1. Unit 3: Chemistry. Development of the Atomic Theory.
The History of the Atom

2. Unit 3: Chemistry. Development of the Atomic Theory.
Atom: smallest particle into which an element can be divided and still be the same substance. Often called the “building block of matter”
Theory: a unifying explanation for a broad range of hypotheses and observations that have been supported by testing

3. Unit 3: Chemistry. Development of the Atomic Theory.
Ancient Greek philosopher
He proposed that all atoms are small, hard particles that are “uncuttable” and always moving
Formed into different shapes and sizes.
Form different materials by joining together.
Word “atom” comes from the word indivisible
Democritus
440 BC

4. Disagreed with Democritus
Unit 3: Chemistry. Development of the Atomic Theory.
Aristotle BC
Greek philosopher
Disagreed with Democritus

5. Atoms join in set ratios with other atoms to make new substances.
Unit 3: Chemistry. Development of the Atomic Theory.
His theory stated…
All substances are made of atoms, which cannot be created, divided, or destroyed.
Atoms of the same element are exactly alike and atoms of different elements are different.
Atoms join in set ratios with other atoms to make new substances.
John Dalton
1803

6. Unit 3: Chemistry. Development of the Atomic Theory.
Conducted Cathode Ray experiment
Known for the “plum-pudding model”
There are small, negatively charged particles called electrons scattered throughout positively charge material inside the atom.
J.J. Thomson
1897
Model: is a representation of an object or system

7. Unit 3: Chemistry. Development of the Atomic Theory.
J.J. Thomson
1897
Cathode Ray Experiment
Electron: negatively charged particles found in all atoms

8. Unit 3: Chemistry. Development of the Atomic Theory.
Thomson’s student from New Zealand
Conducted the gold foil experiment where positively charged particles were aimed at foil.
Atom has a small, dense, positively charged nucleus.
Most of an atom’s mass is in the nucleus
Ernest Rutherford
1911

9. Unit 3: Chemistry. Development of the Atomic Theory.
Niels Bohr
1913
Danish scientist living the early 1900’s
Electrons travel around the nucleus in definite paths.
Electrons jump from a path in one level to a path in another level.

10. Unit 3: Chemistry. Development of the Atomic Theory.
Schrodinger and Heisenberg
1913
Current Model
Twentieth century scientists
There are regions inside the atom where electrons are likely to be found called electron clouds.
Electron paths cannot be predicted.