Transcript History of The Atom2014 (1)

History of The Atom
Early Ideas of The Atom
• Video: Democritus to Dalton
History Of Atom
• 300 B.C. – Democritus: smallest piece of matter
could not be divided any further  “Indivisible”
• 1805 – Dalton: “Atomic Theory”
– Pure substances/elements are made up of atoms
– Atoms of the same element are exactly alike.
– Atoms could not be created or destroyed or
divided into smaller particles.
– Compounds formed by joining 2 or more
elements.
1805: Dalton’s Billiard Ball Model
Atom is “indivisible”
• Video: Introduction to particle accelerators
1875: Crookes
- Crookes used gas discharge tubes, a screen
and a bar magnet to study atoms
- discovered: the magnet could deflect the beam
Video: Cathode tube
gas discharge tube
Closed tube with low pressure
electrode 1
gas-filled tube
electrode 2
cathode ray
Place a paddle wheel in the path of the ray and it moves!
This means the ray must have some mass!
1897: Thomson’s Discovery of
the Electron
• Video: Thomson’s particle accelerator
- Used a particle accelerator and screen to “see”
effects
- varied the voltage and measured the bending
of the beam
- Deduced the mass was 2000X lighter than the
atom….this was the ELECTRON!
- The rest of the atom is a sphere of positive
charge
Thomson’s Raisin Bun Model
Let’s Review:
1909 Millikan’s Oil Drop Experiment
• Determined the CHARGE of an e- by balancing
an electrically charged oil drop and later the
MASS of the e- was determined
1909: Rutherford’s Gold Foil
Experiment
• Video: Gold Foil experiment
What happens when α particles hit the atoms in the gold foil?
radioactive source
gold foil
Something dense and very small must cause the particles to change direction…the
NUCLEUS!
Rutherford’s findings:
1. The atom contains an extremely small,
dense NUCLEUS (positive)
2. The atom is mostly EMPTY SPACE!
(because only some particles (1/8000) were
deflected)
Rutherford’s Nuclear Model
-Electrons orbit the nucleus like planets orbit the Sun
What is the problem with the
Rutherford model?
• Rutherford’s nuclear model does not obey
classical laws of physics….as electrons
orbit around the nucleus they continuously
lose energy and therefore they should spiral
into the nucleus! Atoms can’t exist!?!?
1913: Bohr
• Rescued Rutherford’s model with his
studies of spectroscopy
• Spectroscopy is the analysis of light
The Hydrogen Line Spectrum
violet
blue
green
red
Bohr determined that e- are not allowed to
orbit “anywhere” they must only occupy
certain well defined orbits
-e- exist in defined/fixed orbits
-“nothing in between”
Bohr’s model: similar to planets orbiting around the
Sun
sun
If electrons can be found in discrete places can we know precisely
where an electron is?
The Hydrogen Line Spectrum
violet
blue
green
red
Where does the red line come from?
Where does the green line come from?
Where does the blue line come from?
Where does the violet line come from?
• Electrons exist only at discreet energy levels
•
When excited electrons in H fall back down to
lower energy levels, they emit a photon of light
1932: Rutherford and Chadwick
• Video: Rutherford discovers the nucleus
- Many years later: 1932
Why?
- Discovered that the mass of the nucleus
does not equal the mass of protons only
- This means the nucleus is made up of 2
parts: NEUTRONS and PROTONS!
Let’s Review
• Song: Atomic Theories
1900: Planck
- Determined that electromagnetic energy could
only be emitted in quantized amounts
- Quantum = a unit or packet of energy∴
- There are no partial amounts
- These quanta are now called PHOTONS!
(Einstein)
1905: Einstein
- A “photon” is a unit of light
Heisenberg – not this guy!
Heisenberg
-
Why? Because trying to observe the electron
changes its direction!
Video: Uncertainty Principle
Quantum Mechanics
• Since the mere act of observing an echanges it’s direction or momentum, we can
only measure the PROBABILITY of
finding an electron in a specific location.
• Quantum tells us the statistical probability
of finding an electron at a given location
derived from wave equations and used to
determine the shape of ORBITALS (not
orbits!)
The Wave Nature of Light
• 1926 – Schrödinger – Wave Mechanics
– The Electron is a particle but it’s behaviour is
predicted by a wave
– Electron wave permeated space and time
Video: Quantum Mechanics
Electrons are found in orbitals
not orbits!
Let’s Review
• Song: Atomic Theories
Summary
• Video: The Space Between