Unit 1: Structure & Properties
EARLY HISTORY OF
ATOMIC THEORY
Timeline. . . . .
 Ancient Greeks
 450 BC – Empedocles – Pre-Socratic Philosopher
"There are 4 elements; Earth, Air, Fire and Water"
Benefits:
 Among the first to suggest that substances were
actually made up of a combination of different
"elements."
Democritus:
 400 BC – Democritus
 "The smallest indivisible particle
of matter is the 'atom'"
 No evidence – based on thought
 However, the "4 element" theory
lingered for almost 2000 years!
Democritus’ Model
Alchemists
 Believed that elements were all basically the same,
but with different levels of purity.
 Gold was the purest element, and alchemists tried to
change different metals into gold.
 Considered to be serious science up until the 1600s.
Contributors to the Modern Atomic
Theory….
 Robert Boyle (1660) –
"There are not just 4 elements!“

Controlled experiments, gas law
 Joseph Priestly (1773) – Discovered oxygen, also
discovered pop!
 Antoine Lavoisier (1778)–
 First balance leads to;
Law of Conservation of Mass
 Joseph Proust (1790) –
Law of Definite Proportions
End Result;
 Beginning of the Scientific Method!
John Dalton (1805)
 All matter is composed of indivisible particles called




atoms.
All atoms of a given element are identical; atoms of
different elements have different properties.
Chemical reactions involve the combination of atoms,
not the destruction of atoms.
When elements react to form compounds, they react in
whole-number ratios.
Dalton's atomic theory supported previous research.
Lasted almost a century!
Model: Indivisible billiard ball
J.J. Thomson (1897)
 Used the work of other scientists that showed that
atoms contain charges = modified atomic theory.
 Atoms are positive spheres, with negative particles
(electrons) embedded in them.
 Thomson used a Cathode ray tube & passed rays
between two parallel aluminium plates, which
produced an electric field between them when
they were connected to a battery. The rays were
deflected by the negative plate.
Model: Raisin bun
Ernest Rutherford (1911)
 Student of Thomson – worked further on Thomson’s
model.
Gold Foil Experiment:
 Alpha radiation (positive) were shot at a piece of gold
foil.
 Most of the alpha particles passed through the foil =
most of matter is empty space!
 Some particles were deflected back at angles; they
had come in contact with something very dense!
Gold Foil Experiment:
Leading to…..Nuclear Model:
 Rutherford – dense core of positive charge
(nucleus), with negative electrons orbiting
around the nucleus.
 Positively charged particles = protons, have
equal but opposite charge to the electrons, mass
1836 x greater!
 Completed his work at McGill University
Neutron
 James Chadwick was a student of Rutherford
 He passed alpha particles through positive and
negative charged plates. He discovered a particle that
was not deflected by the positive or negative plate
but passed right through – called this the neutron
 Chadwick won the Nobel
prize in Physics and was
part of the Manhattan
Project
 http://dsc.discovery.com
/tv-shows/othershows/videos/assignmen
t-discovery-shortsdevelopment-of-atomtheory.htm
Timeline
 Using pg 162 – 168, create a
timeline to show the history of the
atom.