Using the number of electrons an element has
And you use them to find the electronic configuration for example if they tell you to find the electronic configuration of an element with 8 electrons then it’s electronic configuration is going to be
2:6 remember the maximum number of electrons on the first shell is 2 and the second is 8 and the third is also 8 that applies to the others
Now if they tell you to find the electronic configuration of an element with 18 electrons then it’s configuration is 2:8:8
I just hope this helps you because am not good at explaining :|