Atomos• small hard particles.• Made of a single material

that’s formed into different shapes and sizes.

• always moving• form different materials

by joining together.

Democritus’ theory was ignored and forgotten for more than 2000 years!

Why?

• The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory.

Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

1808: John Dalton

• Dalton’s atomic model is called the “Billiard Ball Model”

• All substances made of atoms that cannot be created, divided, or destroyed.

• Atoms of the same element are exactly alike and atoms of different elements are different.

• Atoms join with other atoms to form different substances

Thomson’s Plum Pudding Model• 1897 • provided the first

hint that an atom is made of even smaller particles.

CATHODE RAY EXPERIMENT: Thomson concluded that a cathode ray consists of a beam of negatively charged particles (electrons) and that electrons are

constituents of all matter.

http://www.kentchemistry.com/links/AtomicStructure/JJThompson.htm

Thomson’s Atomic Model

• proposed the “Plum Pudding” model. • Atoms made from a positively

charged substance with negatively charged electrons scattered about, like raisins in a pudding.

J. J. Thomson

CONCLUSIONS:• the negative charges came from

within the atom.

• A particle smaller than an atom had to exist.

• The atom was divisible!

• called the negatively charged “corpuscles,” today known as electrons.

• Since the gas was known to be neutral (no charge), he reasoned that there must be positively charged particles in the atom.

Dmitri Mendeleyev

~1868-1870

Subatomic Particles

Particle Charge Mass (g) Location

Electron (e-) -1 9.11 x 10-28 Electron

cloud

Proton (p+) +1 1.67 x 10-24 Nucleus

Neutron (no) 0 1.67 x 10-24 Nucleus

Ernest Rutherford’sGold Foil Experiment - 1911

Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded

Rutherford’s Findings

• The nucleus is small• The nucleus is dense• The nucleus is

positively charged

Observations/Evidence: Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflectedhttp://www.youtube.com/watch?v=bSEOOMs5VNU

“Like howitzer shells bouncing off of tissue paper!”Inference/Conclusions:

The Bohr Model of the Atom (1913)What did Bohr learn about electron movement?

• Bohr proposed that electrons move in paths at certain distances around the nucleus.

• Electrons can jump from a path on one level to a path on another level.

• Click here http://www.colorado.edu/physics/2000/quantumzone/bohr.html

How can bookshelves help you understand the movement of electrons?

•Each shelf represents an energy level

•Each book represents an electron

•You can move a book to a higher or lower shelf with the correct amount of energy.

•A book cannot be between shelves

(An electron can move by gaining or losing energy but can never be between energy levels)

James Chadwick• 1932• discovered neutron in the nucleus.

You can compare the head of a pin to the diameter of a stadium to think about atom

size….

• The diameter of a pinhead is 100,000 times smaller than the diameter of a stadium.

• Likewise the diameter of the nucleus of an atom is 100,000 times smaller than the diameter of an atom

The Modern Theory of the Atom

• Electrons travel in regions called “electron clouds”

• You cannot predict exactly where an electron will be found

http://www.fearofphysics.com/Atom/atom3.html

How small are atoms?

THERE ARE 2 X 1022 ATOMS IN A PENNY. If all the atoms in a penny were blown up to the size of a grain of sand they would cover the entire state of California

What can a scanning tunneling electron

microscope show us?

• These images do not show an actual picture of an atom. They show a color-enhanced image of the surface of a material at the atomic level.

What is the structure of an atom?

Bohr Model“Planetary Model”

Schrödinger Model“Electron Cloud Model”

Quick atomic model

review and the

scientists credited with the model

A review:http://www.youtube.com/watch?v=njGz69B_pUg&feature=related