Atomic Theory

Ancient Greeks

• Democritus’ idea (theory) was: All matter is made of small pieces that cannot be divided any smaller.

• The word ATOM means “indivisible”

Just an idea, no “picture” of what the atom looks like.

Dalton’s Theory

Theory: All matter is made up of individual particles called atoms, which cannot be created, divided, or destroyed. (sound familiar?)

Features:

1. Solid, no “insides”.

2. Atoms of the same element are exactly alike.

(1803)

Thomson’s Theory

Theory: Atoms CAN be divided into smaller pieces. [“Chocolate Chip Cookie Model”]

Feature:

1.Discovered the negatively charged ELECTRON.

(1897)

electrons

Rutherford’s Theory

Theory: Atoms are mostly empty space.[Gold Foil Experiment]

Feature: Proved that atom has POSITIVELY charged NUCLEUS

The Gold Foil Experiment (He shot a stream of positive particles at piece of gold foil- most went straight through and only a few bounced back- this proved that atoms were mostly EMPTY SPACE, the ones that bounced had hit a small solid center, the nucleus.)

(1911)

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nucleus

Gold Foil Experiment

Thomson

Rutherford

Bohr’s Theory

Theory: Electrons move around outside the nucleus in specific patterns called ENERGY LEVELS.

Feature: Electrons orbit around nucleus in specific paths, just like planets around the Sun in the solar system.

(1913)

What does this remind you of?

Energy levels

Electron Cloud (Quantum) Model

Theory: Predicts where electrons MAY be located in an atom. [*Closest to what we think the atom is today!*]

Feature: Cannot know exactly where the electrons are at any time, they are “in the cloud”.

(1926)

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Electrons anywhere at any time

THE BIG IDEA

As technology has improved, our idea of the ATOM has changed over time.

(our models have gotten better!)

Video Links

• History of Atomic Theory Song – Veritasium

• The History of Atomic Chemistry: Crash Course Chemistry #37