Atomic Structure

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Chapter 3

• Atomic Structure

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Section 3-1 Early Models of the Atom (A brief
history)

• 450 B.C.Democritus proposes that all matter is
  composed of tiny, indivisible particles called
  atoms
• 1789Lavoisiers Law of Conservation of Mass
• 1799Joseph Louis Proust, Law of Constant
  Composition a given compound always contains
  the same elements in the same proportions by mass

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• 1803John Dalton, 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 reaction.
• A given compound always has the same relative
  numbers and kinds of atoms.

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Section 3-2 Discovering Atomic Structure

• A long series of experiments proved that atoms
  contain particles that have electrical charge.
• Static electricity has been studied since the
  days of the ancient Greeks. They knew that if
  you rubbed a piece of amber with cloth, the amber
  would attract small dust particles.

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• Benjamin Franklin did his famous kite experiment
  using static electricity on a huge
  scalelightning!
• From this experiment, he learned two things
• There are two kinds of electrical charge and they
  are opposites. He named one kind of charge
  positive and the opposite kind of charge
  negative.
• Like charges repel and opposite charges attract.

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Cathode Rays and Electrons

• The cathode ray tube helped scientists learn more
  about charged particles.

The positively charged end is called the anode.
The cathode rays flow toward this end.
The negatively charged end is called the cathode.
The rays come from this end.
The charged particles make the gas inside the
tube glow.
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• By the end of the 1800s, scientists knew that a
  cathode ray could spin a small paddle wheel, so
  it must have mass. They also knew that a magnet
  could bend the cathode ray, so it must have an
  electric charge.
• 1896J.J. Thomson concluded that atoms contain
  negatively charged particles. He called these
  particles ELECTRONS.

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• Thomson was able to determine the ratio of an
  electrons charge to its mass
• 1.76 x 108 coulombs
• gram

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• 1909Robert Millikan calculated the charge of a
  single electron, 1.60 x 10-19 coulombs. Using
  Thomsons ratio, He calculated the mass of an
  electron.
• 1.60 x 10-19 coulombs X 1 gram
• 1.76 x 108 coulombs
• 9.11 x 10-28 grams

The mass of a single electron!
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• Atoms are electrically neutral, so there had to
  be a positive particle in an atom to cancel out
  the negative charge of the electron.

• scientists thought an atom looked like a ball of
  chocolate chip cookie dough (they called it the
  Plum Pudding Model). The dough would be the
  positive part of the atom, while the chocolate
  chips would be like the electrons.

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• 1909Ernest Rutherford showed that the positive
  charge is concentrated in the middle of the atom,
  and that the rest of an atom is actually lots of
  empty space.

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• During Rutherfords experiment, most of the alpha
  particles went straight through the gold foil.
  This showed that most of the atom is made of
  empty space. A few alpha particles bounced
  straight back, which means they hit something
  solidthe positive particles in the center of the
  atom!
• Rutherford called the center of the atom the
  nucleus.

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Radioactivity

• 1896Henri Becquerel discovered that a chunk of
  uranium could expose photographic film just like
  sunlight can.
• radioactivity the spontaneous emission of
  radiation from an element
• Marie and Pierre Curie discovered two more
  radioactive elementsradium and polonium.

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Section 3-3 Modern Atomic Theory

• Atoms are composed of protons, neutrons, and
  electrons.

Because these particles are so small, we usually
measure their masses in atomic mass units, not
grams.
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Atomic Numbers

• 1913Henry Moseley discovered that each element
  has a unique positive charge in the nucleus.
• An atoms identity comes from the number of
  protons in the nucleus, which is called the
  atomic number.

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Sample Problem 1

• How many protons and electrons are present in an
  oxygen atom?
• Ignore electrons for right now. Lets
  concentrate on protons. If the of protons is
  equal to the atomic number, and oxygens atomic
  number is 8, any oxygen atom has 8 protons.
• In a neutral atom, protons always equal
  electrons, so oxygen also has 8 electrons.

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Isotopes

• One of Daltons postulates that is incorrect says
  that all atoms of a given element are identical.
  Thats not necessarily true.
• Isotopes are atoms that have the same number of
  protons (so theyre the same element), but
  different numbers of neutrons.

