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RELATIVE ATOMIC MASS

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Title: RELATIVE ATOMIC MASS

1
RELATIVE ATOMIC MASS

2
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.

3
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

4
Isotopes of Hydrogen element
Tritium
Deuterium
has the biggest abundance in nature.
5
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.

6
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.

7
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

8

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.

9
To determine the relative isotopic mass values
• an instrument called MASS SPECTROMETER is used.

10
Mass spectrums provide
• Relative abundance of isotopes
• m/z ratio of isotopes
• Since the isotopes detected are 1 charged

m/z ratio isotopic mass
11
Once again, mass spectrums provide
• Relative abundance of isotopes
• Isotopic mass of the isotopes.

12
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
13
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!!!

14
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!!!