Atoms and isotopes

An element's atomic number defines it. An element with 17 protons will always be chlorine.

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.

curriculum-key-fact
Isotopes are forms of an element that have the same number of protons but different numbers of neutrons.

There are three isotopes of hydrogen: hydrogen, deuterium (hydrogen-2) and tritium (hydrogen-3):

Table showing the three isoptopes of hydrogen, the first with 0 neutrons, the second with 1 neutron and the third with 2 neutrons.

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.

Example

How many protons does 146C contain?

The atomic number is 6 so 146C contains six protons.

Question

How many neutrons does 146C contain?

Number of neutrons = mass number - atomic number = 14 - 6 = 8 neutrons.