Defining the Atom

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					                            Defining the Atom
   • All matter is composed of atoms
   • Atoms are the smallest particle of an element
     that retains its identity in a chemical reaction.
   • Atom comes from the Greek word atomos
     meaning indivisible.

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• Democritus (460 BC – 370 BC) was among
  the 1st to suggest the existence of atoms.
• Democritus believed that atoms were
  indivisible and indestructible.
• His approached was not based on scientific
  method and was not accepted until later
  scientific theory.
     Dalton’s Atomic Theory
• The modern atomic thought began with John
  Dalton (1766-1844)
• Dalton used experimental method to
  transform Democritus’s idea of atoms into a
  scientific theory.
• Dalton studied the ratios in which elements
  combine and the result was Dalton’s atomic
      Dalton’s Atomic Theory
• The ideas of Dalton’s Theory are:
  – All elements are composed of tiny particles called atoms.
  – Atoms of the same element are identical. Atoms of
    different elements are different.
  – Atoms of different elements can physically mix together
    or chemically combine in simple whole number ratios to
    form compounds.
  – Chemical reactions occur when atoms are separated,
    joined, or rearranged. Atoms of one element are never
          Sizing Up Atoms
• A pure copper coin the size of a penny
  contains 2.4 x 1022 copper atoms. There are
  only about 6 x 109 people on Earth!!!
• Individual atoms are observable with
  instruments such as scanning tunneling
        Subatomic Particles
• Atoms are now know to be broken down into
  smaller, more fundamental particles called
  subatomic particles.
• There are 3 kinds of subatomic Particles.
  – Electrons
  – Protons
  – Neutrons
• Discovered in 1897 by J.J. Thomson (1856-
  1940) an English physicist.
• They are negatively charged subatomic
• Thomson performed experiments using a
  sealed glass tube with gases in it. He passed
  an electric current through the tube and the
  result was a cathode ray.
        Electrons Continued
• U.S. physicist Robert A. Millikan (1868-1953)
  carried out experiments to find the quantity of
  charge carried out by an electron.
• He calculated the mass of the electron; that
  is very similar to the excepted mass given
• Eugen Goldstein (1850-1930) observed in
  1886 that in a cathode-ray tube there were
  rays going in the opposite direction. He
  concluded they were positively charged
• Protons are positively charged subatomic
• Each proton has a mass about 1840 times
  that of an electron.
• In 1932 English physicist James Chadwick
  (1891-1974) confirmed the existence of
  another subatomic particle.
• Neutrons are subatomic particles with no
  charge but a mass nearly equal to the
      Rutherford’s Gold-Foil
• This experiment change the prevailing
  thought of the structure of the atom.
• Because of this experiment Rutherford
  concluded that most the of alpha particles
  pass through b/c the atom is mostly empty
  space, and the reason that some of the
  particles were deflected were due to the
  concentration of the positive charge in the
  atom which is now known as the nucleus.
       Structure of the atom
• The nucleus of the atom is the tiny central
  core of an atom and is composed of protons
  and neutrons.
• Rutherford’s model is the nuclear atom: In
  the nuclear atom, the proton and neutron are
  located in the nucleus. The electrons are
  distributed around the nucleus.
• This will later be revised, let’s sneak peak @
  page 128.
 Distinguishing Among Atoms
• We learned yesterday that atoms are
  composed of protons, neutrons, and
• Protons and Neutrons are in the nucleus and
  electrons surround the nucleus.
• So how do elements differ if they all contain
            Atomic Number
• Elements are different because they contain
  different numbers of protons.
• The atomic number of an element is the
  number of protons in the nucleus of an atom
  in that element.
  – Ex: The atomic number of Hydrogen (H) is 1, so
    all hydrogen atoms have 1 proton.
  – Ex: All Oxygen atoms (O) have 8 protons, so the
    atomic number of Oxygen is 8.
           Atomic Number
• Remember all atoms are electrically neutral.
• Therefore; the number of Protons equal the
  number of Electrons.
• Meaning the number of negatively charged
  particles must equal the number of postively
  charged particles.
              Mass Number
• Most of the mass of an atom is concentrated
  in its nucleus.
• The mass number of an atom is the total
  number of protons and neutrons in an atom.
  – Ex: Helium (He) has 2 protons and 2 neutrons:
    its mass number is 4.
  – Ex: Carbon (C) has 6 protons and 6 neutrons: its
    mass number is12.
            Mass Number
• If you know the atomic number and mass
  number of an atom, you can determine its
• The number of neutrons in an atom is the
  difference between the mass number and
  atomic number.
• # of neutrons = Mass # - Atomic #
             Mass Number
• The composition of an atom can be written in
  Shorthand notation using atomic number and
  mass number.
  – Ex: Look at Figure 4.8
  – The atomic number is the subscript and the
    mass number is the superscript.
  – How many electrons does a gold atom have?
  – How many neutrons does a gold atom have?

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