Atomic Theory and Law of Definite Proportions

Timeline and Scientists

Democritus (400 B.C. / Greece)

​Known For: naming the atom
Atomos: means uncuttable in Greek
Experiment: Guessed (no scientific data)

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​Visual Representation

Aristotle (384 B.C. to 322 B.C.)

Thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air.

His theory held true for the next 2000 years.

Open Ended

Why do you think Aristotle's theory of the atom was more accepted than Democritus' theory?

John Dalton (1808 / England)

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

The Laws
1. Law of Conservation
2. Law of Definite Proportions
3. Law of Multiple Proportions

Visual Representation

Summarize in your own words

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 (positive charged) and atoms were mostly empty space

Experiment:
Gold Foil Experiment

Visual Representation

Vocabulary

Alpha particles (a)

are composite particles consisting of two protons and two neutrons tightly bound together

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)

A.K.A.: Modern Model of the Atom

​Known For: Quantum mechanics

electrons can only exist in specified energy states

Experiment:
mathematical equations

Visual Representation
"Electron Cloud model"

James Chadwick (1932 / England)

​Known For: Discovered neutrons

neutral particles in the nucleus of an atom

Experiment:
mathematical equations (atom was too heavy with just protons)

Visual Representation
"Quantum model"

Summary of what is known about the current model of the atom:

​Subatomic Particles	​Symbol	​Relative charge	​Relative mass (amu)	​Actual mass
​electrons	​e	-1	​1/1840	​9.11 X 10-28
​protons	​p	​+1	​1	​1.67 X 10-24
​neutrons	o	0 ​(no charge) (neutral)	​1	​1.67 X 10-24

Theories

​Dalton’s Atomic Theory: Dalton

1. All matter is composed of atoms.

2. Atoms of a given element are identical

   • Today: Not true - there are isotopes

3. Atoms cannot be divided, created, or destroyed

   • Technically, no, but there are smaller parts (p, n, and e)

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

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

Quantum Theory
1. Atoms are made of smaller particles

2. Atoms of the same element (isotopes) can vary in mass (neutrons)