# Question #ce59d

Jan 24, 2015

An element's mass number represents the number of protons and neutrons it has in its nucleus.

A quick example:

Helium's mass number is 4, meaning that the number of protons plus the number of neutrons it has in its nucleus is 4. Since helium has an atomic number of 2, which expresses the number of protons it has in its nucleus, you can deduce that it will have

$\text{mass number - [atomic number](http://socratic.org/chemistry/a-first-introduction-to-matter/atomic-number)" = 4 -2 = "2 neutrons}$

Another example of an element's mass number is carbon

Notice that the $\text{_6^12"C}$ isotope has a mass number of 12, meaning it has 6 neutrons and 6 protons in its nucleus, while the $\text{_6^14"C}$ isotpe has a mass number of 14, which means it will have 8 neutrons and 6 protons in its nucleus.