there are so many elements on the periodic table what is it that defines them now that we are aware of the fundamental subatomic particles does the difference between the elements have something to do with these it does in fact an element is defined by the number of protons in its nucleus for example a carbon atom has six protons in its nucleus every carbon atom in the universe has six protons in its nucleus and every atom that does not have six protons is not carbon this is how we define carbon we can say that carbon has an atomic number of six because the atomic number of an atom is equal to the number of protons in its nucleus every element has its own unique atomic number we can also describe the mass number of an atom the mass number of an atom is equal to the sum of the protons and neutrons since each nucleon has a mass of approximately one amu when inside the atomic nucleus however although every element has a particular atomic number elements do not have a specific mass number because unlike the number of protons the number of neutrons in the nucleus can change for any given element for example carbon typically has 6 neutrons corresponding to a mass number of 12 but it can also have 7 neutrons for a mass number of 13 or 8 neutrons for a mass number of 14. atoms of a given element with different numbers of neutrons are called isotopes of a given element so carbon atoms will always have six protons but different isotopes of carbon can have six seven or eight neutrons which correspond to different mass numbers we will talk more about isotopes later so the atomic number of an atom is equal to the number of protons in the nucleus the mass number of an atom is equal to the number of protons plus neutrons in the nucleus this means that we can calculate the number of neutrons in any atom by finding the mass number minus the atomic number these are values that we will use frequently as we explore the world of chemistry