Look at the atomic number of the element(located in upper left corner of the element box).This number represent the number of protons of the element.In same of the elements the number of electrons and protons are the same. in other (negative) you have to calculate 2. Look at the atomical mass( located underneath the element, which is a sum of neutrons and protons.Round the mass to the whole number and substract the atomic number. That will be number of neutrons.