Transcript Atomic Structure – The Nucleus!

Atomic Structure – The
Nucleus!
The Important Idea
• Matter is discontinuous – it cannot be subdivided
infinitely.
• Credited to Democritus of Abdera (ca. 400 BC) –
matter is composed of tiny pieces (atomos –
“indivisible”) that move in a void space.
– Atoms bounce off each other when they collide
– Have tiny velcro-like hooks on their surfaces that
allow them to stick together sometimes
– Change is the result of atoms recombining, not new
atoms being formed
The Modern Version
• John Dalton
(1766-1844)
• Dalton describes behavior
and nature of atoms in his
atomic theory:
1. Atoms are fundamental, indivisible, and
invisible.
2. Atoms of the same element are
identical.
3. Atoms of different elements are unique.
4. Atoms are not created or destroyed but
can recombine. When they combine
they do so in whole number ratios.
The Atom is Cracked
• Pure gases + electricity looks cool:
“cathode”
Evacuated and filled with a low
pressure gas.
Slits.
“anode”
The Atom is Cracked, part I
• Pure gases + electricity looks cool:
“cathode”
“anode”
•The green beam was known as a
“cathode” ray
•Cathode rays can be bent by magnetic
fields
•The cylinder at the lower right
contained an “electrometer”, a primitive
device that showed the presence of
negative charge.
• In this first experiment, Thomson
showed that cathode rays were
negatively charged
The Atom is Cracked, part II
• Electricity and magnetism are related.
• Thomson tries to use electric fields to bend the
beam (he was the first to do this successfully).
• The point is to show that the beam itself carries
the charge, and not that the beam and the
charge are two separate phenomena.
cathode
Electric Plates
Thomson’s Discovery (1898)
• Experiment 1 says cathode rays are negatively
charged
• Experiment 2 says the rays are made of matter
that is negatively charged
• Experiment 3 allows him to measure the mass to
charge ratio (m/z) of the particles that make up
the beam. They are 1000x smaller than the m/z
for hydrogen.
• Conclusion:
– There is a subatomic particle which has a low mass
and is negatively charged. It is common to all
elements: the “electron”
The Plum Pudding Model (1904)
Everyone eats
So everyone knows it
looks like
JJ Thomson (1899)
Electrons are
like fruits,
positive cloud is
“pudding”
Slices of Plum Pudding,
showing the fruits randomly
distributed
New Zealand – Not Just Sheep and
“Lord of the Rings”
• Ernest Rutherford, former student of
Thomson
• Noticed that a beam of alpha particles
became “fuzzy” when directed at a piece of
Mica
α
Fuzzy.
The Gold Foil Experiment
•
Rutherford gives (undergraduate) student Ernest Marsden a little project:
see if alpha particles could be reflected by metals.
+
Why alphas were deflected.
Gold Foil Results/Implications
• Atom is mostly empty space: most alphas were
neither scattered nor deflected.
• Atom has a small, massive, positively charged
object likely at its center (why not the center?).
This is inferred from the number, charge, and
angle of the particles deflected.
• Why gold? Other metals exhibited the same effect, but gold worked
nicely.
• Why alphas and not betas? Betas scattered too much (electron
cloud); beta scattering was well-known but alpha scattering was not.
Models
1. Democritus and the Velcro Ball
3. The Rutherford Atom
2. Thomson’s Plum Pudding