Quantum Mechanics: The Structure Of Atoms
Ανέβηκε στις 2 Ιαν 2010
http://www.facebook.com/ScienceReason ... Quantum Mechanics (Chapter 2): The Structure Of Atoms.
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1. A Brief History Of Quantum Mechanics
http://www.youtube.com/watch?v=B7pACq...
2. The Structure Of Atoms
http://www.youtube.com/watch?v=-YYBCN...
3. Wave Function And Wave-Particle Duality
http://www.youtube.com/watch?v=7GTCus...
4. The Uncertainty Principle
http://www.youtube.com/watch?v=Fw6dI7...
5. The Spin Of Fundamental Particles
6. Quantum Entanglement
---
Atomic Structure
It
was during the early decades of the 19th century that the structure of
atoms was coming into focus. It was known for example that a hydrogen
atom contained one proton and one electron. But the scientists of the
time could think of no stable arrangement of the two particles.
It
was known that protons in any atom were grouped in a small central
region called the nucleus and that the electrons were somehow arranged
at comparatively large distances outside the nucleus. But, in hydrogen,
if the electron were stationary, it would fall into the nucleus since
the charges on the particles would cause them to attract one another.
Yet
the electron couldnt be in an orbit circling the nucleus either.
Circular motion requires constant acceleration of the circling body to
keep it from flying away. But the electron has charge and charged
particles radiate light when they are accelerating. So an electron in a
circular orbit would radiate light and would spiral into the nucleus.
Bohr Atom
Niels
Bohr proposed the first working model of the hydrogen atom. In the Bohr
model, the electron circles the nucleus as if it were a planet going
around the sun. And with a nod to the energy quantization that Max
Planck dreamed up for solving the Ultraviolet Catastrophe, Bohr said
that inside the hydrogen atom, the electron was allowed to have only
discrete values of angular momentum in its orbits around the nucleus.
Translated,
this means the electron can occupy orbits only at a certain distances
from the nucleus. And Bohr simply dismissed the problem of the electron
radiating away its energy by stating that it just didnt happen (even
great scientists cheat sometimes!). He postulated that inside an atom,
electrons only radiate energy when they jump from one allowable orbit to
another, and the energy of this radiation, reveals the allowable
orbits.
The wavelengths of light absorbed by hydrogen when white
light is shined upon it, as well as the wavelengths of light when it is
subsequently re-radiated had been precisely studied at the time but
never explained. Here is a sample of an absorption spectrum and an
emission spectrum.
By predicting the values of orbits that an
electron could have, Bohrs model also predicted the wavelengths of the
lines in the hydrogen spectrum. And his model was tremendously
successful. It explained in exquisite detail the atomic spectra of
hydrogen.
When the energy of the wavelengths of the spectral
lines are compared to the energy differences in orbits allowed in the
Bohr Atom they agree exactly. So the quantum approach worked well in
explaining the allowable orbits, but no one was certain why only those
orbits were allowed.
Particle Waves
In his doctoral
dissertation in 1924, Louis de Broglie put forward a simple idea that
significantly advanced the understanding of the extremely tiny (a
quantum leap forward you might say). Since Einstein and Planck and
Compton had firmly established that light could have characteristics of
both a wave and a particle, de Broglie suggested that matter
particlesprotons, electrons, atoms, billiard balls, etc. could sometimes
act like waves.
And when this idea was applied to the Bohr atom,
it answered many questions. First, the allowed orbits had to be exact
multiples of the wavelengths calculated for the electrons. Other orbits
produced destructive interference of the waves and so the electron
couldnt exist there.
So the circumference of the orbit must equal
the wavelength Or twice the wavelength Or 3 times the wavelength Or,
for that matter, any multiple of the wavelength. Second, these orbits
werent really orbits in the traditional sense. These electrons didnt
travel around the nucleus in a circle. Rather they took the form of a
standing wave that surrounded the nucleus entirely. The exact position
and momentum of the electron particle could not be specified at any
given instant.
• http://www.cassiopeiaproject.com
.
