Development of Atomic Theory

What did Democritus propose?

Matter is made of tiny, indivisible particles called atomos.

What does atomos mean?

Indivisible.

Why was Democritus's idea rejected?

He had no experimental evidence.

Who developed the first modern atomic theory?

John Dalton.

What was Dalton's model called?

The solid ball model.

What did Dalton believe atoms were like?

Tiny, solid, indivisible balls.

Who discovered the electron?

J.J. Thomson.

What experiment did Thomson use?

The cathode ray tube experiment.

What did Thomson's experiment show?

Atoms contain negatively charged electrons.

What was Thomson's model called?

The Plum Pudding Model.

What did the Plum Pudding Model look like?

Negative electrons were spread throughout a positively charged atom.

Who determined the mass and charge of the electron?

Robert Millikan.

What experiment did Millikan use?

The oil-drop experiment.

What did Rutherford's Gold Foil Experiment discover?

The nucleus and that atoms are mostly empty space.

What did Rutherford expect before the experiment?

He expected most alpha particles to pass straight through.

What actually happened in Rutherford's experiment?

Most passed through, some were deflected, and a few bounced back.

What did Rutherford conclude from most particles passing through?

The atom is mostly empty space.

What did Rutherford conclude from particles being deflected?

There is a small, dense, positive nucleus.

Who discovered the neutron?

James Chadwick.

What did Chadwick discover?

The neutron.

Who developed the Bohr model?

Niels Bohr.

What does the Bohr model show?

Electrons in specific energy levels or shells.

What happens when an electron gains energy?

It moves to a higher energy level.

What happens when an electron loses energy?

It moves to a lower energy level and releases energy.

Who developed the quantum mechanical model?

Erwin Schrödinger.

What does the quantum mechanical model show?

The probability of where electrons are likely to be found.

What is the modern model of the atom?

The quantum mechanical/electron cloud model.

Does the modern model show the exact path of an electron?

No, it shows the probability of where the electron is likely to be.

What is the order of atomic theory development?

Democritus → Dalton → Thomson → Rutherford → Bohr → Schrödinger.

Who proposed atomos?

Democritus.

Who proposed the solid ball model?

Dalton.

Who proposed the Plum Pudding Model?

Thomson.

Who discovered the nucleus?

Rutherford.

Who discovered the neutron?

Chadwick.

Who proposed electrons in energy levels?

Bohr.

Who developed the electron cloud model?

Schrödinger.

Why has the atomic model changed over time?

New experiments and evidence allowed scientists to improve the model.