Periodic table
This is the number of the element, and corresponds with the number of these within the atom.
What is proton?
If the symbol has a negative charge of -1, this is the formula used to figure out the number of electrons the atom has.
What is 'atomic number -1'?
This is the number of neutrons that Sodium has with the atomic number of 11 and atomic weight of 22.990.
What is 12?
(22.990 - 11 = 11.990 rounded to nearest whole number)