Relative Atomic Mass

1
Relative Atomic Mass Isotopes
5.1
2
Until recently it was impossible to weigh
individual atoms
3
The relative scale

• English chemist John Dalton (1766-1844) simply
  assigned an arbitrary mass to the carbon atom
• It was assigned an exact mass of 12
• The mass of the elements were calculated
  relative to carbon
• It was attempted using O and H but the relative
  masses were either too small or non-integral
  numbers

4
Relative Atomic Mass

• For example,
• H is 12 times lighter than C
• Mg is 2 times heavier than C

5
Why does carbon weigh 12.01?
6 C 12.01
Carbon is made up of several isotopes, not just
carbon-12
No units because this is NOT the mass of an atom
(relative mass only)
6
Isotopes
Naturally occurring carbon contains 3 isotopes
that are not present in equal amounts
6 C 12.01
Carbon-12
Carbon-13
Average Atomic Mass
Carbon-14
7
Isotopes
Atoms from the same element with different
amounts of neutrons in the nucleus
8
Mass Spectrometer
can find the relative abundance of each isotope
in an element
9
A typical mass spectrum for an element.
How many isotopes does this element have?
10
How would you find the average mass of a group of
objects that have different masses and
abundances?
22g
9 12 22 14.3 g 3 Doesnt take
into account that 80 of the spoons are 9g. Must
calculate the WEIGHTED average. (80 x 9)
(10 x 12) (10 x 22) 10.6 g
11
This applies to atoms and their isotopes
22g
VS
Find the atomic mass for Mg (remember no units!)
12
Relative Atomic Mass (also called Atomic Weight)
13
Relative Molecular Mass

• It is the sum of the relative weights of the
  atoms in the molecular formula
• Eg. H2O

Mass of O 15.9994 Mass of H
1.00794 1.00794 Molecular mass 18.0153
14
Find the molecular weight

• Eg. H2SO4

15
Molecular weight vsFormula weight

• H2SO4 is a discrete covalent molecule
• vs
• NaCl and SiO2 which are infinitely large ionic
  and covalent lattices, respectfully

Because these represent empirical formulas, its
proper term is formula weight for the ratio of
atoms present eg. NaCl is 11 (23 35.5 58.5)