 When you look at the periodic table you'll see that each element has its own box and within that box you'll find two numbers The atomic number or proton number and the mass number But what do these numbers mean? The atomic number has a symbol Z This number tells you how many protons are in one atom of an element The number is always the same for all atoms of a particular element Atoms of different elements have a different atomic number Meaning they have a different number of protons For example an atom of hydrogen has an atomic number of one because it has one proton But an atom of oxygen has an atomic number of eight because it has eight protons The next number we look at is the mass number The mass number has a symbol a The mass number tells you how many protons and Neutrons are in one atom of an element We need to remember that protons and neutrons each have a relative mass of one and That electrons are so small that their mass does not need to be considered in the mass number of an atom So if we know the mass number of an element and we know the atomic number We can calculate the number of neutrons in an atom of a particular element So the mass number equals the atomic number plus the number of neutrons The atomic number is just the number of protons in an atom therefore mass number equals protons plus neutrons So if we take oxygen Oxygen has a mass number of 16 and has eight protons But how many neutrons does it have? Remember mass number equals protons plus neutrons We can rearrange this to show that neutrons equals mass number minus protons neutrons equals 16 minus 8 equals 8 Oxygen therefore has eight neutrons Let's take another example Lithium has a mass number of seven and has three protons How many neutrons does it have? Mass number equals protons plus neutrons We can rearrange this to show that neutrons equals mass number minus protons Neutrons equals seven minus three equals four Lithium therefore has four neutrons So the atomic number is a number of protons in an atom And the mass number is a number of protons and neutrons in an atom