Transcript Atomic Theory

Atomic Theory
Ancient and Modern
Ancient Theory
•Greek philosopher/scientist
•Stated that everything is made up
of “void” in which exists infinite,
tiny, indivisible particles
•“Atom” comes from Gr.
“atomon,” meaning “indivisible”
John Dalton (1766-1844)
5 basic postulates:
1. All matter consists of tiny particles called atoms
2. Atoms are indestructible and unchangeable
3. All atoms of the same element have the same
weight, and all atoms of different elements have
different weights*
4. Atoms in reactions combine in simple, wholenumber ratios (Law of Definite Proportions)
5. Sometimes atoms combine in more than one
simple, whole-number ratio
*We now say that atoms of the same element have the same “nuclear charge”
J.J. Thomson (1856-1940)



“Cathode ray” (stream of electrons)
experiment
“Plum-pudding” model of the atom
Mass-to-charge ratio of electron
J.J. Thomson’s
cathode ray
apparatus—the
negatively-charged
“cathode rays”
(electrons) are
attracted to the
positively-charged
plate
Ernest Rutherford (1871-1937)




Discovered nucleus with gold foil
experiment
Shot He atoms at a piece of gold foil
surrounded by radioactive Zn—if plumpudding model was correct, atoms should go
straight through
Found that some atoms ricocheted!
Concluded that atoms have a dense, positive
core (nucleus)
Rutherford’s Experiment
Max Planck (1858-1947)


Looked at light emitted by heated materials
Came up with an equation that related energy
emitted to the frequency of light emitted
E=h f
h=constant (6.626 x 10-34 J s)
f=frequency (Hz)
E=energy (J)

Discovered that energy can only be emitted in
discrete packets, called “quanta” (sing. “quantum”)
Niels Bohr (1885-1962)


Postulated that atoms have different energy
levels
Theorized that electrons can absorb photons
of light and “jump” from one energy level to
the next
When electrons fall back, they emit photons
of light
Whee!!
4
Energy
level

3
2
1
H atom
Quantum Theories

1)
2)
3)
4)

Each electron in an atom has 4 quantum numbers
to define position and properties:
Principal quantum number (n)—describes e- energy level: n=1,2,3…
Angular momentum quantum number (l)—describes orbital: l=0 for s,
1 for p, 2 for d, etc.
Magnetic quantum number (ml)—describes position (i.e. which “box”
in orbital diagrams): ml= -l to +l
Spin quantum number (ms)—describes movement: -½, +½
Pauli Exclusion Principle: no two electrons can have
the same 4 quantum numbers
More Quantum Stuff
Chromium orbital-filling diagram
ml=0
S orbital:
n=1, l=0
S orbital:
n=2, l=0
ml=-1
S orbital:
n=2, l=0
P orbital:
n=3, l=1
ml=0
ml=1
P orbital:
n=2, l=1
n=3, l=2,
ml=-2,
ms=+½
D orbital:
n=3, l=2
Yet More Quantum Stuff



Albert Einstein (1879-1955): Light has
properties of both particles and waves.
Louis de Broglie (1892-1987): Matter has wavelike properties.
Heisenberg Uncertainty Principle: It is
impossible to know both speed and position of an
electron at the same time.
Bibliography
www.britannica.com/ nobel/cap/oruthef002a4.html
http://www.bookrags.com/sciences/chemistry/atomictheory-woc.html