Explanation:
1) Locate the atomic number in the upper left corner of the square. The atomic number will tell you the number of protons
2) In a neutral state, the element will contain the same number of protons as it will electrons.
--> Boron (B) has an atomic number of 5, so it has 5 protons.
--> Having 5 protons (+1 charge each) means to be balanced, it must have 5 electrons (-1 charge each)
However, what if it's not in a neutral state?
If it's not in a neutral state, it will look something like this:
Ca ^ 2+
Btw, the "^" means that it will be written like it's an exponent. That confuses people sometimes.
Anyways, the "2+" means it's an ion with a positive charge of 2. For us to have a positive charge, it means it's lost two electrons (cause electrons are negative, so taking them away means we are left with a more positive charge).
To get the number of remaining electrons, subtract the 2 from the atomic mass. In this case, that would be 20.
20 - 2 = 18 electrons
Another example:
N ^3-
Atomic Number: 7
7 + 3 = 10 electrons.
Why did I add here instead of subtracting? It's cause the 3- means we have added electrons, resulting in the ion having a negative charge. It's got more electrons than usual.
Hopefully this helps a bit.
