An atom is the smallest particle of an element that retains the chemical properties of that element.

400 B.C. Greek Philosopher Believed matter could not be divided indefinitely Atom - Greek word meaning “indivisible”

350 B.C. Did not believe in atoms Earth, Fire, Wind, and Water His opinion was accepted for nearly 2000 years

1808 Elements are composed of atoms Only whole numbers of atoms can combine to form compounds

All atoms of an element are identical. The atoms of different element are different Atoms can combine to form chemical compounds Atoms cannot be created, divided, nor destroyed. A chemical reaction changes the way atoms are grouped together.

Mass is neither created or destroyed during ordinary chemical reactions or physical changes Carbon, C Mass X Oxygen, O Mass Y Carbon Monoxide, CO Mass X + Mass Y

If element A has an atomic mass of 2 and elements B has an atomic mass of 3, what mass would be expected for compound AB? Mass A 2 Mass B 3 Mass A + Mass B

Mass A 2 units Mass B 6 units ?? Mass A 4 units Mass B ?? 20 units 1. 2.

Carbon, COxygen, OCarbon Dioxide, CO 2 Carbon, C Oxygen, OCarbon Monoxide, CO Oxygen, O

Carbon, C 1 gram Oxygen, O 1.33 gram Carbon Dioxide, CO gram Carbon, C 1 gram Oxygen, O 1.33 g Carbon Monoxide, CO 2.33 gram Oxygen, O 1.33 gram

Not all aspects of Dalton’s Atomic Theory have proved to be correct. They have been modified to explain new observations.

1897 Discovered Electron Cathode Ray Tube

Streams of negatively charged particles were found to emanate from cathode tubes.

J. J. Thomson Positive sphere of matter with negative electrons imbedded in it.

1911 Discovered Nucleus Gold foil experiment –It was “as if you had fired an artillery shell at a piece of tissue paper and it came back and hit you.

Nucleus is a dense small positively charge central region Atom is mostly empty space

Electrons have a defined path

Exact location of electron cannot be determined