SAT Subject Test Chemistry,10 edition

hydrogen    called  tritium.

Note that despite the existence of names like deuterium and tritium, they are all considered
hydrogen because they have the same number of protons (one). The example shown here is a little
bit of an anomaly because in general isotopes do not have specific names of their own.

ATOMIC MASS AND ATOMIC MASS UNIT

Atomic mass is most commonly measured in atomic mass units (abbreviated amu). By definition, 1
amu is exactly one-twelfth the mass of the neutral carbon-12 atom. In terms of more familiar mass
units:

1   amu =   1.66    ×   10–27   kg  =   1.66    ×   10–24   g

ATOMIC WEIGHT

Because isotopes exist, atoms of a given element can have different masses. The atomic weight
refers to a weighted average of the masses of an element. The average is weighted according to the
natural abundances of the various isotopic species of an element. The atomic weight can be
measured in amu.

Example: 99.985499% of  hydrogen    occurs  in  the common  ^1 H    isotope with    a   mass    of

1.00782504  amu.    About   0.0142972%  occurs  as  deuterium   with    a   mass    (including  the

electron)   of  2.01410 amu,    and about   0.0003027%  occurs  as  tritium with    a   mass    of

3.01605 amu.    The atomic  weight  of  hydrogen    is  the sum of  the mass    of  each    isotope

multiplied  by  its natural abundance   (x):

Solution: atomic    weight  of  H   =   m1Hx1H  +   m2Hx2H  +   m3Hx3H