Atoms: The Building Block of Matter

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Atoms The Building Block of Matter 

• 3-1 The Atom From Philosophical Idea to
  Scientific Theory

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From Philosophical Idea to Scientific Theory

• The first idea of matter was simply that all
  matter was infinitely divisible.
• Ex. Folding a piece of paper
• Ex. You could continue to cut a piece of copper
  into smaller and smaller pieces forever.
• The Particle Theory of matter was first
  supported by the Greeks and others scientists
  (Democritus) around 400 B.C.

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From Philosophical Idea to Scientific Theory

• He proposed that all matter that makes up the
  world is composed of small, indivisible
  particles.
• Democritus called the building block of matter,
  atomos, or the atom.
• Interestingly, Aristotle did not agree with
  Democritus because there was no evidence to
  support these claims.

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• Ex. You cannot continue to cut a piece of copper
  into smaller pieces, eventually you get to copper
  atoms which cannot be divided any farther.

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Foundations of Atomic Theory

• One of the biggest speculations at the beginning
  of the 1700s was on whether elements always
  combine in the same ratios when forming
  compounds.
• Chemical Reaction the transformation of one
  substance into new substances.
• Law of Conservation of Matter (Mass) states
  that matter is neither created nor destroyed
  during a physical or chem. Rxn.

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Foundations of Atomic Theory

• Law of Definite Proportions a Chem. cmpd.
  Contains the same elements in exactly the same
  ratio regardless of the source or sample size.
• H2O taken from a stream.
• H2O taken from a paper cup.
• NaCl always contains 39.34 Na by mass 60.66 Cl
  by mass.

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Foundations of Atomic Theory

• Law of Multiple Proportions - if 2 or more
  different cmpds. are composed of the same 2
  elements, then the ratio of the masses of the
  second element combined with a certain mass of
  the first element is always a ratio of small
  whole numbers.
• In CO2 1 g C combines with 2.66 g O

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Daltons Atomic Theory

• I like this guy because he was a schoolteacher.
• Dalton proposed an explanation for the laws
  listed above.
• He believed that elements are composed of atoms,
  and that only whole numbers of atoms can combine
  to form cmpds.
• Ex. Water could never a formula H2.124O
• Look at Daltons Postulates on pg. 66.

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Modern Atomic Theory

• Dalton turned Democrituss idea into a scientific
  theory that could be tested by experiment.
• Some parts of Daltons theory have actually been
  proved NOT to be true. We will discuss these
  later.

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Atoms The Building Block of Matter

• 3-2 The Structure of the Atom

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• Although Dalton thought the atom to be
  indivisible, it is actually composed of other
  subatomic particles.
• Subatomic Particles ? protons, neutrons,
  electrons.

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Discovery of the Electron

• Electron was discovered through experiments using
  cathode-ray tubes.
• A stream of charged particles flows from the
  cathode to the anode in a cathode ray tube,
  causing the fluorescent material inside the tube
  to glow.

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Discovery of the Electron

• The negative electrode is the cathode.
• The positive electrode is the anode.
• Cathode rays were deflected by magnetic fields.
• The ray was deflected away from a negative field
  and toward a positive field.
• Particles that compose cathode rays are
  (-)vely charged.

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Discovery of the Electron

• Thomsons Plum Pudding Model of the atom.
• An atom contains a specific number of electrons
  which are in pool of positive charge.
• Ex. Like the raisins in plum pudding (or the
  chocolate chips in a cookie)

• He knew only that there was positive charge, NOT
  that there were positive particles.

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Discovery of the Atomic Nucleus

• Rutherfords Gold Foil Experiment
• If Thomsons Model was correct, the alpha
  particles should have passed directly through the
  foil with only slight deflections. Most of the
  particles acted this way, but some were deflected
  at wide-angles. The wide angle deflection could
  only have been caused if there was a powerful
  force in the atom. He reasoned their must be a
  small, dense center containing most of the mass
  of the atom nucleus.

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The Gold Foil Experiment (figure 3-14)
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http//micro.magnet.fsu.edu/electromag/java/ruther
ford/

• Most particles passed through gold without a
  problem
• 1 in 8000 alpha particles deflected
• These were sent in ALL directions including
  straight back!

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What does this mean?

• Most of the atoms positive charge, as well as the
  mass is in the middle, called the nucleus.
• Most pass through the empty space but
  occasionally one gets close enough to the
  positive nucleus to deflect it.

