model, Rutherford aimed a beam of
positively charged particles at a thin sheet of gold
foil.
 Rutherford placed a
detector around the gold
foil so that he could tell
where a particle went
after it passed through
the gold foil.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford reasoned that if the Thomson model
accurately described atoms, then all of the particles
should pass straight through the foil.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford reasoned that if the Thomson model
accurately described atoms, then all of the particles
should pass straight through the foil.
 However, he observed something unexpended.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford reasoned that if the Thomson model
accurately described atoms, then all of the particles
should pass straight through the foil.
 However, he observed something unexpended.
 Most of the particles did pass
through the foil, just as
predicted.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford reasoned that if the Thomson model
accurately described atoms, then all of the particles
should pass straight through the foil.
 However, he observed something unexpended.
 Most of the particles did pass
through the foil, just as
predicted.
 But he was very surprised to
observe that many particles
were deflected, some of them
even bouncing straight back at the source.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford reasoned that these particles struck
compact particles in the gold foil that also had a
positive charge.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford reasoned that these particles struck
compact particles in the gold foil that also had a
positive charge.
 There fore, Rutherford concluded that not only was
the mass of an atom found in its nucleus, but also that
the nucleus contained positively charged particles.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford reasoned that these particles struck
compact particles in the gold foil that also had a
positive charge.
 There fore, Rutherford concluded that not only was
the mass of an atom found in its nucleus, but also that
the nucleus contained positively charged particles.
 Rutherford called these positive charged protons, after
a Greek word meaning “first.”
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford reasoned that these particles struck
compact particles in the gold foil that also had a
positive charge.
 There fore, Rutherford concluded that not only was
the mass of an atom found in its nucleus, but also that
the nucleus contained positively charged particles.
 Rutherford called these positive charged protons, after
a Greek word meaning “first.”
 A proton is a positively
charged particle that is
found in the nucleus of an atom.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford’s observations suggested that a new atomic
model was needed.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford’s observations suggested that a new atomic
model was needed.
 In Rutherford’s model, almost all of the mass of the atom
is contained in a tiny, positively charged region in the
center of the atom, called the nucleus.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford’s observations suggested that a new atomic
model was needed.
 In Rutherford’s model, almost all of the mass of the atom
is contained in a tiny, positively charged region in the
center of the atom, called the nucleus.
 According to Rutherford, the elements
move in the vast space that makes up
most of an atom’s size.
Developing a Model of an Atom:
Ernest Rutherford
 Rutherford’s observations suggested that a new atomic
model was needed.
 In Rutherford’s model, almost all of the mass of the atom
is contained in a tiny, positively charged region in the
center of the atom, called the nucleus.
 According to Rutherford, the elements
move in the vast space that makes up
most of an atom’s size.
 In fact, the nucleus is so small that if an
atom were the size of an athletic
stadium, then its nucleus would be only
the size of a pea.
Developing a Model of an Atom:
James Chadwick
 A third subatomic particle was discovered
years later by James Chadwick.
a few
Developing a Model of an Atom:
James Chadwick
 A third subatomic particle was discovered
years later by James Chadwick.
 Chadwick’s experiments demonstrated
the nucleus had more mass than the of
protons suggested that there should be.
a few
that
Developing a Model of an Atom:
James Chadwick
 A third subatomic particle was discovered
years later by James Chadwick.
 Chadwick’s experiments demonstrated
the nucleus had more mass than the of
protons suggested that there should be.
a few
that
 Chadwick made another observation – while the mass
was greater than expected, the charge of the atom did
not change.
Developing a Model of an Atom:
James Chadwick
 A third subatomic particle was discovered
years later by James Chadwick.
 Chadwick’s experiments demonstrated
the nucleus had more mass than the of
protons suggested that there should be.
a few
that
 Chadwick made another observation – while the mass
was greater than expected, the charge of the atom did
not change.
 This suggested to Chadwick that there was another
subatomic particle in the nucleus.
Developing a Model of an Atom:
James Chadwick
 Chadwick described this particle as the
same mass as proton but having no
electrical charge.
Developing a Model of an Atom:
James Chadwick
 Chadwick described this particle as the
same mass as proton but having no
electrical charge.
 He called the particle a neutron.
Developing a Model of an Atom:
James Chadwick
 Chadwick described this particle as the
same mass as proton but having no
electrical charge.
 He called the particle a neutron.
 Because each proton or neutron has a
mass that is
about 2000 times as large as the mass of an electron,
almost all of the mass of the atoms is located in the
nucleus.
Developing a Model of an Atom:
James Chadwick
 Chadwick described this particle as the
same mass as proton but having no
electrical charge.
 He called the particle a neutron.
 Because each proton or neutron has a
mass that is
about 2000 times as large as the mass of an electron,
almost all of the mass of the atoms is located in the
nucleus.
 Although many other subatomic particles have been
discovered, electrons, protons, and neutrons are the
basic structures that make up the modern model of the
atom.
Developing a Model of the Atom:
Niels Bohr
 Rutherford’s model placed the electrons in the area
around the nucleus but did not describe any specific
locations.
Developing a Model of the Atom:
Niels Bohr
 Rutherford’s model placed the electrons in the area
around the nucleus but did not describe any specific
locations.
 As scientists made more observations about atoms and
elements, they were able to add more information to
the model.
Developing a Model of the Atom:
Niels Bohr
 Rutherford’s model placed the electrons in the area
around the nucleus but did not describe any specific
locations.
 As scientists made more observations about atoms and
elements, they were able to add more information to
the model.
 One important observation was that atoms absorb and
release energy.
Developing a Model of the Atom:
Niels Bohr
 Rutherford’s model placed the electrons in the area
around the nucleus but did not describe any specific
locations.
 As scientists made more observations about atoms and
elements, they were able to add more information to
the model.
 One important observation was that atoms absorb and
release energy.
 Niels Bohr used this observation
to develop the revised atomic
mode.
Developing a Model of the Atom:
Niels Bohr
 Niels Bohr used this observation to develop the revised
atomic model.
Developing a Model of the Atom:
Niels Bohr
 Niels Bohr used this observation to develop the revised
atomic model.
 According to the Bohr atomic model, also known as the
planetary model, electrons move around the nucleus in
specific orbits just as the planets move around the Sun in
specific orbits.
Developing a Model of the Atom:
Niels Bohr
 Niels Bohr used this observation to develop the revised
atomic model.
 According to the Bohr atomic model, also known as the
planetary model, electrons move around the nucleus in
specific orbits just as the planets move around the Sun in
specific orbits.
 These orbits are called energy levels.
Developing a Model of the Atom:
Niels Bohr
 Niels Bohr used this observation to develop the revised
atomic model.
 According to the Bohr atomic model, also known as the
planetary model, electrons move around the nucleus in
specific orbits just as the planets move around the Sun in
specific orbits.
 These orbits are called energy levels.
 Electrons can move from one energy level
to another but only if energy is involved.
Developing a Model of the Atom:
Niels Bohr
 Niels Bohr used this observation to develop the reviesed
atomic model.
 According to the Bohr atomic model, also known as the
planetary model, electrons move around the nucleus in
specific orbits just as the planets move around the Sun in
specific orbits.
 These orbits are called energy levels.
 Electrons can move from one energy level
to another but only if energy is involved.
 An electron moving form one orbit to
another absorbs or releases energy.