Transcript History_of_Atomic_Structure

How long have people been interested in
understanding matter and its structure?
A. Thousands of years
B. Hundreds of years
C. A few years
D. Never
History of Atomic
Structure
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 inferrence to explain the
existence of everything.
Democritus
Aristotle
Early Greek Theories
• 400 B.C. - Democritus thought matter
could not be divided indefinitely.
• This led to the idea of atoms in a void.
fire
Democritus
earth
air
water
• 350 B.C - Aristotle modified an earlier
theory that matter was made of four
“elements”: earth, fire, water, air.
Aristotle
• Aristotle was wrong. However, his theory
persisted for 2000 years.
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
John Dalton
• 1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
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All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine in
constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the Law of Conservation of
Mass
The Law of Conservation of Mass
Atoms are neither created nor
destroyed
The Law of Constant Proportions
Atoms in compounds are in fixed
ratios…CO2 is a different ration
than CO.
Discovery of Electrons
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Who: J. J. Thompson
When: 1897
Where: England
What: Thompson discovered that electrons
were smaller particles of an atom and were
negatively charged.
• Why: Thompson knew atoms were neutrally
charged, but couldn’t find the positive
particle.
J. J. Thompson
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.
Ernest Rutherford
Ernest Rutherford
• Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance
path of invisible
-particles
Most particles passed through.
So, atoms are mostly empty.
Some positive -particles
deflected or bounced back!
Thus, a “nucleus” is positive &
holds most of an atom’s mass.
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.
Niels Bohr
Bohr’s model
• Electrons orbit the nucleus in “shells”
• Electrons can be bumped up to a higher shell if
hit by an electron or a photon of light.
There are 2 types of spectra: continuous spectra & line
spectra. It’s when electrons fall back down that they
release a photon. These jumps down from “shell” to
“shell” account for the line spectra seen in gas discharge
tubes (through spectroscopes).
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.
The Atom
• All matter is composed of tiny particles called
Atoms.
• All atoms are composed of smaller particles:
– Protons- atomic particles with a positive charge.
– Neutrons- atomic particles with no charge.
– Electrons- atomic particles with a negative charge
Atomic Number
• The atomic number- depends on the number
of protons in a type of element.
– In an electrically neutral atom, the number or electrons is
the same at the number of protons.
• Number of protons = number of electrons
Atomic Number
Atomic Mass
Atomic Mass
• Atomic mass is based on the number of
protons and neutrons in an atom of an
element.
A certain element has an atomic mass of 16 and an atomic
number of 8. The atomic number equals the number of
protons in the element’s atoms. To find the number of
protons, subtract the atomic number (8) from the atomic
mass (16).
16
- 8
8 neutrons
Dalton’s Theory
• All matter is made of atoms. Atoms are indivisible
and indestructible.
• All atoms of a given element are identical in mass
and properties
• Compounds are formed by a combination of two or
more different kinds of atoms.
• A chemical reaction is a rearrangement of atoms.
J.J. Thompson’s Theory
• Discovered a negatively charged particle in
the atom, that he named the electron.
• Chocolate Chip Cook Model of the Atom.
• Positive and negative charges are distributed
evenly throughout the atom.
Lord Rutherford’s Theory
• Gold Foil Experiment:
– Fired alpha particles at a piece of gold foil.
– Most particles when through the foil, but about 1 in
10,000 got deflected.
• The new evidence allowed him to formulate a
new atomic model with a central nucleus.
• He suggested that the dense positive charge
was in the center and the negatively charged
electrons were being held in place by attractive
forces.
– Remember, opposites attract!