The numbers attached to each element in the periodic table means something. These numbers tell us how many protons, neutrons and electrons there are in an atom of an element.
Let’s take carbon for example, carbon has an atomic number of six.
The atomic number is the smaller of the two numbers shown with each element. The atomic number is the number of protons in that element.
The atomic number of an element never changes as this would change the element itself. In a neutral atom, that is an atom with a neutral or zero charge, the atomic number also tells us how many electrons there are in that element. In a neutral carbon atom, there are six electrons.
Carbon has a mass number of twelve. The mass number is all the protons and neutrons in an element added together.
Therefore, in carbon, there are a total of twelve protons and neutrons.
We can find out the number of neutrons by taking the smaller number, the atomic number, from the bigger number which is the atomic mass.
Six from twelve leaves six, so in carbon there are also six neutrons.
This works for all elements so if you have the periodic table, you can tell how many protons, neutrons and electrons there are in an atom of an element.