An atom can be represented using the symbol notation:
For example, chlorine (Cl) can be shown as:
This symbol shows that chlorine has 35 particles in the nucleus (protons and neutrons), 17 of which are protons. It also tells us that chlorine has 18 neutrons (35 - 17) and, as the number of electrons and protons are equal in a neutral atom, chlorine also has 17 electrons.
An element's atomic number defines it. An element with 17 protons will always be chlorine.
However an element's mass number can vary, which means that it can have different numbers of neutrons. So chlorine has a mass number of 35, which means it has 18 neutrons, but 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 they are not the same element. They have the same mass number but if the number of protons is different, they are different elements.
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.