Atoms and Their
Structure
History of the Atom
Original idea (400 B.C.) came from
Democritus, a Greek philosopher
 Democritus expressed the belief that all
matter is composed of very small, indivisible
particles, which he named atomos.

Who’s Next?

John Dalton (17661844), an English
school teacher and
chemist, studied
the results of
experiments by
other scientists.
Dalton’s Atomic Theory
 John Dalton proposed his atomic theory
of matter in 1803.
 Although his theory has been modified
slightly to accommodate new
discoveries, Dalton’s theory was so
insightful that it has remained
essentially intact up to the present time.
Dalton’s Atomic Theory
1. All matter is made of tiny indivisible
particles called atoms.
2. Atoms of the same element are
identical; those of different atoms are
different.
Dalton’s Atomic Theory, cont.
3. Atoms of different elements combine in
whole number ratios to form
compounds
4. Chemical reactions involve the
rearrangement of atoms. No new atoms
are created or destroyed.
Parts of the Atom
Because of Dalton’s atomic theory,
most scientists in the 1800s believed
that the atom was like a tiny solid ball
that could not be broken up into parts.
 In 1897, a British physicist,
J.J. Thomson, discovered that this solidball model was not accurate.

Parts of the Atom
 Thomson’s experiments used a cathode
ray tube.
 It is a vacuum tube - all the air has been
pumped out.
 J.J. Thomson concluded that cathode
rays are made up of invisible, negatively
charged particles referred to as
electrons
Thomson’s Model
 From Thomson’s
experiments,
scientists had to
conclude that atoms
were not just neutral
spheres, but
somehow were
composed of
electrically charged
particles.
Thomson’s Model
Matter is not negatively charged, so
atoms can’t be negatively charged
either.
 If atoms contained extremely light,
negatively charged particles, then they
must also contain positively charged
particles — probably with a much
greater mass than electrons.

Thomson’s Model
Plum Pudding Model

J.J. Thomson said
the atom was like
plum pudding, a
popular English
dessert.
Draw Diagram in your notes
Millikan’s Oil Drop Experiment

R.A. Millikan found the charge of an
electron to be -1.60 x 10-19 Coulombs.
The Proton
 In 1886, scientists discovered that a
cathode-ray tube emitted rays not only
from the cathode but also from the
positively charged anode.
 Years later, scientists determined that
the rays were composed of positively
charged subatomic particles called
protons.
The Proton
 At this point, it seemed that atoms were
made up of equal numbers of electrons
and protons.
Ernest Rutherford

In 1909, a team of scientists
led by Ernest Rutherford in
England carried out the first
of several important
experiments that revealed
an arrangement far different
from the plum pudding
model of the atom and
discovered the nucleus.
Rutherford’s Experiment

The experimenters set up a leadshielded box containing radioactive
polonium, which emitted a beam of
positively charged subatomic particles
through a small hole.
Rutherford’s Experiment

The sheet of gold foil was surrounded
by a screen coated with zinc sulfide,
which glows when struck by the
positively charged particles of the beam.
Lead
block
Polonium
Florescent
Screen
Gold Foil
What Rutherford Observed
How Rutherford Explained It
To explain the results of the experiment,
Rutherford’s team proposed a new
model of the atom.
 Because most of the particles
passed through the foil, they
concluded that the atom is
mostly empty space.

How Rutherford Explained It
 Alpha particles are deflected by the
nucleus if they get close enough it.
How Rutherford Explained It
Because so few particles were
deflected, they proposed that the atom
has a small, dense, positively charged
central core, called a nucleus.
 The dense part of the atom is where
most of the mass is located, and it has a
positive charge.

The Nuclear Model of the Atom

The new model of the atom as pictured
by Rutherford’s group in 1911 is shown
below.
Model Depicted by the Gold Foil Experiment
Draw Diagram in your notes
Isotopes

In 1910, J.J. Thomson discovered that
neon consisted of atoms of two different
masses.
Isotopes Example
Copy example in your notes
Carbon-12 and
Carbon-14 are
isotopes of one
another.
 They are the
same element
with different
masses.

