It is not hard. Look at the above atom diagram.
In the middle are neutrons and protons. They don't move much. The atomic number is always the number of protons. You just need to remember that point. Neutrons are always in the middle of the atom and equal the number of protons unless it is radioactive (you probably don't need to worry about that for your test).
The electrons are flying around the atom. They normally equal the number of protons too.
If the atom has no charge (like +2 or -2) and is neutral, the number of electrons must equal the number of protons. You can think of protons as being positive charge and electrons negative charge.
They must always cancel out if there is no extra charge. If there is a net negative charge (-2), you must have extra electrons. If it is positive charge (+2), you are missing electrons.
Easy, right?
