How Humans Learned about…
In the late 1800s, scientists such as J.J. Thomson
were using cathode ray tubes to study more about the
nature of matter.
1897: J.J. Thomson experiments with a
cathode ray tube.
Thomson’s experimental results prompted him
to propose a model of matter:
Thomson proposed a
model, sometimes called
the "plum pudding" or
"raisin cake" model, in
which thousands of tiny,
negatively charged spheres
swarm inside a cloud of
mass-less positive charge.
Yes, on a
microscopic
level, all matter
is like a raisin
cake.
Ernest Rutherford and His
Experiment
• In 1911 Ernest Rutherford thought it would prove interesting to
bombard atoms with alpha rays, figuring that this experiment
could yield information about the inside of the atom. He used
radium as the source of the alpha particles and shined them
onto the atoms in gold foil. Behind the foil sat a fluorescent
screen for which he could observe the alpha particles impact.
• Rutherford thought
• he would see this:
Instead, he saw this:
Rutherford Discovers the Nucleus
• Alpha particles that deflected right back must have hit something
with a strong positive charge.
Rutherford then theorized that there was something called a nucleus,
which contained a high density of positively charged particles.
In fact Rutherford speculated that the atom was like
a miniature solar system, where the electrons
orbited like little planets.
The Mystery of Light Emission
and How Niels Bohr Solved It
Scientists of the 19th century discovered that when an
electrical current passes through a small quantity of a gas
in a glass tube, the atoms in the gas emit light. BUT this
radiation occurs only at certain specific, or discrete,
wavelengths, and different elements and compounds emit
different wavelengths, depending on the element.
Argon
Xenon
The lines are at specific wavelengths that
are characteristic of that element.
Niels Bohr, a young scientist
working in Rutherford’s laboratory,
set out to understand the emission of
radiation at these wavelengths based
on the nuclear model of the atom.
The Bohr Atom
Using Rutherford’s model of the atom as a miniature solar system, Bohr
developed a theory by which he could predict the same wavelengths scientists
had measured radiating from atoms with a single electron. He concluded that
because atoms emit light only at discrete wavelengths, electrons could only
orbit at certain designated radii, and light could be emitted only when an
electron jumped from one of these designated orbits to another.
The Bohr atom was an improvement over
the Rutherford atom, and it most certainly
was an improvement over JJ Thomson’s raisin
cake model.
However, some important and VERY odd
phenomena still needed explaining,
and Bohr’s model couldn’t do it.
It seems that every object (including YOU) has a particle nature
AND a wave nature.
For very small objects such as the electron and the photon
(photons are light packets) the wave nature is as significant
as the particle nature.
So how should the electron be described,
as a wave OR a particle???
A new and RADICAL model of the
atom was soon to come along
That would take into account
both the wave and particle nature
of matter…..
the
The work of a number of scientists contributed to
the development of the quantum mechanical model:
Albert Einstein
Louis de Broglie
Wolfgang Pauli
Max Planck
Werner Heisenberg
Enrico Fermi
THE QUANTUM MECHANICAL MODEL
OF THE ATOM
The quantum mechanical view of atomic
structure does keep some of Rutherford and
Bohr’s ideas. The nucleus is still at the
center of the atom and provides the
electrical attraction that binds the electrons
to the atom.
And specific energy levels are associated
with each electron shell.
Contrary to Bohr’s theory, however, the
electrons do not circulate in definite
planet-like orbits.
Rats..
Rats
The quantum-mechanical approach to
the atom provides the framework for
viewing the electrons as…
fuzzy clouds of negative charge.
Electrons still have assigned states of motion, but
these states of motion DO NOT correspond to
fixed orbits.
Instead, they tell us something about the geometry of the
electron cloud—its size and shape and whether it is spherical
or bunched in lobes like a figure eight.
Physicists called
these states of
motion
atomic orbitals.
But how did we come to learn the
shapes of the electron clouds,
or orbitals?
Enter…
Erwin Schroedinger and His Famous Equation
Schroedinger’s
equation, when solved,
gives us the geometry
of the electron orbitals
(clouds).
Four types of orbitals, or electron
cloud structures, exist in the atom:
s orbitals
p orbitals
d orbitals
f orbitals
One final strange feature of the quantum mechanical
model of the atom is that you can never know exactly
where the particle version of the electron is inside its
orbital.
This strange feature is called
the Heisenberg Uncertainty Principle.
Cartoon about
the Heisenberg Uncertainty Principle:
Get it?