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The Nature of Matter
Chapter 1
Inside the Atom
Models of the Atom
• Greek philosophers devised a theory of atoms, or tiny
• They couldn’t perform experiments and collect evidence like
we do now
– They reasoned and formed conclusions
• They reasoned that if you keep cutting matter in half, you
would eventually not be able to cut it anymore
– They called these particles atoms
John Dalton
• He was a school teacher in
England (early 1800s)
• Dalton combined the idea
of elements with the
Greek theory of the atom
• Matter is made up of
• Atoms cannot be divided
into smaller pieces
• All atoms of an element
are exactly alike
• Different elements are
made of different atoms
William Crookes
• Crookes tested Dalton’s theory using
a cathode-ray tube
– Almost all air removed
– Two pieces of metal called electrodes
(conduct electricity) sealed inside and
connected to a battery by wires
– He placed a cross-shaped object between
the electrodes
• When he connected the battery, the
glass tube lit up with a greenish glow
• On the positive side, there was a
shadow of a cross.
• Something was traveling from the
negative side to the positive side and
was being blocked by the cross
J. J. Thomson
• Thomson (1897, England) placed
a magnet by the cathode ray tube
and discovered he could move the
cathode rays.
• He also used different metals and
gasses and discovered that there
are negatively charged particles
(electrons) in every atom
• Thomson revised Dalton’s model
to include a sphere with a positive
charge and negatively charged
electrons spread evenly within the
positive charge
• The negatively charged electrons
and the positive charge in the
sphere neutralized each other
Earnest Rutherford
• Rutherford tested Thomson’s
model in 1906
• His team fired fast-moving,
positively charged bits of matter
(alpha particles) at a thin film of
metal (gold was mainly used)
• If Thomson’s model were true,
there isn’t enough charge
anywhere in an atom to repel an
alpha particle and they would go
straight through the thin film of
– Occasionally on alpha particle
would come directly into contact
with a positive charge and be
repelled, but not often
Earnest Rutherford – The Reality
• When the alpha particles were fired, many
bounced off at large angles
• Rutherford said, “It was about as believable as if
you had fired a 15-inch shell at a piece of tissue
paper, and it came back and hit you.”
• There had a to be a large positive mass somewhere
to deflect the alpha particles
• Thus, Thomson’s model could not be correct
A New Atomic Model
• The idea of a nucleus was
– Rutherford said that almost all
of the mass of an atom must be
in the center (nucleus) and must
be positively charged
– In 1920, scientists identified the
positive charges in the nucleus
as protons
• Most of an atom is empty
space occupied by nearly
massless electrons
• Electrons orbit the nucleus
and the number of electrons
equals the number of protons
in a stable atom
A New Atomic Model
• Since electrons have no mass and there are no other particles,
the mass of an atom must equal the number of protons
• The mass of atoms, however, is at least twice that of the
number of protons
• It was proposed that another particle must be in the nucleus
• Later called the neutron and discovered to have almost the
exact mass of a proton and no charge
Electron Shells
• This is simplified method of
explaining the orbits of electrons
• There are “shells” around the
nucleus kind of like different
orbits around a planet.
• Each shell can hold a different
amount of electrons:
– Shell #1 – 2 electrons
– Shell #2 – 8 electrons
– Shell #3 – 8 electrons
• Not a realistic explanation of how
electrons orbit the nucleus
The Electron Cloud Model
• The electron cloud
model explains the
unpredictable wave
behavior of electrons,
which could be
anywhere in the area
surrounding the nucleus
• These clouds are often
referred to as electron
• There are four
different shapes of
orbitals corresponding
to 4 letters:
– s
– p
• d
• f
• The orbital closest to the nucleus is the 1s orbital
and it can hold 2 electrons
• The 2s is next and can hold two more. Then there
is a 2p orbital that can hold 6 more electrons.
• Then comes a 3s (2), 3p (6) and a 3d (10), 4s (2),
4p(6), 4d (10), and 4f (14)
• But this method is also screwed up. In actuality,
the orbitals fill up in this order:
– 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 5d, 4f, 6p,
7s, 6d, 5f, 7p (or something like that – you don’t need
to remember it for sure)
Section 1 Review
Name three scientists who contributed to current
knowledge of the atom and explain their contributions.
How does the nucleus atom differ from the uniform sphere
model (Thomson’s) of the atom?
