Download The History of the Atom

HISTORY OF THE ATOM
SCH3U
Unit 1: Matter, Chemical Trends and Chemical Bonding
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
Dalton’s Atomic Model

1808: John Dalton
 Described
atoms as solid, indestructible particles that
make up all matter (“billiard ball model”).
Dalton’s Atomic Model

The main postulates of Dalton’s theory:
1.
2.
3.
4.
5.
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.
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.
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”)
Rutherford’s Atomic Model

1910: Ernest Rutherford
 Performed
 He
the gold foil experiment
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
Rutherford’s Atomic Model
Rutherford’s Atomic Model
1.
2.
3.
4.
5.
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
Rutherford-Bohr Model

1913: Neils Bohr
 Bohr
extended
Rutherford’s theory
by stating that
electrons occupy
specific energy levels
surrounding the
nucleus
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
Subatomic Particles
Protons
Electrons
Neutrons
Charge
Positive
Negative
Neutral
Mass
1.67 x 10-24g
(1amu)
9.11 x 10-28g
(0.0005 amu)
1 amu
Location
Nucleus
Orbits the nucleus
Nucleus
Discovered
1886
Eugen Goldstein
1897
JJ Thomson
1932
James Chadwick
Modern Atomic Theory
1.
2.
3.
4.
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