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: 126C, 136C and 146C. They all contain six protons but six, seven and eight neutrons respectively.
147N and 146C 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 146C contain?
The atomic number is 6 so 146C contains six protons.
How many neutrons does 146C contain?
Number of neutrons = mass number - atomic number = 14 - 6 = 8 neutrons.