Prior Knowledge Check
Who first came up with the idea of the
atom?
 What particles are found in atoms?
 Where are these particles found?

Section 5.1
Atoms
 OBJECTIVES:
–Summarize Dalton’s atomic
theory.
–Describe the size of an atom.
History of the atom
 Not
the history of atom, but of the idea
of the atom.
 Original idea Ancient Greece (400
B.C.)
 There were 2 different ideas
 Democritus and Leucippus- Greek
philosophers.
History of Atom
Smallest
at beach
possible piece?
 Made of sand
 Cut sand - smaller Atomos - not to
sand
be cut
 Looked
Another Greek
 Aristotle
- Famous philosopher
 All substances are made of 4
elements
 Fire - Hot
 Air - light
 Earth - cool, heavy
 Water - wet
 Blend these in different proportions to
get all substances
Who Was Right?
Greeks did not experiment.
 Greeks settled disagreements by
argument.
 Aristotle was more famous.
 He won.
 His ideas carried through middle
ages.
 Alchemists change lead to gold.

Who’s Next?
Late 1700’s - John
Dalton- England.
 Teacher- summarized
results of his
experiments and those
of others.
 Dalton’s Atomic Theory
 Combined ideas of
elements with that of
atoms.

Dalton’s Atomic Theory
 All matter is made of tiny indivisible
particles called atoms.
 Atoms of the same element are
identical, those of different elements
are different.
 Atoms of different elements combine in
whole number ratios to form
compounds.
 Chemical reactions involve the
rearrangement of atoms. No new
atoms are created or destroyed.
Just How Small Is an Atom?
 An
atom is the smallest particle of
an element that retains the
properties of that element
 Atoms-very small: Fig. 5.2, p. 108
–still observable with proper
instruments: Fig. 5.3, page 108
Section 5.2
Structure of the Nuclear Atom

OBJECTIVES:
– Distinguish among protons, electrons, and
neutrons in terms of relative mass and charge.
– Describe the structure of an atom, including the
location of the protons, electrons, and neutrons
with respect to the nucleus.
– Describe the experiments of Thomson and
Rutherford.
Parts of Atoms
 J.
J. Thomson English physicist. 1897
 Made a piece of
equipment called a
cathode ray tube.
 It is a vacuum tube - all
the air has been
pumped out.
Thomson’s Experiment
Voltage source
-
+
Vacuum tube
Metal Disks
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field he found
that the moving pieces were negative
Other particles



Proton - positively charged pieces
1840 times heavier than the
electron – by E. Goldstein (~1886)
Neutron - no charge but the same
mass as a proton – by J. Chadwick
(1932)
Where are the pieces?
Rutherford’s experiment






Ernest Rutherford -English
physicist. (1910)
Believed in the plum pudding
model of the atom (discussed
in Chapter 13).
Wanted to see how big they
are.
Used radioactivity.
Alpha particles - positively
charged pieces- helium atoms
minus electrons
Shot them at gold foil which
can be made a few atoms
thick.
Plum Pudding- Yuck!
Rutherford’s experiment
When an alpha particle hits a
fluorescent screen, it glows.
 Here’s what it looked like (page 111)

Lead
block
Uranium
Fluorescent
Screen
Gold Foil
He Expected
 The
alpha particles to pass through
without changing direction very
much.
 Because…?
 …the positive charges were
thought to be spread out evenly.
Alone they were not enough to stop
the alpha particles.
What he expected
Because
He thought the mass was evenly
distributed in the atom
Since he thought the
mass was evenly
distributed in the atom
What he got
How he explained it
Atom is mostly empty.
 Small dense,
positive piece
at center.
 Alpha particles
are deflected by
it if they get close
enough.

+
+
Density and the Atom
 Since
most of the particles went
through, it was mostly empty space.
 Because the pieces turned so much,
the positive pieces were heavy.
 Small volume, big mass, big density.
 This small dense positive area is the
nucleus.
Subatomic particles – p.111
Relative Actual
mass (g)
Name Symbol Charge mass
Electron
e-
-1
1/1840 9.11 x 10-28
Proton
p+
+1
1
1.67 x 10-24
Neutron
n0
0
1
1.67 x 10-24
Section 5.3
Distinguishing Between Atoms
 OBJECTIVES:
–Explain how the atomic number
identifies an element.
Section 5.3
Distinguishing Between Atoms
 OBJECTIVES:
–Use the atomic number and mass
number of an element to find the
numbers of protons, electrons,
and neutrons.
Section 5.3
Distinguishing Between Atoms
 OBJECTIVES:
–Explain how isotopes differ, and
why the atomic masses of
elements are not whole numbers.
Section 5.3
Distinguishing Between Atoms
 OBJECTIVES:
–Calculate the average atomic
mass of an element from isotope
data.
Counting the Pieces
Atomic Number = number of protons in
the nucleus
 # of protons determines kind of atom
(since all protons are alike!)
 the same as the number of electrons in
the neutral atom.
 Mass Number = the number of protons
+ neutrons in a particular isotope of that
element.
 These account for most of mass

