Quiz

1
Quiz

1. Compare Aristotles and Democrituss ideas about
   Matter
2. Why was Aristotle held to be correct?
3. List 3 ideas from Daltons Atomic Theory
4. Who discovered electrons?
5. Draw a model of Thomsons atom.
6. What allowed John Dalton to verify his atomic
   theory?

2

1. Why did the existence of electrons account for
   the existence of protons?
2. Describe Rutherfords experiment thoroughly
3. Explain Rutherfords model of the atom
4. What did Millikan discover?
5. What do opposite charges do to each other?
6. What do like charges do to each other?

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Chemistry February 26, 2007

• 1) Review for Quiz on Factor Label
• 2) Quiz on Factor Label
• 3) Notes on History of Atomic Theory
• 4) HW WS 4.1
• 5) Turn in Vocabulary in box now, please

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Chemistry Unit 4Atomic Structure

• Spring 2007
• 1st, 2nd, 4th Periods

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Answer the following questions in your notes

• 1) Write your definition of an atom
• 2) If you could see atoms, sketch (or describe)
  a model of the atom

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Chapter Objectives

• 1. Discuss early theories about matter
• 2. Discuss the discovery of subatomic particles
• 3. Use terms such as mass number, atomic number,
  atomic mass to describe elements

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Early Theories About Matter

• Democritus (460-370 BC)
• Greek Philosopher
• Around 420 BC, he proposed that matter was
  discontinuous
• There was some point at which matter could NOT be
  divided further

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Early Theories About Matter

• Aristotle (384-322 BC)
• Around 360 BC, he proposed matter was continuous
• Matter could be subdivided indefinitely without
  ever reaching a limit
• He believed there was no ultimate underlying
  structure to matter

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Early Theories About Matter

• Aristotles hypothesis was held to be correct
• In ancient Greece, hypotheses were not tested
• Hypotheses were accepted or rejected based on the
  reputation of the philosopher

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Early Theories About Matter

• In the 1500s and 1600s, a basic change in the way
  science was done occurred.
• Hypotheses were now TESTED!

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Development of the Modern Atomic Theory

• In 1782, a French chemist, Antoine Lavoisier
  (1743-1794), made measurements of chemical change
  in a sealed container.
• He observed that the mass of reactants in the
  container before a chemical reaction was equal to
  the mass of the products after the reaction.

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Development of the Modern Atomic Theory

• Lavoisier concluded that when a chemical reaction
  occurs, mass is neither created nor destroyed but
  only changed.

• Lavoisiers conclusion became known as the law of
  conservation of mass.

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Development of the Modern Atomic Theory
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Development of the Modern Atomic Theory

• In 1799, another French chemist, Joseph Proust,
  observed that the composition of water is always
  11 percent hydrogen and 89 percent oxygen by
  mass.

• Regardless of the source of the water, it always
  contains these same percentages of hydrogen and
  oxygen.

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Development of the Modern Atomic Theory

• Proust studied many other compounds and observed
  that the elements that composed the compounds
  were always in a certain proportion by mass. This
  principle is now referred to as the law of
  definite proportions.

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Daltons Atomic Theory

• John Dalton (1766-1844), an English schoolteacher
  and chemist, studied the results of experiments
  by Lavoisier, Proust, and many other scientists.

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Daltons Atomic Theory

• John Dalton conducted various experiments and
  formulated his atomic theory. He found that
• 1) All matter is composed of extremely small
  particles called atoms

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Daltons Atomic Theory

• 2) Atoms of a given element are identical in
  size, mass, and other properties atoms of
  different elements differ in size, mass, and
  other properties
• 3) Atoms cannot be subdivided, created, or
  destroyed

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Daltons Atomic Theory

• 4) Atoms of different elements can combine in
  simple, whole number ratios to form chemical
  compounds
• 5) And finally, Dalton found that in chemical
  reactions, atoms are combined, separated, or
  rearranged

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Daltons Atomic Theory

• Dalton was able to develop his theory based on
  the invention of the chemical balance
• Although his theory has been modified slightly to
  accommodate new discoveries, Daltons theory was
  so insightful that it has remained essentially
  intact up to the present time.

