ICP

9/10/12Atom and the History of the Atom

Warmup

1. What is an atom?2. What are the parts of an atom?3. Draw what an atom looks like.

The History of the Atom

• We are going to look at how our understanding of the atom developed.

The History of the Atomic Model

DemocritusA Greek philosopher in the year 400BC.

He used the word Atomos to describe the smallest possible piece of matter.

Democritus

Democritus

Just a “thought experiment”

John Dalton

Proposed the first atomic theory in 1803

Dalton’s Theory

1. All elements are composed of indivisible particles.

2. Atoms of the same element are the same.3. Atoms of different elements are different.4. Compounds are formed by joining atoms of

two or more elements.

Dalton’s Theory

Dalton’s Atomic Model

• Atom was represented by a tiny solid sphere of differing mass.

• Atom had no parts or internal structure.

J.J. Thomson

The English scientist who discovered electrons in 1897.

Thomson or “Plum Pudding” Model (1897)

• J.J. Thomson provide evidence that atoms made up of smaller particles (electrons).

• Positively-charge sphere.

• Negative electrons spread throughout the sphere.

Ernest Rutherford

A British physicist who, in 1908, proved the atom had a small, dense, positively charged nucleus.

Rutherford’s “gold foil experiment”

Niels Bohr

A Danish scientist who, in 1913, proposed the planetary model of the atom.

Bohr’s Model

Bohr or “Planetary” Model (1913)

• Niels Bohr developed concept that electrons not moving randomly

• Electrons moved around nucleus in energy levels (fixed orbits).

• Electrons move from between energy levels as atom gains/loses energy

Electron Cloud Model (Wave Model) (1926)

• Erwin Schrödinger develops equations to describe electron motion.

• Electron clouds, or orbitals, represent space where electron is likely to be.

• Nucleus also contains neutrons (no charge).

The Electron Cloud (Wave) Model Electrons travel in no

definite path.

The probable location of an electron is based on how much energy the electron has.

Are There Other Improvements?

You Bet! Just wait and see!

10 Minutes Closing Activity

• With a partner, use your textbook:• Look up today’s scientists in the index. Turn to

the correct pages and read what the book says about each of these scientists.

• From your notes and the book, respond to the following:1) Explain the history of our understanding of the

atom.2) What were the contributions/discoveries of these

scientists?