---
Please SUBSCRIBE to Science & Reason:
• http://www.youtube.com/Best0fScience
• http://www.youtube.com/ScienceTV
• http://www.youtube.com/FFreeThinker
---
1. A Brief History Of Quantum Mechanics
http://www.youtube.com/watch?v=B7pACq...
2. The Structure Of Atoms
http://www.youtube.com/watch?v=-YYBCN...
3. Wave Function And Wave-Particle Duality
http://www.youtube.com/watch?v=7GTCus...
4. The Uncertainty Principle
http://www.youtube.com/watch?v=Fw6dI7...
5. The Spin Of Fundamental Particles
6. Quantum Entanglement
---
Atomic Structure
It
was during the early decades of the 19th century that the structure of
atoms was coming into focus. It was known for example that a hydrogen
atom contained one proton and one electron. But the scientists of the
time could think of no stable arrangement of the two particles.
It
was known that protons in any atom were grouped in a small central
region called the nucleus and that the electrons were somehow arranged
at comparatively large distances outside the nucleus. But, in hydrogen,
if the electron were stationary, it would fall into the nucleus since
the charges on the particles would cause them to attract one another.
Yet
the electron couldnt be in an orbit circling the nucleus either.
Circular motion requires constant acceleration of the circling body to
keep it from flying away. But the electron has charge and charged
particles radiate light when they are accelerating. So an electron in a
circular orbit would radiate light and would spiral into the nucleus.
Bohr Atom
Niels
Bohr proposed the first working model of the hydrogen atom. In the Bohr
model, the electron circles the nucleus as if it were a planet going
around the sun. And with a nod to the energy quantization that Max
Planck dreamed up for solving the Ultraviolet Catastrophe, Bohr said
that inside the hydrogen atom, the electron was allowed to have only
discrete values of angular momentum in its orbits around the nucleus.
Translated,
this means the electron can occupy orbits only at a certain distances
from the nucleus. And Bohr simply dismissed the problem of the electron
radiating away its energy by stating that it just didnt happen (even
great scientists cheat sometimes!). He postulated that inside an atom,
electrons only radiate energy when they jump from one allowable orbit to
another, and the energy of this radiation, reveals the allowable
orbits.
The wavelengths of light absorbed by hydrogen when white
light is shined upon it, as well as the wavelengths of light when it is
subsequently re-radiated had been precisely studied at the time but
never explained. Here is a sample of an absorption spectrum and an
emission spectrum.
By predicting the values of orbits that an
electron could have, Bohrs model also predicted the wavelengths of the
lines in the hydrogen spectrum. And his model was tremendously
successful. It explained in exquisite detail the atomic spectra of
hydrogen.
When the energy of the wavelengths of the spectral
lines are compared to the energy differences in orbits allowed in the
Bohr Atom they agree exactly. So the quantum approach worked well in
explaining the allowable orbits, but no one was certain why only those
orbits were allowed.
Particle Waves
In his doctoral
dissertation in 1924, Louis de Broglie put forward a simple idea that
significantly advanced the understanding of the extremely tiny (a
quantum leap forward you might say). Since Einstein and Planck and
Compton had firmly established that light could have characteristics of
both a wave and a particle, de Broglie suggested that matter
particlesprotons, electrons, atoms, billiard balls, etc. could sometimes
act like waves.
And when this idea was applied to the Bohr atom,
it answered many questions. First, the allowed orbits had to be exact
multiples of the wavelengths calculated for the electrons. Other orbits
produced destructive interference of the waves and so the electron
couldnt exist there.
So the circumference of the orbit must equal
the wavelength Or twice the wavelength Or 3 times the wavelength Or,
for that matter, any multiple of the wavelength. Second, these orbits
werent really orbits in the traditional sense. These electrons didnt
travel around the nucleus in a circle. Rather they took the form of a
standing wave that surrounded the nucleus entirely. The exact position
and momentum of the electron particle could not be specified at any
given instant.
• http://www.cassiopeiaproject.com
.
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