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• To distinguish between isotopes of the same
  element, scientists put a number after the name
  of the element, like chlorine-37. This number is
  the mass number, which is the number of protons
  the number of neutrons in an atom.
• Both the atomic and the mass can be part of
  the chemical symbol for an element.
• Cl

element symbol
mass number
37 17
atomic number
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The Mass of an Atom

• Protons and neutrons both have an atomic mass of
  1 amu, so an atomic mass unit could be defined as
  the mass of a proton or a neutron.
• The problem is, protons and neutrons dont have
  exactly the same mass. There must be a better
  definition of an atomic mass unit.

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• Since atomic mass units are made up anyway,
  scientists got to randomly choose what 1 amu was
  equal to.
• 1 amu is equal to 1/12 the mass of an atom of
  carbon-12 (an isotope of carbon with 6 protons
  and 6 neutrons).

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• Now lets look at the element chlorine. There
  are two isotopes of chlorine, chlorine-37 and
  chlorine-35. They obviously have different
  masses, but if you look at the periodic table,
  the mass number they give you is 35.453. Where
  did that come from???

17 Cl Chlorine 35.453
?
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• The masses they give you on the periodic table
  are average atomic masses. They take the mass
  of every atom for each element in the world and
  average them together. Its not as hard as it
  sounds.
• 75 of chlorine atoms are chlorine-35
• 25 of chlorine atoms are chlorine-37
• (.75 x 35) (.25 x 37)
• 100
• 35.453

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Ions

• Ions are atoms that have either gained or lost
  electrons, so the protons and electrons are no
  longer equal.
• If an ion loses electrons, it now has more
  protons, so it has a positive charge.
• If an ion gains electrons, it now has more
  electrons, so it has a negative charge.
• number of protons
• ? number of electrons
• charge of ion

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Sample Problem 2

• Write the chemical symbol for the ion with 9
  protons and 10 electrons.
• 9 protons goes with the element fluorine
• F
• The protons and electrons are not equal, so it
  must have a charge.
• 9 protons
• ? 10 electrons
• -1
• F-1

Final answer
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Sample Problem 3

• How many protons, neutrons, and electrons are
  present in the Fe ion?
• Protons equal the atomic number, which is
• 26
• Neutrons equal the mass ? the atomic
• 56 ? 26 30
• Protons ? Electrons charge
• 26 ? e 2 so e 24

56 26
2
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Section 3-4 Changes in the Nucleus

• In chemical reactions, only the electrons change
  the nucleus of an atom is unaffected.
• If the nucleus is affected, the reaction is
  called a nuclear reaction.
• Most atoms have stable nuclei, which means the
  nucleus isnt going to change.
• A few atoms, however, are radioactive.
• What makes some atoms radioactive and others
  stable???

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• We know that protons and neutrons are both
  squeezed into the nucleus. The neutrons dont
  have charges, so they basically just sit there.
  The protons are all positively charged, and what
  do like charges do?
• REPEL
• As long as you dont have too many protons, the
  strong nuclear force can keep the nucleus
  together.

More strong internuclear forces overcome the
repulsive force between the protons.





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• For elements 1 to 20, stable nuclei have
  basically equal numbers of protons and neutrons.
• After element 20, you need to have more and more
  neutrons to be stable.
• After element 83, it doesnt matter how many
  neutrons you have. There are just too many
  protons. Everything after element 83 is
  radioactive.
• You can also be radioactive if you have too many
  neutrons. Some isotopes with either too many or
  too few neutrons are considered radioactive
  isotopes.

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Types of Radioactive Decay
4 2
4 2
0 -1
0 -1
0 0
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• Radioactive decay when an atom gives off
  radiation
• Nuclear equation an equation that keeps track
  of what atom you started out with, what type of
  radiation it gives off, and what the atom turns
  into
• Example

226 88
222 86
4 2
Ra ? Rn ?
Atom we start with
What it turns into
Type of radiation given off
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Sample Problem 4

• Write the nuclear equation for beta decay of
  sodium-24.
• First, write the symbol on the left of the arrow
  for what we start with, sodium-24
• Na ?
• Next, write the symbol on the right of the arrow
  for the type of radiation given off
• Na ? ______ ?
• Last, write the symbol in the empty space for
  what the atom turns into. The numbers on the
  right side of the arrow have to add up to equal
  the numbers on the left side of the arrow.
• Na ? Mg ?

24 11
24 11
0 -1
24 11
0 -1
24 12