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Copy Answer in Notes

• Answer the following questions on a separate
  sheet of paper
• 1) Compare and contrast Democrituss and
  Aristotles early theories about the atom
• 2) Which philosopher (Democritus or Aristotle)
  was correct Hint Look at Daltons Atomic
  Theory

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Questions Continued

• 3) On what basis did Aristotle and Democritus
  propose hypotheses about the nature of matter?
• 4) How did Daltons Atomic Theory differ from
  Democrituss or Aristotles Theories?

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Homework

• Construct complete a chart with the following
• 1) Individuals Who Proposed Various Atomic
  Theories
• 2) Approximate Date Theory Proposed
• 3) Theory
• 4) Description of Experiment Used
• 5) Any discoveries made
• 6) Other important information

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Discovery of the Electron

• Because of Daltons atomic theory, most
  scientists in the 1800s believed that the atom
  was like a tiny solid ball that could not be
  broken up into parts.
• In 1897, a British physicist, J.J. Thomson,
  discovered that this solid-ball model was not
  accurate.

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JJ Thomsons Experiment

• JJ Thomson (1856-1940)-British Physicist
• Credited with discovery of electrons in 1897
• Reasoned that negative particles resided in the
  atom
• He called these particles electrons

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The Electron

• Because of Daltons atomic theory, most
  scientists in the 1800s believed that the atom
  was like a tiny solid ball that could not be
  broken up into parts.

• In 1897, a British physicist, J.J. Thomson,
  discovered that this solid-ball model was not
  accurate.

• Thomsons experiments used a vacuum tube.

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The Electron

• A vacuum tube has had all gases pumped out of it.

• At each end of the tube is a metal piece called
  an electrode, which is connected through the
  glass to a metal terminal outside the tube.

• These electrodes become electrically charged when
  they are connected to a high-voltage electrical
  source.

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Cathode-Ray Tube

• When the electrodes are charged, rays travel in
  the tube from the negative electrode, which is
  the cathode, to the positive electrode, the
  anode.

• Because these rays originate at the cathode, they
  are called cathode rays.

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Video of Thomsons Experiment
Click box to view movie clip.
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Cathode-Ray Tube

• Thomson found that the rays bent toward a
  positively charged plate and away from a
  negatively charged plate.

• He knew that objects with like charges repel each
  other, and objects with unlike charges attract
  each other.

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Cathode-Ray Tube

• Thomson concluded that cathode rays are made up
  of invisible, negatively charged particles
  referred to as electrons.

• These electrons had to come from the matter
  (atoms) of the negative electrode.

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Animation of Thomsons Experiment

• http//www.dlt.ncssm.edu/core/Chapter3-Atomic_Str_
  20Part1/Chapter3-Animations/Canal_Ray-CRT.html

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• He reasoned that since atoms are NEUTRAL, there
  must be some positive charge to balance out this
  negative charge
• He proposed a dense cloud of positive charge with
  the negative charge randomly embedded within the
  cloud
• His model is called the plum pudding model
• Similar to a pepperoni pizza model

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Thomsons Plum Pudding Model
The dense cloud of positive charge
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Thomsons Experiment

• 1) What did he discover?
• 2) In what year did he make his discovery?
• 3) Adequately describe the experiment that he
  used.
• 4) How did his discovery support or disprove
  Daltons atomic theory?

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Thomsons Experiment

• 5) Why were the electrons in the cathode tube
  attracted to the positive plates?
• 6) Draw a model of what Thomson believed the
  atom looked like.

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Millikans Oil Drop Experiment

• Robert Millikan (1868-1953)
• American scientist, University of Chicago
• Discovered the charge on an electron using the
  oil drop experiment

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Animation of Millikans Experiment

• http//www.dlt.ncssm.edu/core/Chapter3-Atomic_Str_
  20Part1/Chapter3-Animations/OilDrop.html

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Millikans Experiment

• Liquid goes through an atomizer to produce
  droplets
• These droplets enter a chamber
• The bottom of the chamber has a hole in the
  center
• The droplets fall through the hole due to gravity

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• An electrical field is produced using electricity
• This field offsets the effect of gravity causing
  the droplets to be suspended in mid air
• Through a series of known relationships and
  mathematical equations, Millikan calculates the
  charge of the electron in Coulombs.

