The Atom
Chapter 4
I.
II.
III.
IV.
History of the Atomic Theory
A. Democritus
B. Aristotle
C. Lavoisier
D. Proust
E. Dalton
F. Modern Atomic Theory
History of Atomic Structure
A. Thomson
B. Milikan
C. Rutherford
D. Bohr
E. Chadwick
F. Quantum Atom
Subatomic Particles
A. Atomic Number
B. Mass Number and Isotopes
C. Electrons and Ions
D. Nuclear and Hyphenation Notation
E. Average Atomic Mass
Weighing and Counting Atoms
A. Mole
Atoms
B. Mole
Mass
C. Mass Atoms
I. History of the Atomic Theory
Remember: a scientific theory explains
behaviors and the ‘nature’ of things
 Theories can be revised when new
discoveries are made
 The theory describing the composition of
matter has been revised many times

I. History of the Atomic Theory
Democritus (460-370 BC)
1.Matter is made up of “atoms”
that are solid, indivisible and
indestructible
2.Atoms constantly move in space
3.Different atoms have different
size and shape
4.Changes in matter result from
changes in the grouping of
atoms
5. Properties of matter result from
size, shape and movement
A.
I. History of the Atomic Theory
B. Aristotle (384-322 BC ) & Others
1. Four kinds of matter
a. Fire – Earth – Water – Air
2. One kind of matter can transform
into another
3. Rejected idea of the “atom” (idea then
ignored for almost 2000 years
4. This theory was more popular and
it was easier to accept
Aristotle’s Theory of Matter
I. History of the Atomic Theory
C. Antoine Lavoisier (1770s)
1. Experiment:
2 Sn + O2

2 SnO
tin
oxygen
tin (II) oxide
mass before reaction = mass after reaction
2. Law of Conservation of Mass
a. Matter cannot be created or destroyed
(in a chemical or physical change)
I. History of the Atomic Theory
D.
Joseph Proust (1779)
1. Develops Law of Definite Composition- all
samples of a specific substance contain the
same mass ratio of the same elements
a. ex: all samples of CO2 contains 27.3%
carbon and 72.7% oxygen
b. therefore ‘elements’ are combining
in a whole number ratio – WHY????
I. History of the Atomic Theory - Dalton
E.
John Dalton (1803)
1. Develops Law of Multiple Proportions
a. describes the ratio of elements by mass in
two different compounds composed of the
same elements
2. Example: carbon monoxide carbon dioxide
1 part oxygen : 2 parts oxygen
*when compared to the same amount of
carbon in each compound
I. History of the Atomic Theory- Dalton
3. Dalton collects data and develops his
atomic theory in 1803
4. Dalton’s Background
a.
b.
c.
d.
Dalton became a school teacher at the
age of 12 (he left school at age 11)
loved meteorology - pioneer in this field
studied works of Democritus, Boyle and
Proust
Wrote New System of Chemical
Philosophy in 1808
5. Dalton’s Atomic Theory
1. Matter is made of small particles-atoms
2. Atoms of a given element are identical in
size, mass, but differ from those of other
elements*.
3. Atoms cannot be subdivided or destroyed*.
( supports law of conservation of mass)
4.Atoms combine in small whole number ratios
to form compounds. (def comp,Mult prop)
5. Atoms combine, separate, or rearrange in
chemical reactions.
* Modified in Modern Atomic Theory
JUST A THEORY…….

But it lead to the
Modern
Atomic theory
F. Modern Atomic Theory
1. All matter is made up of small particles
called atoms.
2. Atoms of the same element have the same
chemical properties while atoms of different
elements have different properties
3. Not all atoms of an element have the same
mass, but they all have a definite average
mass which is characteristic. (isotopes)
F. Modern Atomic Theory
4.
5.
Atoms of different elements combine to
form compounds and each element in
the compound loses its characteristic
properties.
Atoms cannot be subdivided by chemical
or physical changes – only by nuclear
changes
I. History of the Atomic Theory
1803
1897
1909
1913
1935
Today
solid
particle
electron
proton
e- orbit
nucleus
neutron
Quantum
Atom
theory
Dalton
Thomson
Rutherford
Bohr
Chadwick
Schrodinger
and others
II.
History of the Atomic Structure
A. J.J. Thomson (1856-1940)
J.J. Thomson (1887)
1.
Experiments with cathode ray tubes
a. atoms have (-) charged particles
which are smaller than atoms
b. determined charge/mass ratio of the
“electron”
Voltage source
-
+
Vacuum tube
Metal Disks
Voltage source
-
+
Voltage source
-
+
Voltage source
-
+
Voltage source

