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History of Atomic Models
Democritus
(460-360 BC)
Democritus
All
things are composed of
minute, invisible and
indivisible particles, atoma,
(indivisibles) in constant
motion through otherwise
empty space, kenon.
Aristotle
Argued against atomic
theory and believed
everything was
composed of the 5
elements.
 Highly influential for
2000 years.

Dalton 1800s
Chemicals are
composed of
elements combined
together in certain
proportions.
 billiard ball
model of atoms

Sir Joseph John Thomson
1900
Analyzed the nature
of cathode rays.
 He noticed that they
had particle like
properties and were
identical regardless
of the material
producing the rays.

Plum-Pudding Model
 Lord
Kelvin and JJ Thomson
concluded that the atom was
composed of tiny negatively
charged electrons and positive
charge mixed together. This was
called the Plum-Pudding Model.
Plum-Pudding Model
Ernest Rutherford
Rutherford Gold Foil (Geiger
Muller) Experiment
1911
 Ernest Rutherford proposed shooting
alpha particles at extremely thin gold
foil.
 Most went right through with little
deflection, but some bounced back.
 This result was thought to be like
cannon balls bouncing off tissue paper!

Alpha particles should be undeflected but observed to bounce back
Rutherford Atomic Model
 The
alpha particles must have
bounced off an area of the atom
that is positively charged and
extremely dense.
 Rutherford concluded that the atom
has a dense positive nucleus with
electrons orbiting around.
Rutherford Model of the Atom
Niels Bohr
Bohr Model 1913
 Light
(electromagnetic radiation) is
emitted from matter at particular
energies, or quanta (from Planck).
 Light is produced when electrons
move from high energy states to
low energy states. E=hf
 Electrons exist in particular orbits.
Bohr Energy E=-13.6eV/n2

What is the energy of the emitted
photon when an electron drops from
the 3rd to the first orbital? What is the
wavelength? is it visible?
De Broglie said that electrons act like waves.
Waves resonate if the frequency is just right
for the container (musical instrument).
The orbital is the container for the electron
and only certain orbitals will work for
electrons of a certain energy.
the wavelength of a particle l=h/p plank’s
constant divided by its momentum.
De Broglie derived this relationship from
E=hf, the energy of a photon and p=E/c =h/l
the momentum of a photon so that the waveparticle duality that Einstein applied to
photons also applied to particles.
Modern Atomic Model
Quantum Mechanics shows how there is a
limit to our ability to determine the motion
and position of particles. (Heisenberg)
 The motion of objects is described using a
wave equation, y , where y2 is the probability
distribution of the object. Graphing y2 for an
electron around the hydrogen atom we obtain
orbital shapes. (Schrödinger)

Inside the Nucleus
Chadwick 1931 discovered that the
nucleus contains protons and neutrons.
 Protons and neutrons are composed of
quarks (1964 Gell-Mann and Zweig).
 The strong force between the quarks is
what holds the nucleus together and
produces massive amounts of energy in
a nuclear reaction.

Homework p1049 q 46-48, 54,