1. How to figure number of protons, neutrons, and electrons: Each element on the periodic table has an atomic number (the large number in the box, ex. Oxygen’s atomic number is 8) and an atomic mass number (usually smaller number in decimal form, always the same or greater than the atomic number) The atomic number represents the number of protons and electrons of an element The atomic mass number can tell you how many neutrons are present by using the following equation: Atomic mass number (rounded to the lowest whole number) minus the atomic number equals the number of neutrons. Ex: Carbon (C on the periodic table of elements) has an atomic number of 6 and an atomic mass of 12.011. This means that Carbon has 6 protons, 6 electrons and how many neutrons??? 12.011 rounded to the lowest whole number is 12. 12 – 6 = 6 , therefore Carbon has 6 neutrons.