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Composition of the Atomic Nucleus

• Nucleus contains positive protons, neutral
  neutrons.
• The nucleus has a net positive charge.
• Atoms are electrically neutral because the
  positive nucleus is surrounded by a sea of
  negative electrons.
• The number of protons in an atom determines the
  atoms identity.
• See Table 3-1 pg. 74

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Question If an atom contains positive
particles, what keeps the atom together? Dont
like charges repel each other?

• Strong Nuclear Force
• Binding Energy
• Size of the atom

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Atoms The Building Block of Matter

• 3-3 Counting Atoms

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The Structure of the Atom

• The atom has a positively changed central core
• Contains Protons and Neutrons
• Protons are positive, equal and opposite to
  electrons
• Neutrons do not carry a charge and are slightly
  more massive

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• 1 proton has the mass of about 2000 electrons

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• Electrons move in space around the nucleus
• Rutherford visualized it as a mini solar system.

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Atomic Numbers

• Henry Moseley found that atoms contain unique
  positive charge in their nucleus.
• The number of protons is called the atomic number.

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• The atomic number indicates protons.
• Chlorine has 17 protons atomic number

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Practice

• How many protons and electrons are in a magnesium
  atom?
• What is the name of the element that has atoms
  that contain 11 protons.

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Ions

• When an atom gains or loses electrons it acquires
  a charge
• Fewer electrons means positive charge
• More electrons means negative charge
• Charge of ion protons - electrons

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Sample

• Write the chemical symbol for the ion with 9
  protons and 10 electrons
• Answer F-
• What is the symbol of the ion with 13 protons and
  10 electrons?
• Answer Al3
• 7 Protons and 10 electrons?
• N 3-

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Isotopes

• Dalton said all atoms of an element are the same.
• Not quite true, ISOTOPES have a different number
  of neutrons

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• In nature, elements are almost always found as a
  mixture of isotopes
• Isotopes are usually in the same percentages.

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• To identify isotopes more specifically
• Use the Mass Number
• Mass Number ( protons) ( neutrons)

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• To identify an isotope chemists write the mass
  number behind the element symbol for example
  Cl-37 indicates that this chlorine has 20
  neutrons, it is written in symbol form as 3717Cl
• Cl-35 has 18 neutrons and is written as 3517Cl

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The Mass of an Atom

• Measured by Atomic Mass Units (AMU)
• The atomic mass is approximately the same as the
  sum of protons and neutrons
• This is not very precise so Scientists define it
  more precisely.

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• 1 amu is equal to 1/12 the mass of a carbon-12
  atom.
• 1.66 x 10 24 grams
• Carbon 12 is the only element with an AMU equal
  to protons and neutrons, because of isotopes

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• The average mass of an elements atoms is called
  the atomic mass.
• AM (mass isotope x abundance)(mass isotope x
  abundance)...

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Fundamental Subatomic Particles
Particle Location Charge (C) Mass (g) Mass (AMU)
Proton Inside nucleus 1.602 x 10-19 1.673 x 10-24 1
Neutron Inside nucleus 0 1.675 x 10-24 1
Electron Outside nucleus -1.602 x 10-19 9.109 x 10 28 0
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Relative Mass to Numbers of Atoms

• Mole is the amount of a substance that contains
  as many particles as there are atoms in exactly
  12 g of carbon-12.
• SI unit for amount.
• It is a unit which relates atoms and masses.

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Relative Mass to Numbers of Atoms

• Avogadros number is the number of particles in
  exactly 1 mole of a substance.
• 6.02 x 1023
• 1 mole of carbon 6.02 x 1023 atoms
• 2 moles of silver 1.204 x 1024 atoms
• 1 mole of water 6.02 x 1023 water molecules
• 2 moles of marshmallows 1.204 x 1024
  marshmallows.

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Relative Mass to Numbers of Atoms

• Heres where it could get tricky
• 1 mole of water (H2O) 6.02 x 1023 molecules
• How many atoms are in 1 mole water?
• H2O is composed of 2 H and 1 O atoms 3 total
  atoms.
• 3 x (6.02 x 1023) 1.81 x 1024 atoms in water

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Relative Mass to Numbers of Atoms

• Molar Mass the mass of one mole of a substance.
• The amount of a substance that contains
  Avagadros number of particles.
• Usually written with unit g/mol.
• Molar masses are on the periodic table.
  Numerically they are the same as atomic mass.
• Ex. Molar mass of He 4.00 g/mol
• Molar mass of Al 26.98 g/mol

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Relative Mass to Numbers of Atoms

• Now is when it gets fun! Gram/mole
  ConversionsReady?
•  
• Be sure to look at the chart on pg.82. This is
  one of the most important ideas which we cover
  this year. The only way to understand this is to
  practice, practice, practice.
• Practice Problems pgs. 82-85