1. SCH3U Unit 1: Matter, Chemical Trends and Chemical Bonding
History of the Atom
SCH3U
Unit 1: Matter, Chemical Trends and Chemical Bonding

2. The History of the Atom
Atomic theory has evolved significantly over time to our modern theory
400 BC: Democritus
If matter was divided into smaller and smaller pieces, eventually a point would be reached where it could no longer be divided
Atoms (greek – atomos) – small indivisible particles

3. Dalton’s Atomic Model 1808: John Dalton
Described atoms as solid, indestructible particles that make up all matter (“billiard ball model”).

4. Dalton’s Atomic Model The main postulates of Dalton’s theory:
All matter is made of tiny, indivisible particles called atoms
All the atoms of any given element are identical in mass and size
The atoms of any given element are different masses and size of atoms of any different elements
Atoms of different elements combine in small whole number ratios to form compounds
Atoms remain unchanged in chemical reactions. They only join together or separate from one another.

5. Dalton’s Atomic Model
Dalton’s theory helped to explain two previously discovered laws:
The Law of Conservation of Mass:
Matter cannot be created nor destroyed. The mass of the products is equal to the mass of the reactants.
The Law of Definite Proportions:
The elements of a compound are always present in the same proportion by mass.

6. Thomson’s Atomic Model
1897: J.J Thomson
Discovered the electron, which was found to have a negative charge
He proposed the atom was made of negatively charged electrons embedded in a sphere of positive charge (“raisin bun model”)

7. Rutherford’s Atomic Model
1910: Ernest Rutherford
Performed the gold foil experiment
He directed alpha-particles (positively charged particles from a radioactive source) at a sheet of gold foil
He predicted that based on Thomson’s model, all the particles would pass through the foil undeflected

8. Rutherford’s Atomic Model

9. Rutherford’s Atomic Model
Most of the atom is made up of empty space
An atom contains an equal number of positively charged protons and negatively charged electrons
The dense, positive nucleus of the atom accounts for most of its mass
The nucleus contains protons (and neutrons, later discovered by James Chadwick in 1932)
Very small, negative electrons orbit the nucleus

10. Rutherford-Bohr Model
1913: Neils Bohr
Bohr extended Rutherford’s theory by stating that electrons occupy specific energy levels surrounding the nucleus

11. Modern Atomic Theory
Atom: the smallest particle of an element that has all of the chemical properties of that element
Molecule: the smallest particle of a compound that has all of the chemical properties of that compound

12. Subatomic Particles Protons Electrons Neutrons Charge Positive
Negative
Neutral
Mass
1.67 x 10-24g (1amu)
9.11 x 10-28g ( amu)
1 amu
Location
Nucleus
Orbits the nucleus
Discovered
1886
Eugen Goldstein
1897
JJ Thomson
1932
James Chadwick

13. Modern Atomic Theory
All matter is made up of tiny particles called atoms. Each atom is made of subatomic particles including protons, electrons and neutrons
The atoms of any given element remain unchanged during a chemical reaction
Atoms of the same element have the same properties, which differ from properties of different elements
Atoms of different elements combine in specific proportions to form compounds