Relative Atomic Mass & Isotopes 5.1
Dec 13, 2015
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
Why does carbon weigh 12.01?
6
C12.01 No units because
this is NOT the mass of an atom
(relative mass only)
Carbon is made up of several isotopes, not just carbon-12
Isotopes
6
C12.01 Average
Atomic Mass
Carbon-12
Carbon-13
Carbon-14
Naturally occurring carbon contains 3 isotopes that are not present in equal amounts
How would you find the average mass of a group of objects that have different masses and abundances?
80% 10% 10%
9g
12g22g
= 9 + 12 + 22 = 14.3 g 3
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
This applies to atoms and their isotopes
80% 10% 10%
9g
12g22g
Mg-24 Mg-26Mg-25
80% 10% 10%
VS
Find the atomic mass for Mg (remember: no units!)
Relative Molecular Mass
• It is the sum of the relative weights of the atoms in the molecular formula
Eg. H2OMass of O: 15.9994
Mass of H: 1.00794 1.00794
Molecular mass: 18.0153
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)