Download Atomic Physics

Atomic Physics
Demo. Young’s double-slit interference. Discuss.
Young’s Double-Slit Experiment
Laser light is passed through
two slits not far from one
another. An interference
pattern is observed on a screen
placed at about 1.0 m away.
Two slits act as new sources of
spherical waves. These new
waves interfere to produce
constructive interference
(bright spots) and destructive
interference (dark spots). This
experiment was proof that light
behaved like a wave.
Davisson-Germer Experiment
This experiment demonstrated the wave nature of
the electron, confirming the earlier hypothesis of
deBroglie. Putting wave-particle duality on a firm
experimental footing, it represented a major step
forward in the development of quantum mechanics.
Phet animation: Electron Diffraction. Discuss.
Photon
𝐸 = ℎ𝑓
Photoelectric Effect
1.8𝑒𝑉
2.3𝑒𝑉
𝑲𝑬 = 𝒉𝒇 − 𝑾
3.1𝑒𝑉
𝑾 = 𝟐. 𝟎𝒆𝑽
The photoelectric effect is the observation that
many metals emit electrons when light shines upon
them. Electrons emitted in this manner can be
called photoelectrons.
Phet animation: Photoelectric effect. Discuss.
Thompson’s Model for the Atom
J.J. Thompson’s plum
pudding model consists
of a sphere of positive
charge with electrons
embedded inside.
This model would
explain that most of
the mass was positive
charge and that the
atom was electrically
neutral.
Positive
pudding
Electron
Thompson’s
plum pudding
The size of the atom
(10-10 m) prevented
direct confirmation.
Rutherford’s Experiment
The Thompson model was abandoned in
1911 when Rutherford bombarded a thin
metal foil with a stream of positively charged
alpha particles.
Rutherford Scattering Exp.
Alpha source
Gold foil
Screen
Most particles
pass right
through the
foil, but a few
are scattered
in a backward
direction.
Phet animation: Rutherford’s experiment. Discuss.
The Nucleus of an Atom
If electrons were distributed uniformly, particles
would pass straight through an atom. Rutherford
proposed an atom that is open space with positive
charge concentrated in a very dense nucleus.
Alpha scattering
-
+
Gold foil Screen
-
Electrons must orbit at a distance in order not to
be attracted into the nucleus of atom.
Failure of Classical Model
When an electron is accelerated
by the central force, it must
radiate energy.
v
+
Nucleus
-
e-
The loss of energy should
cause the velocity v to decrease, sending the electron
crashing into the nucleus.
This does NOT happen and
the Rutherford atom fails.
The Bohr Atom & De Broglie
Atomic spectra indicate that atoms emit or absorb
energy in discrete amounts. In 1913, Niels Bohr
explained that classical theory did not apply to
the Rutherford atom.
An electron can only have
certain orbits and the atom
must have definite energy
levels which are analogous
to standing waves.
De Broglie 𝜆 = ℎ = ℎ
𝑝 𝑚𝑣
wavelength
e+
Electron orbits