atomic theory ancient and modern. ancient theory d e m o c r i t u s greek philosopher/scientist...
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Atomic TheoryAtomic Theory
Ancient and ModernAncient and Modern
Ancient TheoryAncient Theory
Democritus
•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)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, whole-number 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)J.J. Thomson (1856-1940)
“Cathode ray” (stream of electrons) experiment
“Plum-pudding” model of the atomMass-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)Ernest Rutherford (1871-1937)
Discovered nucleus with gold foil experiment
Shot He atoms at a piece of gold foil surrounded by radioactive Zn—if plum-pudding model was correct, atoms should go straight through
Found that some atoms ricocheted! Concluded that atoms have a dense,
positive core (nucleus)
Rutherford’s ExperimentRutherford’s Experiment
Looked at light emitted by heated materials Came up with an equation that related energy
emitted to the frequency of light emitted
E=h fh=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”)
Max Planck (1858-1947)Max Planck (1858-1947)
Niels Bohr (1885-1962)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
H atom
Ene
rgy
leve
l
4
3 2
1
Whee!!
Quantum TheoriesQuantum Theories
Each electron in an atom has 4 quantum numbers to define position and properties:
1) Principal quantum number (n)—describes e- energy level: n=1,2,3…
2) Angular momentum quantum number (l)—describes orbital: l=0 for s, 1 for p, 2 for d, etc.
3) Magnetic quantum number (ml)—describes position (i.e. which “box” in orbital diagrams): ml= -l to +l
4) Spin quantum number (ms)—describes movement: -½, +½
5) Pauli Exclusion Principle: no two electrons can have the same 4 quantum numbers
More Quantum StuffMore Quantum Stuff
D orbital: n=3, l=2
S orbital: n=1, l=0
ml=0
P orbital: n=2, l=1
ml=0ml=-1 ml=1
S orbital: n=2, l=0
P orbital: n=3, l=1
S orbital: n=2, l=0
Chromium orbital-filling diagram
n=3, l=2, ml=-2,
ms=+½
Yet More Quantum StuffYet More Quantum Stuff
Albert Einstein (1879-1955): Light has properties of both particles and waves.
Louis de Broglie (1892-1987): Matter has wave-like properties.
Heisenberg Uncertainty Principle: It is impossible to know both speed and position of an electron at the same time.
www.britannica.com/ nobel/cap/oruthef002a4.html
http://www.bookrags.com/sciences/chemistry/atomic-theory-woc.html
BibliographyBibliography