Atomic Theory and Law of Definite Proportions

Atomic Theory

• Scientists

• Experiments

• Laws and Theories

• The atom

Timeline and Scientists

Vocabulary

Alpha particles (a)

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

Democritus (400 B.C. / Greece)

​Known For: naming the atom Atomos
Experiment: Guessed (no scientific data)

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​Visual Representation

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

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

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"

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"

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

Laws

​Law of Conservation of Mass	​Law of Definite Proportions	​Law of Multiple Proportions
​Mass is neither created nor destroyed during ordinary chemical reactions or physical changes.	​A chemical compound contains the same elements in exactly the same proportions by mass.	​When two elements, A and B form two or more different compounds, the masses of element B, that combine with a fixed mass of element A, can be expressed as a ratio of small whole numbers.

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 made of smaller particles
2. they can vary in mass (neutrons)