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• Thomsons and Millikans work allowed for the
  mass of the electron to be determined using known
  relationships and equations

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Discovery of Neutron

• James Chadwick (1891-1974) is credited with the
  discovery of the neutron in 1932

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Rutherfords Model of the Atom

• Ernest Rutherford (1871-1937) from New Zealand
• Thomsons atomic model had not yet been tested
  due to the tiny size of the atom

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Rutherfords Model of the Atom

• Rutherford had been studying radioactivity
• In 1911, he bombarded a thin sheet of metal with
  alpha particles
• Alpha particles are positively charged

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The Gold Foil Experiment
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• http//micro.magnet.fsu.edu/electromag/java/ruther
  ford/

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Video of Rutherfords Experiment
Click box to view movie clip.
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• If Thomsons plum pudding model was correct, he
  expected the alpha particles to crash through the
  gold foil (with only minor deflections)
• He placed a screen around the foil. After the
  alpha particles passed through the gold foil,
  they would light up the screen. Most of the
  particles should pass through unaffected

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Rutherfords Results

• Some of the alpha particles passed through and
  hit the screen as predicted
• BUT a high number of alpha particles were
  deflected. This indicated that there must be
  something in the center of the atom that caused
  deflection

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Rutherfords Conclusion

• He reasoned that a positive charge center rested
  in the atom
• He called this center the nucleus

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Thomsons Movie
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Video of Rutherfords Experiment
Click box to view movie clip.
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Rutherfords Experiment
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Neils Bohrs Model

• His model is called the solar systemmodel
• The center of the atom contains protons and
  neutrons
• This area is collectively called the nucleus
• The electrons orbit around the nucleus like the
  planets do the sun

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Question (copy and answer in your notes)

• The planets orbit around the sun due to gravity.
  Why do electrons orbit the nucleus?

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The Three Subatomic Particles

• After various molecular models of the atoms had
  been tested, it was determined that three
  subatomic particles made up the atom
• Protons
• Neutrons
• Electrons

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Protons

• Protons are found in the nucleus
• Protons have an actual charge of 1.6 x
  10-19 C and a relative charge of 1
• The actual mass of a proton is 1.67 x 10-24 g
• The relative mass of a proton is 1 atomic mass
  unit (amu)
• The symbol is p

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Neutrons

• Neutrons are found in the nucleus
• Neutrons have an actual charge of 0 C and a
  relative charge of 0
• The actual mass of a neutron is 1.67 x 10-24 g
• The relative mass of a neutron is 1 atomic mass
  unit (amu)
• The symbol is n0

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Electrons

• Electrons are found outside the nucleus
• Electrons have an actual charge of -1.6 x 10-19 C
  and a relative charge of -1
• The actual mass of an electron is 9.11 x 10-28 g

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Electrons

• The relative mass of a electron is 1/1840 atomic
  mass unit (amu)
• The electron has a very small mass
• The symbol is e-

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Nuclear Forces

• Neutrons exert an attraction toward protons and
  other neutrons
• Overall the forces produce enough attraction to
  keep protons relative close together in the
  nucleus

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Construct and complete a chart with the following
information

• List the particles of the atoms
• List the particles location in the atom
• List the relative and actual charge of the
  particle
• List the relative and actual mass of the
  particles
• List the symbol of the particle

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Atomic Number

• Each element has a certain number of protons in
  its nucleus
• The number of protons in the nucleus is called
  the atomic number
• Each element has its own atomic number because
  each element has its own, unique number of protons

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• An element has certain chemical properties based
  on the number of protons in its nucleus
• Elements are numbered in the periodic table based
  on the atomic number
• Example If an element has two protons, what is
  its atomic number? What is the identity of the
  element?

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On Your Own

• Which element has
• a) 12 protons
• b) 35 protons
• c) 50 protons
• d) 92 protons

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• Since atoms are neutral, the number of protons
  must equal the number of electrons
• Example If an element (atom) has 7 electrons,
  how many protons does it have? What is the
  elements identity?

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• On Your Own
• Determine the number of protons in the following
  atoms as well as each atoms identity
• a) 6 electrons b) 14 electrons
• c) 72 electrons d) 55 electrons

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Mass Number

• Mass Number the number of protons and neutrons
  in an atom added together
• Mathematically
• Mass Number Protons Neutrons

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Example

• What is the mass number of an atom with 16
  protons and 16 neutrons?

