Transcript Isotopes and Average Atomic Masses

5.1
Relative Atomic Mass &
Isotopes
Until recently it was impossible to weigh
individual atoms
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
Relative Atomic Mass
• For example,
– H is 12 times lighter than C
– Mg is 2 times heavier than C
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)
Isotopes
6
C
12.01
Naturally
occurring carbon
contains 3
isotopes that are
not present in
equal amounts
Average
Atomic Mass
Carbon-12
Carbon-13
Carbon-14
Isotopes
Atoms from the same element with different amounts of
neutrons in the nucleus
Mass
Spectrometer
can find the
relative
abundance
of each
isotope in an
element
A typical mass spectrum
for an element.
How many
isotopes does
this element
have?
How would you find the average mass of a group of
objects that have different masses and abundances?
22g
12g
= 9 + 12 + 22 = 14.3 g
3
9g
Doesn’t 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
80%
10%
10%
This applies to atoms and their isotopes
22g
12g
9g
VS
80%
10%
10%
Mg-24
Mg-25
Mg-26
80%
10%
10%
Find the atomic mass for Mg
(remember: no units!)
Relative Atomic Mass
(also called Atomic Weight)
Relative Molecular Mass
• It is the sum of the relative weights of
the atoms in the molecular formula
Mass of O:
Eg. H2O
Mass of H:
15.9994
1.00794
1.00794
Molecular mass: 18.0153
Find the molecular weight:
Eg.
H2SO4
Molecular weight
vs
Formula 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, it’s
proper term is “formula weight” for the ratio of
atoms present eg. NaCl is 1:1 (23 + 35.5 = 58.5)