# What is the difference between atomic mass and mass number?

As total number of nucleons in nucleus is equal to mass number. And is also called atomic mass number. I want to know that is there any difference between the two? Because in nuclear reactions mass number differs from atomic mass and the difference between two provides binding energy.

The mass number is the number of nucleons (protons and neutrons) in the nucleus of an atom.
For example $^{12}_{\,\,6}\rm C$ has a mass number of $12$ as it has $6$ protons and $6$ neutrons in its nucleus and $^{35}_{17}\rm Cl$ has a mass number of $35$ as it has $17$ protons and $18$ neutrons in its nucleus.

The atomic mass of an atom is the mass of the atom in atomic mass units (amu or u).
The atomic mass of a $^{12}_{\,\,6}\rm C$ atom is defined as being $12$ atomic mass units.
The atomic mass of a $^{35}_{17}\rm Cl$ atom is $34.96885268$ atomic mass units ie a $^{35}_{17}\rm Cl$ atom is $\dfrac{34.96885268}{12}$ heavier than a $^{12}_{\,\,6}\rm C$ atom.

Note that the atomic mass is approximately equal to the mass number.

There is a third quantity which you should be aware of which is used by Chemists and that is the (standard) atomic weight.
The unit is again the atomic mass unit but it is the average atomic mass of the different types of atoms (isotppes) of which an element is composed.
For example in nature carbon exists as the isotopes $^{12}_{\,\,6}\rm C, \, ^{13}_{\,\,6}\rm C$ and $^{14}_{\,\,6}\rm C$ and so you will see the atomic weight of carbon listed as $12.011$ to reflect the fact that natural carbon is $75\%\,^{12}_{\,\,6}\rm C,\, 24\%\,^{13}_{\,\,6}\rm C$ with a trace of $^{14}_{\,\,6}\rm C$.
However the isotopic composition of Carbon may vary and so you will also see the standard atomic weight listed as a range of values such as $[12.0096 \,\,\,12.0116]$ when greater accuracy is required.

• Do you mean to say that atomic mass and mass number has no difference?But ,in nuclear reactions, why there is a difference between the two? – Rabik John Apr 1 '18 at 11:11
• RabikJohn They are different but I also wrote “Note that the atomic mass is approximately equal to the mass number. Mass number must be an integer whereas atomic mass is not. – Farcher Apr 1 '18 at 11:15
• Ok ,you mean they are not equal only"approximately" equal and the rest of their mass is converted into binding energy of nucleus.One more thing;Does atomic mass includes mass of electrons? – Rabik John Apr 1 '18 at 11:48
• @RabikJohn Atomic mass is the mass of a stationary neutral atom. – Farcher Apr 1 '18 at 11:50
• You mean, yes ;it includes electron. – Rabik John Apr 1 '18 at 12:21