How atoms differ

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Section 4.3

• How atoms differ

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Atomic Number

• Represents three things in a neutral atom
• What element it is
• The number of protons in each atom
• The number of electrons in each atom

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Atoms

• Must have neutral charge so positive charge
  negative charge
• This means
• p e-

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Finding Mass Number

• Mass p mass n0
• A proton or neutron has a mass of 1 amu
• Mass e- 0

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Mass number

• Will be equal to the number of protons number
  of neutrons
• If not given to you, round the atomic mass off
  to a whole
• Atoms of the same element can have different mass
  s

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Neutrons

• Since mass number
• of p of no
• Then of no
• mass number of p

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Gold
Atomic number
Element symbol
79
Au
How many p? e-? n0?
196.967
Atomic Mass
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Isotopes

• Atoms of the same element with a different number
  of neutrons (different mass )
• Still have same of protons (that determines the
  element)
• Same atomic number

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Neon (atomic 10)

• 3 isotopes
• Ne-20 10p 10n0
• Ne-21 10p 11n0
• Ne-22 10p 12n0

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Isotopes of H

• H-1 hydrogen
• H-2 deuterium
• H-3 tritium

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Atomic Mass

• Different from mass
• A weighted average of all isotopes of that
  element
• The mass given on the
• periodic table

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Atomic Mass

• Isotopes occur in different amounts or - not
  in equal amounts
• So atomic mass has to be a weighted average

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Finding a weighted average atomic mass

• ( abundance isotope 1 x mass isotope 1 )
• ( abundance isotope 2 x mass isotope 2)
• ( abundance isotope 3 x mass isotope 3)
• ..Continue for all the isotopes you have

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Finding a weighted average atomic mass

• Element X has 2 isotopes
• X-6 has a mass of 6.015 amu and is 7.5 abundant
• X-7 has a mass of 7.016 amu and is 92.5
  abundant
• Find the average atomic mass for element X

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Abundance

• Which isotope must be more abundant Cu-63 or
  Cu-65?
• How did you decide?

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The Standard

• C-12 is the standard that atomic mass is based on
• C 12 amu (atomic mass units)