The Structure of the Atom
Chapter 4
4.1 Early Theories of Matter
The Philosophers
• Speculated about the nature of matter
• Many concluded that matter was composed of
earth, water, air, and fire
4.1 Early Theories of Matter
Democritus (460-370 BC)
• First to propose the idea that matter was not
infinitely divisible
• “atomos”
•
•
•
•
Atoms 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 differing properties of matter are due to the size, shape, and
movement of atoms.
• Apparent changes in matter results from changes in the groupings
of atoms and not from changes in the atoms themselves.
4.1 Early Theories of Matter
Aristotle (384-322 BC)
• Influential Greek philosopher
• Rejected Democritus’ ideas
• Thought matter was continuous
• Aristotle’s influence so great and the science so
primitive (lacking!) his denial for the existence of
atoms went largely unchallenged for 2000 years.
4.1 Early Theories of Matter
John Dalton (1766-1844)
• Revived and revised Democritus’s ideas based on research.
ATOMIC THEORY (1803)
1. All matter is composed of extremely small particles called
atoms.
2. All atoms of a given element are identical, having the
same size, mass, and chemical properties. Atoms of a
specific element are different from those of any other
element.
3. Atoms cannot be created, divided into smaller particles,
or destroyed.
4. Different atoms combine in simply whole-number ratios
to form compounds.
5. In a chemical reaction, atoms are separated, combined, or
rearranged.
4.1 Early Theories of Matter
John Dalton cont.
• Studied numerous chemical reactions… and
determined mass ratios of the elements involved in
the reactions.
Atoms explain the Law of Conservation of Mass!
Dalton has three errors in his theory:
1. Atoms ARE divisible.
2. Atoms of the same elements are not all identical.
3.
4.1 Early Theories of Matter
Defining the Atom
Atom – the smallest particle of an element that
still retains the properties of that element
Can see them with a scanning tunneling
microscope!
4.2 Subatomic Particles and the
Nuclear Atom
• Charles Goodyear’s discovery of rubber
vulcanization was an accident just like many
scientific discoveries!
4.2 Subatomic Particles and the
Nuclear Atom
• Partially evacuated
tube
• Pass electricity
through tube
• Metal electrodes at
each end
• William Crookes
• Light-producing
coating at end
• “Cathode ray”
• TV’s use cathode
rays!
4.2 Subatomic Particles and the
Nuclear Atom
4.2 Subatomic Particles and the
Nuclear Atom
• By the end of the 1800’s…
1. Cathode Rays were actually a stream of charged
particles.
2. The particles carried a negative charge (actual charge
not known).
4.2 Subatomic Particles and the
Nuclear Atom
They determined that the ray’s negative
particles were found in all forms of matter
because…
1. Changing the type of electrode …
2. Varying the gas in the tube…
did not affect the cathode ray produced
Fig. 4-8 Multiple Experiments helped determine
the properties of cathode rays.
Observation
Interpretation
A.
B.
C.
i.e. Electrons were
discovered!
4.2 Subatomic Particles and the
Nuclear Atom
JJ Thomson (1856-1940)
1890’s – completed a series of experiments that
determined the charge-to-mass ratio of the electron.
- Compared…to other known ratios. Concluded
that the mass of the charged particle was much
less than that of a hydrogen atom.
- SHOCKING! Because… there are particles smaller
than atoms! Atoms are divisible!
4.2 Subatomic Particles and the
Nuclear Atom
Robert Milikan (1868-1953)
1909 – determined the charge of the electron
Within 1% of currently accepted value!
Knowing JJ’s charge-to-mass ratio he calc’d mass
of an electron
9.2 x 10-28 g = __1__ mass of H atom
1840
4.2 Subatomic Particles and the
Nuclear Atom
4.2 Subatomic Particles and the
Nuclear Atom
Some questions Thomson’s and Milikan’s work
raised…
1. If electrons are part of all matter an they
possess a negative charge, how is it that all
matter is neutral?
2. If the mass of an electron is so extremely
small, what accounts for the rest of the mass
in a typical atom?
4.2 Subatomic Particles and the
Nuclear Atom
JJ Thomson’s Model:
Fig. 4-9
Credited with discovery
of the electron due to
his ratio.
4.2 Subatomic Particles and the
Nuclear Atom
The Nuclear
Atom –
Ernest
Rutherford
(1871-1937)
Gold Foil Expt.
1911
4.2 Subatomic Particles and the
Nuclear Atom
Rutherford predicts
the alpha particles
go straight through
because JJ’s model
has diffuse positive
charge.
Fig. 4-10 Expected Results
4.2 Subatomic Particles and the
Nuclear Atom
Fig. 4-11
Set-up &
Results
Diffuse positive
charge cannot
explain
deflections at
large angles.
Fig. 4-10 Expected Results
Fig. 4-12 Explanation of
Results
Observation
1)
2)
Interpretation
4.2 Subatomic Particles and the
Nuclear Atom
Rutherford’s Model:
• Nucleus at center (positive)
• Mostly empty space in which electrons travel
• Atom diameter is two football fields; nucleus
is a nickel!
Completing the atom – The discovery
of protons and neutrons
1920 – Rutherford discovers the proton
1932 – James Chadwick discovers the neutron
Atom Basics:
nucleus – 99.97% atom’s mass
protons = electrons (atoms are neutral)
Table 4-1: Properties of Subatomic
Particles
Particle
Symbo Location Relative
l
electrical
charge
Electron
e-
Proton
p+
Neutron
no
Outside
nucleus
In
nucleus
In
nucleus
Actual
Charge
(C)
Relative
mass
Actual
Mass
-1
1/1840
+1
1
9.11 x
10-28
1.673 x
10-24
0
1
1.675 x
10-24