SCH4C
UNIT 1: MATTER AND QUALITATIVE ANALYSIS
ATOMIC THEORY
Development of the Rutherford Model of the
Atom
Early Atomic Theory
• Aristotle and other Greek philosophers thought that matter
was composed of 4 classical elements was accepted
until the late Middle Ages.
• Democritis proposed that if matter is divided into smaller
and smaller pieces, a finite limit known as the atom is
ultimately reached.
Dalton’s Atomic Theory (1803)
Dalton revived the atomic theory to explain several
observations and chemical laws:
• Elements cannot be broken down
• Compounds can be broken down into elements
• Law of Conservation of Mass
This led to the “Billiard Ball” model of indivisible
atoms of elements. These combine to form all
known compounds.
Dalton’s Billiard Ball Model
Dalton’s Atomic Theory
1. All matter is made of tiny indivisible particles
2.
3.
4.
5.
called atoms.
Atoms cannot be created or destroyed.
All atoms of a particular element are identical.
Compounds are formed through the
combination of elements.
Chemical reactions occur when atoms in
compounds join together or separate to form
new compounds.
Thomson’s Model of the Atom (1897)
• The study of electricity and batteries
(electrochemistry) suggested that matter contains
electric charges.
• Thomson detected and measured the mass of a
beam of particles in a vacuum tube (or cathode
ray tube, CRT) .
• He called the very tiny particles that emerged
from a metal cathode electrons.
Thomson’s Plum Pudding Model
• Thomson explained electrical conduction in metals by
the movement of subatomic electrons in a solid.
• Ions (charged atoms) are formed by losing or gaining
electrons.
Rutherford’s Atomic Model
• The discovery of radioactivity (,  and  rays) allowed
Ernest Rutherford to probe inside the atom.
• Based on Thomson’s model, Rutherford proposed that
alpha rays (high speed helium ions, He2+) should pass
through the positive pudding in a very thin sheet of gold
foil.
• The results were slightly different…
Gold Foil Experiment
Expected result:
Actual result:
Rutherford’s Planetary Model (1911)
• Rutherford concluded that the atom consists of a
very tiny, dense nucleus composed of the positive
charge.
• >99.99% of the atom consists of empty space.
• Tiny electrons are found orbiting the nucleus.
• A teaspoon on the atomic nuclei would weigh six
billion tonnes (6 x 1012 kg)!!!!
900 x
• In 1914, Rutherford proposed that there existed a
positively charged particle called a proton.
• In 1932, Chadwick & Rutherford found that the
nucleus also contained a neutral particle called a
neutron.
Revisiting Dalton’s Theory…
1.
All matter is made of tiny indivisible particles called
atoms.
FALSE [Atoms consist of p, n and e]
2.
Atoms cannot be created or destroyed.
FALSE [Nuclear reactions create or destroy atom]
3.
All atoms of a particular element are identical.
FALSE [Elements consist of several forms called isotopes]
4.
Compounds are formed through the combination of
elements.
TRUE!
5.
Chemical reactions involve atoms recombining to form
new substances.
TRUE!
The Modern Atomic Theory
1. All matter is made of atoms. Atoms are made of
protons, neutrons and electrons.
2. Atoms cannot be created or destroyed in chemical
reactions.
3. Atoms of one element have the same average
properties.
4. Atoms combine in specific proportions to form
compounds.
Summary of the Subatomic Particles
Particle
Symbol
Location
Charge
Mass (amu)
Proton
p
Nucleus
+1
1.0073
Neutron
n
Nucleus
0
1.0087
Electron
e-
Outside
Nucleus
-1
0.00055
amu = atomic mass unit = 1.66 x 10-24 g