However an element's mass numbers can vary, which means that it can have different numbers of neutrons. So although chlorine has a mass number of 35, which means it has 18 neutrons, it can also have a mass number of 37, which means it has 20 neutrons. The different types of chlorine are called isotopes.
There are three isotopes of hydrogen: hydrogen, deuterium (hydrogen-2) and tritium (hydrogen-3):
Carbon has three isotopes: , and . They all contain six protons but six, seven and eight neutrons respectively.
and are not isotopes because although they have the same mass number, they are not the same element. If the number of protons changes, then it is a different element.
How many protons does contain?
The atomic number is 6 so contains six protons.
How many neutrons does contain?
Number of neutrons = mass number - atomic number = 14 - 6 = 8 neutrons.