Symbols
 Contain
the symbol of the element,
the mass number and the atomic
number.
Symbols
 Contain
the symbol of the element,
the mass number and the atomic
number.
Mass
number
Atomic
number
X
Symbols
 Find
the
–number of protons
–number of
neutrons
–number of
electrons
–Atomic number
–Mass Number
19
9
F
Symbols
 Find
the
–number of protons
–number of neutrons
–number of electrons
–Atomic number
–Mass Number
80
35
Br
Symbols
 if
an element has an atomic
number of 34 and a mass number
of 78 what is the
–number of protons
–number of neutrons
–number of electrons
–Complete symbol
Symbols
 if
an element has 91 protons and
140 neutrons what is the
–Atomic number
–Mass number
–number of electrons
–Complete symbol
Symbols
 if
an element has 78 electrons and
117 neutrons what is the
–Atomic number
–Mass number
–number of protons
–Complete symbol
Isotopes
 Dalton
was wrong.
 Atoms of the same element can
have different numbers of neutrons.
 different mass numbers.
 called isotopes.
Naming Isotopes
 We
can also put the mass number
after the name of the element.
 carbon- 12
 carbon -14
 uranium-235
Atomic Mass
 How
heavy is an atom of oxygen?
–There are different kinds of oxygen
atoms.
 More concerned with average atomic
mass.
 Based on abundance of each element
in nature.
 Don’t use grams because the numbers
would be too small.
Measuring Atomic Mass
 Unit
is the Atomic Mass Unit (amu)
 One twelfth the mass of a carbon12 atom.
 Each isotope has its own atomic
mass, thus we determine the
average from percent abundance.
Calculating averages
 Multiply
the atomic mass of each
isotope by it’s abundance (expressed
as a decimal), then add the results.
 Sample 5-5, p.120
Atomic Mass
 Calculate
the atomic mass of copper if
copper has two isotopes. 45.00% has a
mass of 63 amu and the rest has a
mass of 64 amu.
Atomic Mass
 Magnesium
has three isotopes.
79.10% magnesium 24, 10.00%
magnesium 25, and the rest
magnesium 26. What is the atomic
mass of magnesium?
 If not told otherwise, the mass of the
isotope is the mass number in amu
Atomic Mass
 Is
not a whole number because it is
an average.
 The average atomic mass or the
atomic mass is the decimal number
on the periodic table for each
element.
On The Periodic Table

Atomic Number

Average Atomic Mass
Isotopes are not the only ones!
Atoms of the same element may also
have different numbers of electrons.
 Atoms of the same element with
different numbers of electrons are called
ions.
 Ions can either have more or fewer
electrons than a neutral atom.
 We will discuss this more at a later time.

Section 5.4
The Periodic Table: Organizing
the Elements
 OBJECTIVES:
–Describe the origin of the periodic
table.
Section 5.4
The Periodic Table: Organizing
the Elements
 OBJECTIVES:
–Identify the position of groups,
periods, and the transition metals
in the periodic table.
Development of the Periodic
Table
 mid-1800s,
about 70 elements
 Dmitri Mendeleev – Russian
chemist
 Arranged elements in order of
increasing atomic mass
 Thus, the first “Periodic Table”
Mendeleev
 Left
blanks for undiscovered
elements
 When discovered, good prediction
 Problems?
–Co and Ni; Ar and K; Te and I
New way
Moseley – British physicist
 Arranged elements according to
increasing atomic number
 The arrangement today
 P.124 – long form
 Symbol, atomic number & mass
 Henry
Periodic table
 Horizontal
rows = periods
–There are 7 periods
 Periodic law: when the elements are
arranged in order of atomic number, then
there are periodic repetition of physical and
chemical properties
 Vertical
column = group (or family)
–Similar physical & chemical prop.
–Identified by number & letter
Areas of the periodic table
 Group
A elements = representative
elements
–Wide range of phys & chem prop.
–Metals, nonmetals, metalloids
(semimetals)
–Very reactive, not reactive
–Solids, liquids and gases
Metals
IA – alkali metals
 Group 2A – alkaline earth metals
 Transition metals and Inner
transition metals – Group B
 Metals: electrical conductors, have
luster, ductile, malleable
 All metals are solids at room
temperature, except _____.
 Group
Nonmetals
 Nonmetals:
generally nonlustrous,
poor conductors of electricity
–Some gases (O, N, Cl); some are
brittle solids (S); one is a fuming
dark red liquid (Br)
 Group 7A – halogens
 Group 0 – noble gases
Division between metal &
nonmetal
 Heavy,
stair-step line
 Metalloids border the line
–Properties intermediate between
metals and nonmetals
 Learn the general behavior and
trends of the elements, instead of
memorizing each element property