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The Structure of Matter
Unit 2
Atomic Structure
Chapter 3
Section 1
EARLY MODELS OF THE ATOM
Objectives
• Define the term “atom”
• Explain how the idea of the atom
originated
• List the postulates of Dalton’s atomic
theory
Greeks
• 450 BC—Greek philosopher
Democritus
• Proposed that all matter is made of
tiny, indivisible particles called
“atomos” (atom in English)
• Theory was that if you cut a material
over and over again, eventually you
would have a piece that cannot be cut
Atom
• An atom is the smallest particle of an
element that retains the chemical
identity of that element
• Democritus’ ideas were rejected at
the time (what keeps atoms
together?), but revisited in the 17th
and 18th centuries…talk about waiting
to be justified!
Laws of Science
• Late 1700s—Antoine Lavioser
proposed the law of conservation of
matter
• No mass is gained or lost in the rxn
• 1799—Joseph Louis Proust proposed
the law of constant composition
• A given compound always contains
the same elements in the same
proportions by mass
John Dalton
• 1803—Proposed the atomic theory of
matter
• Each element is composed of extremely
small particles called atoms
• All atoms of a given element are
identical, but they differ from those of
any other element
• Atoms are neither created nor destroyed
in any chemical rxn
• A given compound always has the same
relative numbers and kinds of atoms
Elements
• There are over 100 elements,
therefore there are over 100 kinds of
atoms
• Do we only have 100 substances in
the world?
• Atoms combine to make up every
substance in the world around us
Atoms
• Can we be absolutely 100% sure
atoms exist?
• Atoms are too small to be seen with a
microscope
• 1981—Scanning tunneling microscope
gave us our closest look at an atom,
but shows virtually nothing about
what an atom is like on the inside
Two(?) Worlds
• The macroscopic world—ordinarysized objects that we can see
• The submicroscopic world—smaller
than we can see
• Microscopic world—Need special
techniques to see, but can be
relatively easy to work with
Chemist’s Work
• Chemists work on the macroscopic
and microscopic worlds and use their
findings to understand the
submicroscopic world
In Summation
• What is an atom?
• List the postulates of Dalton’s atomic
theory of matter
• Why did the ancient Greeks reject
Democritus’ ideas about atoms?
• Why is there such diversity in nature if
there are only about 100 different
kinds of atoms?
Section 2
DISCOVERING ATOMIC
STRUCTURE
Faraday Again
• Proposed that atoms and electricity
are somehow connected
• Led to the discovery that atoms
contain particles with electric charges
Benjamin Franklin
• Wait, this isn’t history class!
• Franklin was a scientist as well
• Flew a kite with a key attached to it
during a thunderstorm
• …The idiot
• Lightning struck the key
Benjamin Franklin
• Concluded that an object could have
one of two kinds of electric charge:
positive (+) or negative (-)
• Like charges repel each other,
opposite charges attract
Cathode Rays and Electrons
• Using a vacuum sealed tube, scientists
were able to observe a type of
radiation
• Called a cathode ray tube, used to be
common in TVs (you can tell the age
of the book from this comment)
Cathode Rays and Electrons
• 1896—J. J. Thomson worked with the
cathode ray tube through several
experiments
• Concluded that the particles of
radiation are negative
• Named them electrons (e—)
Cathode Rays and Electrons
• Thomson determined that the total
electrical charge to its mass is 1.76 x
108 coulombs per gram
• Robert Millikan was able to determine
the mass of the electron by
determining a single electron’s charge
• Charge: 1.60 x 10-19 coulomb
• Mass: 9.11 x 10-28 gram
Radioactivity
• 1896—Henri Becquerel
• Accidentally discovered that uranium
is radioactive
• Radioactivity: the spontaneous
emission of radiation from an element
• Marie and Pierre Curie isolated two
other radioactive elements: radium
and polonium
Radioactivity
• Radioactive decay causes
fundamental changes in the atom
• Rutherford studied this radiation and
realized that there are three types
– α radiation, which has a charge of +2
– β radiation, which has a charge of -1
– γ radiation, which has no charge or mass
The Nuclear Atom
• Thomson had shown the existence of
electrons, but atoms are neutral
• Rutherford fired alpha rays at gold foil
• Most particles went straight through
• About 1/8000 were deflected, and in
all directions!
The Nuclear Atom
• Rutherford eventually realized that
the “plum pudding” model of the
atom was incorrect
• Proposed that most of an atom’s
mass, including its positive charge, is
at a very small core in the center
• Called this core the nucleus
• EXTREMELY tiny!