+
Passing an electric current makes a beam
appear to move from the negative to the
positive end
Voltage source

+
Passing an electric current makes a beam
appear to move from the negative to the
positive end
Voltage source

+
Passing an electric current makes a beam
appear to move from the negative to the
positive end
Voltage source

+
Passing an electric current makes a beam
appear to move from the negative to the
positive end
Voltage source

By adding an electric field
Voltage source
+
 By adding an electric field
Voltage source
+
 By adding an electric field
Voltage source
+
 By adding an electric field
Voltage source
+
 By adding an electric field
Voltage source
+
 By adding an electric field
Voltage source
+
 By adding an electric field he found that the
moving pieces were negative

Demonstration of the cathode ray
experiment.
2. Thomson’s Model
The Pudding Model
a. electrons present
 b. atom is like plum
pudding - bunch of
positive stuff
(pudding), with the
electrons suspended
(plums)

II.
History of the Atomic Structure
B. Robert Milikan (1868-1953)
1. Oil Drop Experiment (1909)
a. Discovered mass and actual charge of
electron (-1)
b. Mass is 1/1840 the mass of a hydrogen
atom
1) e – has a mass of 9.11 x 10-28 g
Oil Drop
II.

History of the Atomic Structure –
Summary thus far
So, at this point we know:
- Atoms are divisible into smaller particles
– Electrons are negatively charged
– The mass of an electron is very small
HOWEVER
– Atoms should have a (+) portion to balance
the negative part
- Electrons are so small that some other
particles must account for mass
II. History of the Atomic Structure
Ernest Rutherford (1871-1937)
C. Ernest Rutherford (1909)
1. Discovered the proton p+
2. Received Nobel Prize in Chemistry
3. Gold Foil Experiment (Expectations)
a. Shot alpha particles at atoms of gold
b. expected them to pass straight
through
Lead
block
Uranium
Florescent
Screen
Gold Foil
He thought this would happen:
According to Thomson Model
He thought the mass of the positive charge was
evenly distributed in the atom
Here is what he observed:
4. Gold Foil Experiment Results
a. Most positive alpha particles pass right
through
b. However, a few were deflected
c. Rutherford reasoned that the positive
alpha particle was deflected or repelled
by a concentration of positive charge
The positive region accounts for deflection
5. Gold Foil Experiment Conclusions
a. the atom is mostly empty space
b. the atom has a small, dense positive center
surrounded by electrons
Rutherford Model of the Atom
II. History of the Atomic Structure

At this point in 1909, we know:
– p+ = 1.67 x 10-24 g
– e- = 9.11 x 10-28 g
– The charges are balance!

But,
– How are the electrons arranged?
– There is still mass that is unaccounted for
II. History of the Atomic Structure
D.
Niels Bohr (1913)
1. Electrons orbit nucleus in
predictable paths
II.
E.
History of the Atomic Structure
E. Chadwick (1891 – 1974)
Chadwick (1935)
1. Discovers neutron in
nucleus
2. Neutron is neutral - does
not have a charge n0
3. Mass is 1.67 x 10-24 g
a. slightly greater than
the mass of a proton
II. History of the Atomic Structure
F. The Quantum Atom Theory
1. The atom is mostly empty
space
2. Two regions:
a. Nucleus- protons and neutrons
b. Electron cloud- region where
you have a 90% chance of
finding an electron
II. History of the Atomic Structure
Charges balanced
 Mass accounted for
 However –
what about the
behavior of the
electrons?