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On Your Own

• Determine the Mass Number for the following atoms
• a) 17 protons and 18 neutrons
• b) 11 protons and 12 neutrons
• c) 1 proton and NO neutrons
• d) 3 protons and 4 neutrons

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Nuclide Symbols

• Mass Numbers are written in the upper left
  preceding the chemical symbol
• Atomic Number is written directly under the mass
  number

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• Example Write the correct nuclide symbol for an
  element with 79 p and 118 n0

Mass Number
197
Au
Element Symbol
79
Atomic Number
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Copy and Answer in Notes

• 1. List the three subatomic particles, their
  relative charge, relative mass, and location
• 2. What does atomic number mean?
• 3. What atom has an atomic number of 32?
• 4. Determine the number of protons and electrons
  ina. Ne b. Po c. Pt

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Review-Complete the chart
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Isotopes

• Atoms of the same elements have the same number
  of protons
• HOWEVER there may be different numbers of neutrons

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Isotopes

• When an elements atom has different numbers of
  neutrons, it is said to have isotopes
• Isotopes are common in nature

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Hydrogens Three Isotopes

• Hydrogen has the following isotopes
• Protium-a hydrogen atom with one proton and NO
  neutrons

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Hydrogens Three Isotopes

• Deuterium-a hydrogen atom with one proton and
  only one neutron
• Tritium-a hydrogen atom with one proton and two
  neutrons

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On Your Own

• Determine the mass and atomic numbers for each of
  hydrogens isotopes. Put this information into
  nuclide symbol form

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Average Atomic Mass

• Atomic mass is the mass of an atom expressed in
  atomic mass units or amu
• The atomic mass unit is based in relation to the
  standard of carbon-12

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Average Atomic Mass Continued

• Carbon-12 has a mass of 12.000 00 amu
• If an atoms weighs half as much as carbon-12, its
  atomic mass will be 6.000 00 amu

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• If an atom weighs four times as much as
  carbon-12, it will have a mass of 48.000 00 amu
• The atomic mass that is reported in the periodic
  table is a weighted average based on the relative
  abundance of each element

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Review - Answer in Notes

• 1. What mass would an atom have that has a mass
  equal to 1/12 that of carbon-12? What element is
  this?
• 2. Determine the mass each atom would have if
• a. It weighed 12 times as much as C-12
• b. It weighed 3/17 as much as C-12
• 3. How do all isotopes differ from each other?

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• Relative abundance refers to how commonly the
  isotope occurs in nature
• Certain isotopes will occur more commonly than
  other isotopes

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To Determine Avg. Atomic Mass

• 1) First convert relative abundance () to
  decimal equivalent
• 2) Multiply mass (in amu) by decimal equivalent
• 3) Add the numbers together
• 4) The sum (in amu) is the average atomic mass

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Copy Answer in Notes

• 1. What is an isotope?
• 2. How is mass number different from atomic
  number?
• 3. How is relative () abundance determined?

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• For example, an element has two naturally
  occurring isotopes. One isotope has a relative
  abundance of 19.91 and a mass of 10.012 amu. A
  second isotope has a relative abundance of
  80.08 and a mass of 11.009 amu. Calculate the
  atomic mass

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Additional Example

• Calculate the average atomic masses for the
  following
• Isotope Rel. Abund. Rel. Mass
• hydrogen-1 99.985 1.007 825
• hydrogen-2 0.015 2.0140
• Ans) 1.008 amu

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Copy Answer in Notes

• 3. Determine Avg. Atomic Mass for oxygen
• Isotope Rel. Abund. Actual Mass
• O-16 99.762 15.994 915
• O-17 0.038 16.999 13
• O-18 0.200 17.999 160

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• Isotope rel. rel. mass abund.
  () (in amu)
• b. C-12 98.90 12.0 C-13
  1.10 13.003 355
• c. U-235 0.720 235.043 U-238 99.280
  238.050 78
• d. O-16 99.762 15.994 915 O-17 0.038 16.999
  131 O-18 0.200 17.999 160