Transcript History of Atomic Structure

History of Atomic
Theory
Essential Question
• How did the atomic theory evolve from
Dalton's Postulates to the current modern
atomic theory?
Ancient Philosophy
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Who: Aristotle, Democritus
When: More than 2000 years ago
Where: Greece
What: Aristotle believed in 4 elements: Earth,
Air, Fire, and Water. Democritus believed
that matter was made of small particles he
named “atoms”.
• Why: Aristotle and Democritus used
observation and inference to explain the
existence of everything.
Democritus
• Matter is composed of empty space through which
atoms move.
• Atoms are solid, homogeneous, indestructible,
and indivisible.
• Different kinds of atoms have different sizes and
shapes.
• The different properties of matter are due to the
size, shape, and movement of atoms.
• Apparent changes in matter result from changes
in the groupings of atoms and not from changes
in the atoms themselves.
Democritus
Aristotle
Alchemists
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Who: European Scientists
When: 800 – 900 years ago
Where: Europe
What: Their work developed into what is now
modern chemistry.
• Why: Trying to change ordinary materials
into gold.
Alchemic Symbols
Particle Theory
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Who: John Dalton
When: 1808
Where: England
What: Described atoms as tiny particles that
could not be divided. Thought each element
was made of its own kind of atom.
• Why: Building on the ideas of Democritus in
ancient Greece.
John Dalton
• Matter composed of small particles (atoms)
• Atoms of an element have the same size, mass, and
chemical properties.
• Atoms cannot be created, divided into smaller particles,
or destroyed.
• Atoms of different elements combine in whole number
ratios to form compounds (H2O, CO2, MgO, etc.).
• In a chemical reaction, atoms are separated, combined
or rearranged.
John Dalton
Discovery of Electrons
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Who: J. J. Thomson
When: 1897
Where: England
What: Thomson discovered that electrons
were smaller particles of an atom and were
negatively charged.
• Why: Thomson knew atoms were neutrally
charged, but couldn’t find the positive
particle.
Cathode Ray Tube
- Cathode rays are a stream
of charged particles
- Particles carry a negative
charge (electrons)
- Thomson concluded that
electrons were smaller
than a hydrogen atom
(lightest known atom)
- Conclusion: Electrons are
smaller than atoms
- Dalton was wrong: Atoms
were divisible into smaller
subatomic particles
The Atom, circa 1900:
• “Plum pudding” model,
put forward by Thomson.
• Positive sphere of
matter with negative
electrons imbedded in
it.
Plum
Pudding
J. J. Thomson
Atomic Structure I
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Who: Ernest Rutherford
When: 1911
Where: England
What: Conducted an experiment to isolate
the positive particles in an atom. Decided
that the atoms were mostly empty space, but
had a dense central core.
• Why: He knew that atoms had positive and
negative particles, but could not decide how
they were arranged.
Discovery of the Nucleus:
Gold Foil Experiment
Ernest Rutherford
shot  particles at a
thin sheet of gold foil
and observed the
pattern of scatter of
the particles.
(Zinc sulfide)
The Nuclear Atom
Since some particles
were deflected at
large angles,
Thomson’s model
could not be correct.
THE NUCLEAR MODEL
According to Rutherford,
most of the atom
consists of electrons
moving rapidly through
empty space and a
very dense, tiny,
positively charged
nucleus.
Ernest Rutherford
Other Subatomic Particles
• By 1920 , Rutherford had refined his
concept of the nucleus: He concluded
that the very dense nucleus contained
positively particles called protons.
• James Chadwick (1932), a coworker,
showed that the nucleus also contained
a neutral particle. This was the neutron
– a particle with nearly equal mass as a
proton.
Atomic Structure II
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Who: Niels Bohr
When: 1913
Where: England
What: Proposed that electrons traveled in
fixed paths around the nucleus. Scientists
still use the Bohr model to show the number
of electrons in each orbit around the nucleus.
• Why: Bohr was trying to show why the
negative electrons were not sucked into the
nucleus of the atom.
Bohr Model
THE BOHR MODEL
Niels Bohr
Electron Cloud Model
• Electrons travel around the nucleus in
random orbits.
• Scientists cannot predict where they will be
at any given moment.
• Electrons travel so fast, they appear to form
a “cloud” around the nucleus.
Electron Cloud Model
Properties of Subatomic Particles
Symbol Location Charge
Relative
mass
Actual mass
(g)
Electron
e-
Around
nucleus
1–
1
1840
9.11×10-28
Proton
p+
Nucleus
1+
1
1.673×10-24
Neutron
n0
nucleus
0
1
1.675×10-24
Particle
Atomic Structure Timeline
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Atomic Structure and the
Periodic Table
Symbol
14
Si
28.086
Silicon
Atomic number = # protons
Name
Atomic Mass: Round to a whole #:
28 = Relative mass of nucleus
-14 = # protons
14 = # neutrons
Practice From the Periodic
Table
Complete the following chart:
Name
Relative
Symbol
mass
#
#
#
protons neutrons electrons
Lithium
Mg
26
16