Section 3
MODERN ATOMIC THEORY
Objectives
• Name and describe the three subatomic
particles of the atom
• Explain how to determine the number of
protons, neutrons, and electrons in an
atom
• Explain how an ion differs from an atom
• Explain how isotopes of the same
element differ
• Explain the relationship between atomic
mass and the relative abundance of
isotopes
The Structure of the Atom
• Remember that the central core of an
atom is a nucleus
• Contains two types of particles:
protons and neutrons
• Protons have an equal charge as an
electron, only positive
• Neutrons have no charge and are very
slightly heavier than a proton
The Structure of the Atom
• Rutherford visualized the atom as a
miniature solar system with electrons
circling the nucleus
• A more accurate model shows the
likely positions of electrons
The Structure of the Atom
Particle
Location
Charge (C)
Mass (g)
Mass (amu)
Proton
Inside
nucleus
+1.602 x 10-19
1.673 x 10-24
1.0073 = 1
Neutron
Inside
nucleus
0
1.675 x 10-24
1.0087 = 1
Electron
Outside
nucleus
-1.602 x 10-19
9.109 x 10-28
0.0006 = 0
The Structure of an Atom
• For simplicity’s sake, an electron has a
1- charge and a proton has a 1+
charge
• Mass is expressed in atomic mass
units (amu)
• Protons and neutrons are
approximately 1 amu, and electrons
are essentially 0 amu
The Structure of an Atom
• The diameter of atoms range from
0.100 to 0.500 nm
• The nucleus of the atom is very much
smaller!
Atomic Numbers
• Henry Moseley—Discovered that each
element has a unique positive charge
in the nucleus
• The number of protons in the nucleus
is equal to the atomic number
• Neutral atoms have the same number
of electrons
Ions
• When an atom gains or loses one or
more electrons, it gains a net
electrical charge
• This charge causes the atom to
become an ion
• Charge of ion = # of protons - # of e—
Isotopes
• All atoms of an element always have
the same protons in their nuclei
• Not all have the same number of
neutrons
• Atoms with the same number of
protons but a different number of
neutrons are called isotopes
Isotopes
• Nature contains a mixture of isotopes,
typically found in the same
percentages
• An atom’s mass number is the sum of
the isotope’s protons and neutrons
• Naming an isotope
The Mass of an Atom
• Because protons and neutrons have
slightly different masses, the amu
needed to be adjusted
• Scientists assigned carbon-12 to be
the standard at 12 amu
The Mass of an Atom
• Average mass of an element’s atoms
is called the atomic mass
• Listed on the periodic table
Objectives in Review
• What are the names and properties of
the three subatomic particles?
• How can you determine the number
of protons, neutrons, and electrons in
an atom or ion?
• What is an isotope? What is atomic
mass?
Section 4
CHANGES IN THE NUCLEUS
Objectives
• Describe the changes that may occur
in the nucleus of a radioactive
element
• Write a nuclear reaction to represent
radioactive decay
• Compare and contrast alpha, beta,
and gamma radiation
Nuclear Reactions
• Neither physical nor chemical
reactions cause changes in the
nucleus
• A nuclear reaction changes the
composition of an atom’s nucleus
Nuclear Stability
• Most atoms are stable (not
radioactive)
• Instability is controlled by the number
of protons and neutrons
Nuclear Stability
• If protons all have a positive charge,
what is stopping the nucleus from
repelling its own particles?
• A strong nuclear force is an attractive
force that overcomes the electric
repulsion between protons
• Only works when subatomic particles
are extremely close
Nuclear Stability
• Neutrons add a net attractive force to
the nucleus
• Neutrons = glue
Nuclear Stability
• Look at the belt of stability on pg. 113
• More neutrons are needed for larger
elements to be stable
• ALL elements beyond bismuth (83)
are radioactive
Nuclear Stability
• A nucleus can become unstable if it
has too few neutrons, but also if it has
too many
• Excess neutrons often release beta
radiation
Types of Radioactive Decay
• What are the three types of
radiation?
• Alpha (α), beta (β), gamma (γ)
Types of Radioactive Decay
• An alpha particle consists of two
protons and two neutrons
• Equivalent to a helium-4 nucleus
• Not much power
• Symbolized by:
Types of Radioactive Decay
• Radiation can be useful!
• Read the article about smoke
detectors on page 114
Types of Radioactive Decay
• Beta radiation is a stream of highspeed electrons
• Still a nuclear reaction; comes from a
change in the nucleus
• A neutron changes into a proton and
an electron
• The proton stays in the nucleus; the
beta particle (electron) is fired out at
high speed
Types of Radioactive Decay
• Much more penetrating than an alpha
particle (100x)
• Can penetrate 1-2 mm into solid
material
• Represented by:
Types of Radioactive Decay
• Gamma rays are a very energetic form
of light
• Outside of the visible spectrum
• Does not consist of particles
• Represented by:
Types of Radioactive Decay
• Accompanies the other two types of
radiation
• Much more penetrating; can only be
stopped by substances such as
concrete or lead
Types of Radioactive Decay
• When an atom emits one of these
kinds of radiation, it is undergoing
radioactive decay
• Scientists use nuclear equations to
keep track of a reaction’s components
Practice!
1. Write a nuclear equation for the
alpha decay of Protactinium-231
2. Write a nuclear equation for the beta
decay of Francium-223
Other Nuclear Reactions
• Radioactive decay is only one type of
nuclear rxn
• Fusion: the collision and joining of
two nuclei
• Fission: the splitting of a nucleus
• More will be covered in Chapter 24
Objectives in Review
• What changes accompany nuclear
reactions?
• What is radioactivity?