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: 

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