If a neutral atom has 49 protons, how many electrons does
it have?
Can you see at atom using just your eye? Is it bigger than
Describe the three kinds of particles found in atoms. Where
are they located in the atom and what are their charges?
In Rutherford’s experiment, why wouldn’t the electrons in
the atoms of the gold foil affect the paths of the alpha
What is an electron cloud?
How many orbitals are there? What are their names and
how many electrons can they hold?
The Elements
• Elements are materials that cannot
be broken down into simpler
materials without becoming a
different kind of atom
• As of 2002, there were 115 known
– 90 are naturally occurring
– 25 are synthetic elements (made in a
• 4 most common elements in life:
– Carbon, Hydrogen, Oxygen, Nitrogen
• 4 most common elements in the
– Oxygen, silica, aluminum, iron
The Periodic Table
• The Periodic Table is a chart that organizes
and displays information about the elements
• The elements represented by the symbols on
the periodic table are placed purposely in
their position on the table
• Even the shape of the periodic table is that
way on purpose
The Periodic Table
• The rows are called periods
– They have the same number of energy levels (1
in the 1s, etc.)
• The columns are called groups
– They have similar properties and tend to form
similar bonds
Atomic Number
• The atomic number is the top number in the
element’s periodic table block
• Tells the number of protons in the nucleus of each
atom of an element
– Also the number of electrons in an electronically neutral
• The number of protons remains constant in every
atom of an element
Isotopes and Ions
• Isotopes are atoms of the
same element that have
different numbers of
• Ions are atoms of the
same element that have
different numbers of
– Except for the elements in
Group 18, all atoms have
an empty spot in their
outermost electron orbital
and “nature” hates this
– So atoms either lose or
gain electrons to fill these
Mass Number and Atomic Mass
• Mass number is the number of protons plus the number of
neutrons (different mass numbers for different isotopes)
• Atomic mass is the average mass of all the isotopes of an
• The atomic mass is found below the element symbol
• The unit used for atomic mass is the atomic mass unit, or “u”
• The majority of
elements are metals
(blue in your book)
• Metals have a shiny
• Good conductors of heat
and electricity
• Solids at room
• Malleable (can be
• Ductile (can be drawn
into wires without
• Nonmetals are found on the
right side of the periodic table
(yellow in your book)
• Dull luster
• Poor conductors of heat and
• Many are gases at room
• Brittle (cannot change shape
without breaking)
• 96% of the human body is
made up of nonmatals
• Metalloids are found
between the metals and
nonmetals on the periodic
table (green in your book)
• Have characteristics of both
metals and nonmetals
• Do not conduct heat and
electricity as well as metals
• All are solids at room
Section 2 Review
What is an element?
What are the four most common elements in Earth?
Describe the difference between atomic number and
atomic mass.
What are isotopes? What is different in each isotope?
Explain some uses of metals.
What are the three different kinds of elements?
What is an isotope?
What is an ion?
Hector is new to your class today. He missed the lesson
on how to use the periodic table to find information
about the elements. Describe how you would teach
Hector to find the atomic number for the element
oxygen. Explain what this information tells him about
• Matter that has the
same composition and
properties throughout
is known as a
• A compound is a substance
whose smallest unit is made up
of atoms of more than one
• Written in formulas
– The subscript number tells how
many atoms of the preceding
element are in the compound
• Ex: CO2
– No subscript is used when only one
atom of the element is present
• Ex: HCl
• A given compound is always
made of the same elements in the
same proportion
– Ex: Water is always H2O, never
HO2 or H3O
• Mixtures occur when two or more substances mixed
together which don’t make a new substance
• Unlike compounds, the proportions of the substances can
be changed
• Examples: air, blood
• Mixtures can be separated easily
• Homogeneous mixtures are the same throughout the whole
– Ex: Kool-aid, soup broth
• In heterogeneous mixtures, you can see the different parts
– Ex: most rocks, tacos
Section 3 Review
1. List three examples of compounds and three
examples of mixtures. Explain your choices.
2. How can you tell that a substance is a compound
by looking at its formula?
3. Which kind of mixture is sometimes difficult to
distinguish from a compound? Why?
4. What is the difference between homogeneous
and heterogeneous mixtures?
5. Was your breakfast a compound, a homogeneous
mixture, or a heterogeneous mixture? Explain.