Atomic Theory History

• Scientists

• Experiments

• Laws and Theories

• The atom

Timeline and Scientists

atoms

The basic building blocks of matter consist of a nucleus containing protons and neutrons,

surrounded by electrons.
nucleus
The positively charged central core of an atom, containing protons and neutrons. (*mass of atom)

Democritus (400 B.C. / Greece)

​Known For:
Atomism
Everything is made of invisible spheres
Experiment: Guessed (no scientific data)

​“Nothing exists except atoms and empty space; everything else is opinion.” – Democritus.

​Aristotle

​Directly argued against atomism. More popular philosopher

John Dalton (1808 / England)

​Known For: Atoms were indivisible (cannot be divided)

1. All matter is composed of atoms.

2. Atoms of a given element are identical.

3. Atoms cannot be divided, created, or destroyed.

4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

Visual Representation

J.J. Thomson (1897 / England)

​Known For: electrons were smaller particles of an atom and were negatively charged.

Experiment:
Cathode Ray Tube Experiment

Visual Representation

Ernest Rutherford (1911 / England)

​Known For: Protons (positively charged) and atoms were mostly empty space

Experiment:
Gold Foil Experiment

Visual Representation

Niels Bohr (1913 / England)

​Known For: Energy levels of electrons
Electrons can jump across energy levels

Experiment:
Emissions Spectrum (color of light)

Visual Representation
"Planetary model"

Erwin Schrödinger (1926 / Austria)

​Known For: Quantum mechanics

Electrons can only exist in specified energy states.

Experiment:
mathematical equations

Visual Representation
"Electron Cloud model"

Heisenberg’s Uncertainty Principle (1927)

Known For: Quantum Physics
The more precisely the position of a particle is known, the less precisely its momentum can be known, and vice versa

James Chadwick (1932 / England)

​Known For: Discovered neutrons

neutral particles in the nucleus of an atom

Experiment:
Mathematical equations (the atom was too light with just protons)

Visual Representation
"Quantum model"

​https://www.youtube.com/watch?v=R8r9kXwgdbA

Law

​​Matter/Mass is neither created nor destroyed

Theories

​Dalton’s Atomic Theory: Dalton

• All matter is composed of atoms.

• Atoms of a given element are identical.

• Atoms cannot be divided, created, or destroyed.

• Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

• In chemical reactions, atoms are separated, rearranged, or combined.

Quantum Theory:

1. Atoms are composed of smaller particles.
2. Atoms can vary in mass due to the presence of neutrons.)