Chemistry Chapter 4

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Chemistry Chapter 4
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The Structure of the Atom

• About 2500 years ago, Greek philosophers thought
  about matter and its composition
• 4th Century B.C. Democritus first suggested the
  idea of atoms (indivisible particles)
• Aristotle did not believe in atoms

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Antoine Lavoisier (1782)

• Father of Modern Chemistry
• French chemist
• The first to use truly quantitative research
• Observations led to the Law of Conservation
  of Mass
• Identified components of water as hydrogen and
  oxygen

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Joseph Proust (1799)

• French chemist
• Observed composition of water is always 11
  hydrogen and 89 oxygen by mass
• Studied many other compounds and always found a
  constant composition by mass for a given compound
• This is the Law of Definite Proportions

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John Dalton (1808)

• English schoolteacher
• Studied the results of Lavoisier and Proust and
  many other scientists
• He wanted an atomic theory to explain the
  experimental evidence
• His theory led to the solid ball model of the
  atom

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Daltons Atomic Theory

• All matter is composed of indivisible particles
  called atoms. Atoms cannot be subdivided,
  created, or destroyed
• Atoms of the same element are identical atoms of
  different elements are different
• Atoms of different elements combine in small
  whole number ratios to form compounds
• Chemical reactions occur when atoms are combined,
  separated, or rearranged

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Crookes Experiment (1870s)

• Gas Tubes w/2 electrodes (conductors)
• Anode positive
• Cathode negative
• Cathode ray tube (CRT) when voltage was applied
  a beam of light composed of particles was
  deflected by a magnet determined they were
  charged particles

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J.J. Thomson (1897)

• Was investigating the relationship between matter
  electricity
• Cathode ray tube experiment with a fluorescent
  screen allowed him to measure deflection when a
  magnet was used
• Measured ratio of charge to mass and determined
  particles were identical regardless of the gas
  used or the material of the cathode
• These particles were later named electrons
• This led to the plum pudding model

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Thomsons Plum Pudding Model

• Nobel Prize 1907
• Pudding was charge and most of the mass of the
  atom
• Plums - charged electrons spread throughout to
  make the atom neutral
• Ions / - charged atoms result from the loss
  or gain of electrons
• Cations positive charge / lost electrons
• Anions negative charge / gain electrons

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Robert A. Millikan (1909)

• Oil drop experiment suspended fine mist of oil
  droplets between charged plates
• Calculated the mass of an electron to be
  approximately 1/2000 the mass of an H atom
• Currently known to be 1/1837 of a H atom
• 9.109 x 10-31 kg

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Protons

• Since atoms are neutral, a positive charge must
  also exist in the atom
• Thomson showed that positively charged rays
  existed in the CRT (also seen by Goldstein in
  1886 canal rays)
• Protons finally identified by 1920
• Proton mass is 1836/1837 of H atom
• Mass of Proton 1.673 x 10-27 kg

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Radioactivity Discovered (Ch 25.1)

• 1896 Radioactivity discovered in Uranium by
  Becquerel
• Marie/Pierre Curie radium polonium
• Radiation energy that is emitted from a source
  and travels through space
• Radioactivity spontaneous radiation from the
  nucleus of an atom

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Radioactivity (Ch 25.1)

• By 1900 3 types of radiation identified
• Alpha (a) He nuclei(2p 2n0) 1/10th the
  speed of light stopped by paper or clothing
• Beta (ß) electrons at high speeds / stopped by
  a few mm of Al
• Gamma (g) high energy photon, form of
  electromagnetic radiation more energetic than
  X-rays stopped by several cm of Pb or more
  concrete/ no mass or charge

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Rutherford (1911)

• Gold foil experiment with alpha particles
• Led to the nuclear model of the atom
• Atoms contain a small dense nucleus
• Electrons move around like bees in a hive
• Diameter of nucleus 1/100,000 the size of the
  atom most of the atom is empty space
• 1920 Rutherford proposed neutral particles with
  the same mass as protons

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Chadwick - 1932

• Credited with the discovery of neutrons
• Nobel Prize 1935
• Neutron Mass 1.675 x 10-27 kg

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Forces in the Nucleus

• Like charges normally repel
• However, protons are strongly attracted to one
  another in the nucleus
• Also neutron/neutron and neutron/proton
  attractions
• These are the result of
• NUCLEAR FORCES

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Atomic Number Mass Number

• Atomic number (Z) the number of protons in the
  nucleus defines what element an atom is
• Mass number
• of protons(p) of neutrons(n0) Mass
  Number
• Atomic mass units (amu) 1/12 the mass of a
  carbon-12 atom
• 1 proton 1.007276 amu
• 1 neutron 1.008665 amu

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Isotopes

• Atoms of the same element with different numbers
  of neutrons
• Nuclide general term for any isotope of any
  element
• Each naturally occurring isotope has a
  abundance
• Symbols for isotopes
• Hyphen notation Lithium 6 Lithium 7
• Nuclear symbol 6Li 7Li
• 3 3
• Isotopes differ by
• Number of neutrons and therefore,
• Mass number and therefore,
• Atomic mass

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Isotopes (Ch 25.2)

• Of 1500 known isotopes, only 264 are stable
  others are radioactive
• Radioactive decay alpha, beta, or other
  particles are emitted and the nucleus changes to
  form a new element or isotope this continues
  until a stable form is reached

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Isotopes Cont

• Atomic mass is an average of the masses of an
  elements isotopes based on their abundance
• Carbon 12.011 amu
• Carbon 13 1.11
• Carbon 12 98.89
• Take relative abundance x mass of isotope and add
  together
• Example Problem Find average atomic mass of
  carbon
• .0111 x 13amu .1443 amu
• .9889 x 12 amu 11.8668 amu
• 12.011 amu

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Example Problem

• Using the following information, determine the
  atomic mass of chlorine Two isotopes are know
  chlorine-35 (mass35.0 amu) and chlorine-37
  (mass37.0 amu). Their relative abundances are
  75.4 and 24.6 respectively.

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Sample Problems
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Some possibly useful websites

• Lavoisier
• http//encarta.msn.com/encnet/refpages/RefArticle.
  aspx?refid761571807
• http//encarta.msn.com/encnet/refpages/RefMedia.as
  px?refid461541281
• Atomic Structure Time line
• http//www.watertown.k12.wi.us/hs/teachers/bues
  cher/atomtime.asp
• Rutherford
• http//micro.magnet.fsu.edu/electromag/java/rut
  herford
• Atomic theory http//www.brainpop.com/science/ma
  tter/atomicmodel/index.weml?tried_cookietrue
• http//www.sci.tamucc.edu/pals/morvant/genchem/at
  omic/index.htm
• http//www.sci.tamucc.edu/pals/morvant/genchem/at
  omic/page6.htm
• http//www.sci.tamucc.edu/pals/morvant/genchem/at
  omic/page8.htm
• http//www.sci.tamucc.edu/pals/morvant/genchem/at
  omic/page9.htm