Developing the Atomic Model
History of the Atom VideoAluminum Foil Activity
• At your lab bench take your piece of foil,
and tear it into 3-4 smaller pieces.
• Continue to tear your foil into smaller and
smaller pieces.
• What is left over after you cannot tear the
foil any longer?
Defining the Atom
The Greek philosopher Democritus was among
the first to suggest the existence of atoms (from the Greek word “atomos”)
Atoms were indivisible and indestructible
Different kinds of atoms for every substance His ideas did agree with later scientific theory,
Dalton’s Atomic Theory
(experiment based!)
• Matter made of particles that cannot be split into smaller
particles
• Atoms cannot be created or destroyed
John Dalton (1766 – 1844)
Dalton’s Atomic Theory
(experiment based!)
• Atoms of same element have same properties
Different element – different properties
• In chemical rxns, atoms are combined, separated, or
rearranged – but never changed into atoms of another element.
John Dalton (1766 – 1844)
Think About it
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What are similarities or differences
between Democritus and Dalton?
Structure of Atoms
One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles:
Electrons (e-),
protons (p+),
neutrons(n0)
are examples of these fundamental particles There are many other types of particles, but
Discovery of the Electron (e
-)
In 1897, J.J. Thomson used a cathode ray
tube to deduce the presence of a negatively charged particle: the electron (e-)
Modern Cathode Ray Tubes
Cathode ray tubes pass electricity through a gas that is contained at a very low
pressure.
Television Computer Monitor
Thomson’s Atomic Model
Thomson believed that the e- were like
plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
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Plum Pudding Model
• Developed by J.J. Thomson (early 1900’s) • Negative charge in atom
• Reasoned there is also positive charge to
balance the negative charge
• Proposed a model of a positively charged
sphere with electrons spread throughout
Discovery of the electron video
Sticky Tape Activity
• DO NOT write on the class copy.
• You may write on the other handout I
provide.
• Be sure to completely answer all of the
questions, then see me to have it checked off.
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Ernest Rutherford’s
Gold Foil Experiment - 1911
Alpha particles are He nuclei - The alpha
particles were fired at a thin sheet of gold foil
Particles that hit on the detecting screen
(film) are recorded
Rutherford’s problem:
In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?
Target #1
Target #2
The Answers:
Rutherford’s Experiment video
The Rutherford Atomic Model
Based on his experimental evidence:
– The atom is mostly empty space – All the positive charge (protons),
and almost all the mass is
concentrated in a small area in the center. He called this a “nucleus”
– this model is the first to establish the
nucleus in the center of the atom, with negatively charged electrons orbiting it, as planets orbit the sun.
Rutherford’s Findings
a) The nucleus is small b) The nucleus is dense
c) The nucleus is positively charged
Most of the particles passed right through. A few particles were deflected.
VERY FEW were greatly deflected.
“Like howitzer shells bouncing off of tissue paper!”
The Rutherford Atomic Model
Based on his experimental evidence:
– The e- orbited around the nucleus, and
occupy most of the volume
The Atom
• If a hydrogen atom’s nucleus was the size
of a marble the outer edge of the atom would be nearly a football field away!
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The Atom
• So what is most of an atom made up of?? • So what is our brain made out of?
In a picture
Draw what you think an
atom looks like based on
what you now know.
Scale Visual
• The Scale of the Universe!
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Nuclear Model Review
• Developed by Sir Ernest Rutherford in
1911
• Rutherford Backscattering Experiment • Nucleus found to carry atom’s mass
• Positively charged nucleus surrounded by
electrons
• Electrons occupy the bulk of the atom's
Bohr Model
• After Rutherford established that atoms
consisted of a cloud of negatively charged electrons surrounding a small, dense,
positively charged nucleus, there was still
confusion as to how the electrons surrounded the nucleus.
• Therefore, in 1913 Niels Bohr suggested that
electrons could only have certain motions.
Bohr Model
• The Bohr model states that electrons orbit
the nucleus at set distances.
• Electrons are in ‘energy levels’ or ‘shells’
• Electrons can jump energy levels and emit
light!
• Specific number of electrons are allowed
in each energy level
Bohr Model
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James Chadwick
• Won Nobel Prize in 1935 for
“Discovering the Neutron”
• Student of Rutherford; they noticed that the atomic
# (# of protons) was less than atomic mass.
• Neutron made up rest of mass of nucleus
• Actually discovered the neutron in 1932
• Chadwick put a piece of beryllium in a vacuum
chamber with some polonium. The polonium emitted alpha rays, which struck the beryllium.
When struck, the beryllium emitted the mysterious neutral rays.
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Quantum Model
• Developed by Edwin Schrodinger in 1926
• Energy levels are actually ‘clouds’
• Electrons are in orbitals
• Only 2 electrons per orbital
Quantum Model
• The Bohr model was a one-dimensional
model.
• Schrödinger's model allowed the electron
to occupy three-dimensional space. It therefore required three coordinates, or three quantum numbers, to describe the orbitals in which electrons can be found.
Summary of Important People
• Create a chart in your notes of the
different people we have talked about • Example below:
32
Name Model Name Experiment Contribution to Atomic Theory
Democritus Based on his
mental thoughts No experiment -1
st person to talk
about atoms
- Said they were indivisible, and the smallest piece of matter
