I use atomic/molecular weight and
atomic/molecular mass interchangeably.
Is this the "correct" thing to do?
It depends who you ask.
Some sources declare that atomic
mass refers to a single isotope and
atomic weight refers to the abundance-
weighted average of naturally occurring
isotopes. Other sources say atomic mass
and atomic weight are synonymous and
make the distinction between single
isotope and natural isotope by the
presence or absence of a mass number.
If you make this mass/weight distinction
you either have to specify the mass number
to accompany your atomic mass, or you need
to assume that your reader will assume
your atomic mass refers to the most
prevalent isotope.
Here are two examples of this school
of thought...
Example 1
The atomic mass of 18O is 17.99916
The atomic weight of O is 15.99941
Example 2
The atomic mass of O is 15.99492
The atomic weight of O is 15.99941
In example 1 it is clear the mass reference
is for the specific isotope 18O, and it
is implied that the weight reference is
the natural weighted average.
In example 2 it is implied that the mass
reference is to 16O (the most abundant) and
it is implied that the weight reference is
the natural weighted average.
I do not like either of these. First, I
think this is a bad distinction between
weight and mass that does not correspond
to the distinction we make in our
general physics classes. Second, I don't
like the assumption that a non-specified
mass number implies the most abundant. If
the mass number is not specified then it
seems more reasonable that it would refer
to the natural mix.
Before I state what I think is best, let
me also state that when we use the term
atomic mass and the number 15.99492 for 16O
that we are implying units of amu, not units
of kilogram which is the standard unit for
mass. And when we use weight, we are likewise
using units of amu rather than the standard
unit of newton, and we have not multiplied
the mass by g to get the weight.
Thus, I think the terms mass and weight are
both problematic, especially for students, who
are still trying to justify this atomic physics
and atomic chemistry stuff to what they are
learning about mass and weight and SI units
in their general physics classes.
I prefer to use mass when dealing with
atoms/molecules, and I prefer to be explicit.
Example 3
The atomic mass of 16O is 15.99492
The atomic mass of (nat)O is 15.99941.
In these cases the 16 and the nat would be
superscripts to the left of the chemical
symbol, and this notation is very clear.
I am implying that the units are amu.
I am willing to accept
Example 4
The atomic mass/weight of 16O is 15.99492
The atomic mass/weight of O is 15.99941.
I accept that many people, especially chemists,
use the word weight rather than mass. If I can
think fast enough, I tend to use weight when I am
with chemists and mass when I am with physicists.
That makes me sound more like the group I am with.
If I use weight around physicists they sometimes
look down their noses at me because I am not
distinguishing the difference between
mass and weight. If I use mass around chemists
they think I am being a highbrowed physicist.
I accept that an element without a mass number
ought to refer to the natural mix... otherwise
a specific isotope should have been specified.
Although I don't have the privilege of setting
the definitions, I think you will find that
my preferences are widely used.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu