History of atoms

What did Democritus believe about matter?

All matter is made of indestructible atoms that differ in size, shape, mass, and arrangement.

Which philosopher argued against atomic theory, and what did he propose instead?

Aristotle; he proposed matter is made of four elements: Earth, Air, Fire, and Water.

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

1. Atoms of the same element are identical.

2. Atoms of different elements have distinct weights.

3. Atoms rearrange during chemical reactions (Law of Conservation of Matter).

What does the Law of Constant Composition state?

A compound always contains the same elements in the same proportion by mass.

What did JJ Thomson discover using the cathode ray tube?

The electron, a negatively charged particle 2000 times lighter than a hydrogen atom.

What is the Plum Pudding Model?

JJ Thomson’s model of the atom where electrons are scattered in a positively charged “soup.”

What experiment did Rutherford conduct, and what were its findings?

Gold foil experiment.

Findings: Atoms are mostly empty space with a dense, positively charged nucleus.

Who discovered the neutron, and how?

James Chadwick, by bombarding beryllium with alpha particles to detect neutral particles in the nucleus.

What did Niels Bohr discover about electrons?

Electrons exist at specific energy levels, and transitions between levels emit discrete wavelengths of light.

What was the significance of Bohr’s observations of hydrogen’s light spectrum?

It showed that electrons emit energy as light when they move from higher to lower energy levels.