RELATIVE ATOMIC MASS

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RELATIVE ATOMIC MASS

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In order to produce correct amount of chemicals

• We have to know about masses of atoms and
  molecules, how these are used in chemical
  calculations.

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Masses of atoms and molecules Relative isotopic
mass

• Isotopes
• Atoms with the same number of protons, but
  different numbers of neutrons.
• Atoms of the same element (same atomic number)
  with different mass numbers (nucleon numbers).
• Isotopes of chlorine
• 35Cl 37Cl
• 17 17
• chlorine - 35 chlorine - 37

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Isotopes of Hydrogen element
Tritium
Deuterium
has the biggest abundance in nature.
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Relative isotopic mass

• Is the mass of a particular isotope of an element
  on a scale where an atom of carbon-12 has a mass
  of exactly 12 units.

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Why C-12?

• 12C is chosen as the STANDARD atom and given
  the mass of exactly 12 units.
• The mass of other atoms is found by comparing
  their mass w/ the mass of Carbon-12 atoms.

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C-12 is the standard atom

• An atom twice as heavy as one C-12 has a mass of
  24.00 (Mg24.00). An atom 12 times lighter than
  one C-12 atom is 1.00 (H1.00

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Relative isotopic mass

• There are three naturally occurring isotopes of
  neon (Ne)
• 20Ne isotopic mass 19.99244018 amu
• 21Ne isotopic mass 20.9938467 amu
• 22Ne isotopic mass 21.9913855 amu
• 1 atomic mass unit 1/12th of the mass of
  one carbon-12 tom.

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To determine the relative isotopic mass values

• an instrument called MASS SPECTROMETER is used.

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Mass spectrums provide

• Relative abundance of isotopes
• m/z ratio of isotopes
• Since the isotopes detected are 1 charged

m/z ratio isotopic mass
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Once again, mass spectrums provide

• Relative abundance of isotopes
• Isotopic mass of the isotopes.

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Relative Atomic Mass, Ar

• is the weighted average of the masses of the
  isotopes of an element on a scale on which a
  carbon-12 atom has a mass of exactly 12 units.
• Ar

(IM 1x its RA) (IM2 x its RA)
The sum of RA of all isotopes
IM Isotopic mass RArelative abundance
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IMPORTANT!!!

• Since Ar is the ratio of the average mass per
  atom of an element to 1/12th of the mass of an
  atom of the C-12 isotope, IT HAS NO UNITS!!!
• Ar is for
• 1 ATOM!!!

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What mass values of atoms are given in the data
booklet?

• Any atom with whole number Ar?
• What does having whole number Ar mean for an
  atom?
• It has no isotopes!!!