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