Introduction to Atomic Theory

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Introduction to Atomic Theory
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History of the atom

• Democritus (400 BC) suggested that the material
  world was made up of tiny, indivisible particles
• atomos, Greek for uncuttable
• Aristotle believed that all matter was made up
  of 4 elements, combined in different proportions
• Fire - Hot
• Earth - Cool, heavy
• Water - Wet
• Air - Light
• The atomic view of matter faded for centuries,
  until early scientists attempted to explain the
  properties of gases

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Re-emergence of Atomic Theory

• John Dalton postulated that
• All matter is composed of extremely small,
  indivisible particles called atoms
• All atoms of a given element are identical (same
  properties) the atoms of different elements are
  different

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• 3. Atoms are neither created nor destroyed in
  chemical reactions, only rearranged
• 4. Compounds are formed when atoms of more than
  one element combine
• A given compound always has the same relative
  number and kind of atoms

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Make-up of the Atom

• By the 1850s, scientists began to realize that
  the atom was made up of subatomic particles
• Thought to be positive and negative

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Cathode Rays and Electrons

• Mid-1800s scientists began to study electrical
  discharge through cathode-ray tubes. Ex neon
  signs
• Partially evacuated tube in which a current
  passes through
• Forms a beam of electrons which move from
  cathode to anode
• Electrons themselves cant be seen, but certain
  materials fluoresce (give off light) when
  energised

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Oh there you are!

• JJ Thompson observed that when a magnetic or
  electric field are placed near the electron beam,
  they influence the direction of flow
• opposite charges attract each other, and like
  charges repel.
• The beam is negatively charged so it was repelled
  by the negative end of the magnet

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• http//www.chem.uiuc.edu/clcwebsite/video/Cath.mov
  
• Magnetic field forces the beam to bend depending
  on orientation
• Thompson concluded that
• Cathode rays consist of beams of particles
• The particles have a negative charge

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• Thompson understood that all matter was
  inherently neutral, so there must be a counter
• A positively charged particle, but where to put
  it
• It was suggested that the negative charges were
  balanced by a positive umbrella-charge
• Plum pudding model chocolate chip cookie
  model

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Rutherford and the Nucleus

• This theory was replaced with another, more
  modern one
• Ernest Rutherford (1910) studied angles at which
  a particles were scattered as they passed through
  a thin gold foil
• http//www.mhhe.com/physsci/chemistry/essentialche
  mistry/flash/ruther14.swf

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Rutherford expected

• Rutherford believed that the mass and positive
  charge was evenly distributed throughout the
  atom, allowing the a particles to pass through
  unhindered

a particles
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Rutherford explained

• Atom is mostly empty space
• Small, dense, and positive at the center
• Alpha particles were deflected if they got
  close enough

a particles
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The modern atom is composed of two regions

• Nucleus Containing protons and neutrons, it is
  the bulk of the atom and has a positive charge
  associated with it
• Electron cloud Responsible for the majority of
  the volume of the atom, it is here that the
  electrons can be found orbiting the nucleus
  (extranuclear)

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Major Subatomic Particles
Name Symbol Charge Relative Mass (amu) Actual Mass (g)
Electron e- -1 1/1840 9.11x10-28
Proton p 1 1 1.67x10-24
Neutron no 0 1 1.67x10-24

• Atoms are measured in picometers, 10-12 meters
• Hydrogen atom, 32 pm radius
• Nucleus tiny compared to atom
• If the atom were a stadium, the nucleus would be
  a marble
• Radius of the nucleus is on the order of 10-15 m
• Density within the atom is near 1014 g/cm3

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Elemental Classification

• Atomic Number (Z) number of protons (p) in
  the nucleus
• Determines the type of atom
• Li atoms always have 3 protons in the nucleus,
  Hg always 80
• Mass Number (A) number of protons neutrons
  Sum of p and nº
• Electrons have a negligible contribution to
  overall mass
• In a neutral atom there is the same number of
  electrons (e-) and protons (atomic number)

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Nuclear Symbols

• Every element is given a corresponding symbol
  which is composed of 1 or 2 letters (first letter
  upper case, second lower), as well as the mass
  number and atomic number

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• Find the
• number of protons
• number of neutrons
• number of electrons
• atomic number
• mass number

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• If an element has an atomic number of 34 and a
  mass number of 78 what is the
• number of protons in the atom?
• number of neutrons in the atom?
• number of electrons in the atom?
• complete symbol of the atom?
• If an element has 91 protons and 140 neutrons
  what is the
• atomic number?
• mass number?
• number of electrons?
• complete symbol?

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Isotopes

• Atoms of the same element can have different
  numbers of neutrons and therefore have different
  mass numbers
• The atoms of the same element that differ in the
  number of neutrons are called isotopes of that
  element
• When naming, write the mass number after the
  name of the element

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How heavy is an atom of oxygen?

• There are different kinds of oxygen atoms
  (different isotopes)
• 16O, 17O, 18O
• We are more concerned with average atomic
  masses, rather than exact ones
• Based on abundance of each isotope found in
  nature
• We cant use grams as the unit of measure
  because the numbers would be too small
• Instead we use Atomic Mass Units (amu)
• Standard amu is 1/12 the mass of a carbon-12
  atom
• Each isotope has its own atomic mass

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Calculating Averages

• Average ( as decimal) x (mass1) ( as
  decimal) x (mass2) ( as decimal) x
  (mass3)

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Try AgainTHINK!!

• Your test and quiz mark is 30. 10 is made up
  of quiz, 20 is made up of tests
• If you made 85, 78 and 79 on your quizzes
• If you made 87 on your only test
• What would be your test quiz mark for your report
  card?

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Calculating Averages

• Calculate the atomic mass of copper if copper has
  two isotopes
• 69.1 has a mass of 62.93 amu
• The rest (30.9) has a mass of 64.93 amu
• Magnesium has three isotopes
• 78.99 magnesium 24 with a mass of 23.9850 amu
• 10.00 magnesium 25 with a mass of 24.9858 amu
• The rest magnesium 26 with a mass of 25.9826 amu
• What is the atomic mass of magnesium?

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Average Atomic Masses

• If not told otherwise, the mass of the isotope is
  the mass number in amu
• The average atomic masses are not whole numbers
  because they are an average mass value
• Remember, the atomic masses are the decimal
  numbers on the periodic table