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WHAT IS AN ATOM?
Unit #3 Atomic Structure
Review from 8th Grade
 OBJECTIVES:
 Identify the subatomic particles
 Explain how the atomic number
identifies an element.
 Use the atomic number and mass
number of an element to find the
numbers of protons, electrons, and
neutrons
 Review key terms
Subatomic particles
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
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.
These account for most of mass
Counting the Pieces
 Protons: equal to atomic number
 Neutrons: Mass Number – Atomic
Number
 Electrons: In a neutral atom equal to
atomic number
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
What if Atoms Aren’t Neutral
 Ions: charged atoms resulting from the
loss or gain of electrons
What if Atoms Aren’t Neutral
 Anion: negatively charged ion; result from
gaining electrons
 Take the number of electrons in a neutral atom
and add the absolute value of the charge
81
35
Br
1-
Identify:
Number of Protons
Number of Neutrons
Number of Electrons
What if Atoms Aren’t Neutral
 Cation: positively charged ion; result from
the loss of electrons
 Take the number of electrons in a neutral atom
and subtract the value of the charge
27
13
Al
3+
Identify:
Number of Protons
Number of Neutrons
Number of Electrons
Distinguishing Between Atoms
 OBJECTIVES:
 Explain how isotopes differ, and
why the atomic masses of elements
are not whole numbers.
Distinguishing Between Atoms
 OBJECTIVES:
 Calculate the average atomic mass
of an element from isotope data.
Isotopes
 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
 We are more concerned with average
atomic mass
 Average atomic mass is based on
abundance of each element in nature.
 We don’t use grams because the numbers
would be too small
Measuring Atomic Mass
 Unit is the Atomic Mass Unit (amu)
 It is one twelfth the mass of a carbon-
12 atom
 Each isotope has its own atomic
mass, thus we determine the average
from percent abundance
Calculating averages
 Convert the percent to a decimal (move
the decimal 2 places to the left or divide
by 100)
 Multiply the atomic mass by it’s percent
(expressed as a decimal), then add the
results.

Calculating Averages
 Calculating the Average Atomic Mass is
similar to calculating your grade in this class
Percent of Grade
Tests 35%
Quizzes 25%
Labs 20%
Homework 20%
Student's
Average
89
95
35
100
Calculating Averages
 If Bromine-79 has an abundance of 50.69%
and Bromine-81 has an abundance of
49.31% what is the average atomic mass of
Bromine?
 Why is it better to use the mass of an
individual isotope for the mass number
rather than round the number from the
periodic table?
Atomic Mass
 Magnesium has three isotopes. 78.99%
magnesium-24 with a mass of 23.9850
amu, 10.00% magnesium-25 with a mass
of 24.9858 amu, and the rest magnesium26 with a mass of 25.9826 amu. 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.
 are the decimal numbers on the
periodic table.
History of the atom
 Not the history of atom, but the idea of
the atom.
 Original idea Ancient Greece (400 B.C.)
 Democritus and Leucippus- Greek
philosophers.
Democritus and Leucippus
 Leucippus was the first person to come up
with the idea of the atom
 Democritus was a student of Leucippus and
expanded on his idea
Democritus
 Democritus added:
 Matter is composed of atoms which
move through empty space
 Atoms are solid, homogeneous,
indestructible, and indivisible
 Different atoms have different shapes
and sizes
 The size, shape, and movement of
atoms determine their properties
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
Aristotle
 There was no scientific evidence to back up
either Democritus or Aristotle, but people
believed Aristotle based on reputation
 Aristotle’s theory persisted for 2,000 years
Leading to the modern theory
 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.
 Saw atoms as small solid spheres.
Billiard Ball Model.
Dalton’s Atomic Theory
 All matter is made of tiny indivisible particles



called atoms.
Atoms of the same element are identical,
those of atoms 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.
Laws Related to Atomic Theory
 Law of Conservation of Mass- Antoine
Lavoisier; states that matter is neither
created nor destroyed in chemical reactions
 Law of Definite Proportions- Joseph Proust;
states that all compounds contain the same
elements in the same ratio
 Law of Multiple Proportions- John Dalton;
elements combine in small whole number
ratios
Just How Small Is an Atom?
 Think of cutting a piece of lead into
smaller and smaller pieces
 How far can it be cut?
 An atom is the smallest particle of an
element that retains the properties of
that element
 Atoms-very small
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.
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
Plum Pudding Model
 Proposed by JJ
Thomson
 Said the atom had a
uniform positive
charge in which the
negatively charged
electrons resided
Other particles
 Proton - positively charged pieces
1840 times heavier than the electron
– by E. Goldstein
 Neutron - no charge but the same
mass as a proton – by J. Chadwick
 Where are the pieces?
Millikan’s Oil Drop
 JJ Thomson used the cathode ray to find the
mass to charge ratio of an electron.
 He did not know the mass or charge, only the
ratio
 Millikan’s Oil Drop experiment determined
the charge of an electron
Millikan’s Experiment
Atomizer
Oil droplets
+
-
Oil
Telescope
Millikan’s Experiment
X-rays
X-rays give some electrons a charge.
Millikan’s Experiment
From
thedrops
masswould
of the hover
drop and the charge on
Some
the plates, he calculated the mass of an electron
Rutherford’s experiment
 Ernest Rutherford -English physicist. (1910)
 Believed in the plum pudding model of the




atom
Wanted to see how big they are.
Used radioactivity.
Alpha particles - positively charged pieceshelium atoms minus electrons
Shot them at gold foil which can be made a
few atoms thick.
Rutherford’s experiment
 When an alpha particle hits a fluorescent
screen, it glows.
 Here’s what it looked like
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
center.
 Alpha particles
are deflected by
it if they get close
enough.
at
+
+
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.
Bohr Model
 Bohr worked with the concepts of energy,
wavelength and frequency
 Each color of light is associated with a
different energy
 Each atom gives off its own unique color so..
 Electrons of diiferent atoms have different
energies
Bohr Model
 Each atom has its own specific electron
arrangement
 Electrons are in Energy levels in the atom
 When an electron goes from an excited state
back down to its ground state the atom emits
light
The Bohr Ring Atom
n=4
n=3
n=2
n=1
The Bohr Model
 Doesn’t work.
 Only works for hydrogen atoms.
 Electrons don’t move in circles.
 The quantization of energy is right, but not
because they are circling like planets.
 Back to the drawing board
The Quantum Mechanical Model
 A totally new approach.
 De Broglie said matter could be like a wave,
like standing waves.
 The vibrations of a stringed instrument.
 More on this in Unit # 4
Models of the Atom to Know
 Daltons
 Thomsons
 Rutherford
 Bohr
 Quantum