Isotopes are atoms of a chemical element with varying numbers of neutrons in their nuclei. All atoms of a specified element have the same number of protons. While electrons are present in many atoms, because they have so little mass, only the protons and neutrons are considered when measuring the mass of an atom. Because the number of protons does not vary from atom to atom of an element, that number is designated the atomic number. Neutrons can vary from atom to atom, and are calculated by comparing the mass of an isotope to the standard mass of an atom containing only its characteristic number of protons.
Locate the element for the isotope in question on the periodic table. Note the atomic number for that element. The atomic number is usually written in relatively large print above the element's chemical symbol. This number tells you the number of protons in an atom of that element.
Determine the mass number of the isotope by examining the name of the isotope. A particular isotope of an element is usually labeled with the name of the element and the mass number for that isotope. The number is either in superscript above the element symbol or written like this: carbon-12. The presence of a mass number with an element designates it as an isotope of that element.
Subtract the atomic number of the element from the mass number of the isotope to obtain the number of neutrons in that isotope. For example, the atomic number for Silicon (Si) is 14. A particular isotope of silicon is silicon-30. To find the number of neutrons in this Si isotope, subtract the atomic number from the mass number: 30 - 14 = 16. Therefore, Silicon-30 has 16 neutrons and 14 protons.