12
14
6
6
C-12
12
C
6
C-14
Isotopes
Atoms of an element that are chemically
alike but differ in mass are called
isotopes of the element.
 Because of the discovery of isotopes,
scientists hypothesized that atoms
contained still a third type of particle that
explained these differences in mass.

The Neutron
Calculations showed that such a particle
should have a mass equal to that of a
proton but no electrical charge.
 The existence of this neutral particle,
called a neutron, was confirmed in the
early 1930s.
 James Chadwick is given credit for
discovering the neutron.

The Neutron

Isotopes of an element differ in the
number of the subatomic particle called
neutrons.
Naming Isotopes

Put the mass number as a whole number
after the symbol of the element.
 Carbon – 12.000000 amu
C-12
 Carbon
– 14.003242 amu
C-14
 Lead
– 209.98418 amu
Pb-210
 Lead
– 211.99188 amu
Pb-212
Subatomic Particles
Name
Electron
Relative
mass
Symbol Charge
(amu)
e-1
1/2000
Proton
p+
+1
1
Neutron
n0
0
1
***REMEMBER THIS CHART***
Modern View of the Atom
The atom has two regions and is
3-dimensional.
 The nucleus is at the center and
contains the protons and neutrons.

Modern View of the Atom

The electron
cloud is the
region where you
might find an
electron and
most of the
volume of an
atom.
Atomic Number
The atomic number (Z) of an element
is the number of protons in the nucleus
of an atom of that element.
 The number of protons determines the
identity of an element, as well as many
of its chemical and physical properties.

Atomic Number
Because atoms have no overall
electrical charge, an atom must have as
many electrons as there are protons in
its nucleus.
 Therefore, the atomic number of an
element also tells the number of
electrons in a neutral atom of that
element.

Masses
The mass of a neutron is almost the
same as the mass of a proton.
 The sum of the protons and neutrons in
the nucleus is the mass number of that
particular atom.
 The mass number (A) is the atomic
mass rounded to a whole number.

SYMBOLS
Isotopes

Remember, isotopes of an element have
different mass numbers because they
have different numbers of neutrons, but
they all have the same atomic number.
Isotopes
Subtract the atomic number from the
mass number to determine the number
of neutrons.
 How many neutrons are in each lithium
isotope below?
4 neutrons

3
neutrons
5
neutrons
Summary of Isotopes
(copy in your notes)

# protons = atomic #

# electrons = atomic # (in neutral atoms)

# protons = # electrons (in neutral atoms)

mass # = # protons + # neutrons

# neutrons = mass # - # protons
Information in the Periodic Table

The average atomic mass is the
weighted average mass of all the
naturally occurring isotopes of that
element. Average atomic mass is based
on an isotope’s natural abundance.
Symbols Example
19
9

F
Determine the complete symbol for a
fluorine atom with a mass number of 19.
Symbols Example

Determine the following for the
fluorine atom depicted below.
a) number of protons (9)
19
b) number of neutrons (10)
c) number of electrons (9)
d) atomic number (9)
e) mass number (19)
F
9
Symbols Example
80
35

Br
Determine the complete symbol for a
bromine atom with a mass number of
80.
Symbols Problem

Determine the following for the
bromine atom depicted below.
a) number of protons (35)
80
b) number of neutrons (45)
c) number of electrons (35)
d) atomic number (35)
e) mass number (80)
Br
35
Symbols Problem

If an element has an atomic number
of 34 and a mass number of 78 what
is the
a) number of protons (34)
b) number of neutrons (44)
c) number of electrons (34)
d) complete symbol
78
34
Se
Symbols Problem

If an element has 91 protons and
140 neutrons what is the
a) atomic number (91)
b) mass number (231)
c) number of electrons (91)
d) complete symbol
231
91
Pa
Symbols Problem

If an element has 78 electrons and
117 neutrons what is the
a) atomic number (78)
b) mass number (195)
c) number of protons
d) complete symbol
(78)
195
78
Pt

Fill in the chart below.
Element
Potassium
Argon
# of
# of
# of
Protons Neutrons Electrons
Mass
#
Atomic
#
19
20
19
39
19
18
22
18
40
18