Guide for the Use of the International System of Units (SI)

Guide for the Use of the International System of Units (SI) 
 
24 
Inasmuch as NIST is a scientific and technical organization, the word “weight” used in the everyday 
sense (that is, to mean mass) should appear only occasionally in NIST publications; the word “mass” 
should be used instead. In any case, in order to avoid confusion, whenever the word “weight” is used, it 
should be made clear which meaning is intended.
8.4 Relative atomic mass and relative molecular mass 
 
The terms atomic weight and molecular weight are obsolete and thus should be avoided. They have 
been replaced by the equivalent but preferred terms relative atomic mass, symbol A
r
, and relative molecular 
mass, symbol M
r
, respectively [4: ISO 31-8], which better reflect their definitions. Similar to atomic weight 
and molecular weight, relative atomic mass and relative molecular mass are quantities of dimension one 
and are expressed simply as numbers. The definitions of these quantities are as follows [4: ISO 31-8]: 
Relative atomic mass (formerly atomic weight): ratio of the average mass per atom of an element to 1/12 
of the mass of the atom of the nuclide 
12
C. 
Relative molecular mass (formerly molecular weight): ratio of the average mass per molecule or specified 
entity of a substance to 1/12 of the mass of an atom of the nuclide 
12
C. 
Examples: A
r
(Si) = 28.0855
M
r
(H
2
) = 2.0159
A
r
(
12
C) = 12 exactly 
 
Notes: 
1. It follows from these definitions that if X denotes a specified atom or nuclide and B a specified 
molecule or entity (or more generally, a specified substance), then A
r
(X) = m(X) / [m(
12
C) / 12] 
and M
r
(B) = m(B) / [m(
12
C) / 12], where m(X) is the mass of X, m(B) is the mass of B, and m(
12
C) 
is the mass of an atom of the nuclide 
12
C. It should also be recognized that m(
12
C) / 12 = u, the 
unified atomic mass unit, which is approximately equal to 1.66 × 10
−27
kg [see Table 7, 
footnote (d)]. 
2. It follows from the examples and note 1 that the respective average masses of Si, H
2
, and 
12
C are 
m(Si) = A
r
(Si) u, m(H
2
) = M
r
(H
2
) u, and m(
12
C) = A
r
(
12
C) u. 
3. In publications dealing with mass spectrometry, one often encounters statements such as “the 
mass-to-charge ratio is 15.” What is usually meant in this case is that the ratio of the nucleon 
number (that is, mass number—see Sec. 10.4.2) of the ion to its number of charges is 15. Thus 
mass-to-charge ratio is a quantity of dimension one, even though it is commonly denoted by the 
symbol m / z. For example, the mass-to-charge ratio of the ion 
12
C
7
1
H
7
+ +
is 91/2 = 45.5.