Chemistry - History of the Atomic Theory

Who proposed that matter is made of indivisible particles called “atomos”?

Democritus (460–370 B.C.)

What did Democritus believe about atoms?

They are small, hard, indivisible, indestructible, and made of a single material in different shapes and sizes.

Which philosopher believed all matter was made of four elements: fire, air, earth, and water?

Aristotle (384–332 B.C.)

What did Aristotle believe about the properties of matter?

Matter has four properties: moist, hot, dry, and cold.

When did ideas about atoms shift from philosophy to science?

In the 19th century.

Who proposed the first atomic theory in 1803?

John Dalton

What are the key points of Dalton’s atomic theory?

All matter is made of atoms; atoms can’t be created or destroyed; atoms of the same element are identical; atoms combine to form new substances.

Who discovered the electron?

J.J. Thomson (1856–1940)

What was J.J. Thomson’s key contribution?

Atoms contain negatively charged electrons and must also contain positive charges.

Who performed the gold foil experiment?

Ernest Rutherford (1871–1937)

What did Rutherford’s experiment reveal about the atom?

It has a dense, positively charged nucleus with electrons surrounding it.

Who introduced the term “proton”?

Ernest Rutherford

What did Niels Bohr propose in his atomic model?

Electrons orbit the nucleus in fixed paths and can jump between energy levels.

What is Heisenberg’s Uncertainty Principle?

It’s impossible to know both the exact position and velocity of a particle at the same time.

What did Heisenberg’s model lead to?

The development of the Quantum Mechanical Model—electrons don’t move in fixed orbits.