Mass Number
The sum of the number of protons and neutrons in an atom is called the mass number.
Be careful! It sounds so close to molar (or atomic) mass,
but the idea is very different. Mass number is always an integer.
Mass Number = Protons + Neutrons
Atom |
Protons |
+ Neutrons |
= Mass Number |
Hydrogen-1 |
1 |
0 |
1 |
Hydrogen-2 |
1 |
1 |
2 |
Oxygen-16 |
8 |
8 |
16 |
Oxygen-17 |
8 |
9 |
17 |
Oxygen-18 |
8 |
10 |
18 |
Chlorine-35 |
17 |
18 |
35 |
Chlorine-37 |
17 |
20 |
37 |
Atomic mass is never an integer number for several reasons:
- The atomic mass reported on a periodic table is the weighted average of all the
naturally occuring isotopes. Being an average it would be most unlikely to be a whole
number.
- The mass of an individual atom in atomic mass units is the mass relative to
carbon-12. Only carbon-12 has a mass exactly equal to itself.
- Even the mass of a single proton (1.00727661 u), or neutron (1.00866520 u) is not a
whole number. These are also the ratio of their masses to carbon-12 (since carbon-12 is
the reference standard, its the only atom whose atomic mass, and mass number are the same
-- both are exactly 12).
- The mass of an atom isn't what you'd get by adding up the masses of its individual
protons, neutrons and electrons. This may seem strange, but the cause is the binding
energy required to hold the atom together. This energy must come from somewhere, and is
equal to E = mc2. Because energy is required to hold the nucleus
together in all atoms except Hydrogen-1, a small loss of mass (the m in the
equation) occurs.
Considering the possibility of confusion between the terms mass number and atomic
mass, you can probably see why a lot of chemists still like to use the archaic term
atomic weight instead of atomic mass.
Atomic mass (the weighted average of all naturally occurring isotopes) is not given for
those atoms which have no stable isotopes. In these cases the mass number of the most
stable isotope is reported, often in brackets, for example Technetium (98).
Copyright © 1998 - 2008 David Dice