![]() Its atomic number is 2, which is not always included in the notation because He is defined by the atomic number 2. Each isotope has a certain percentage abundance found in nature, and these are added and averaged to obtain the average mass number.įor example, 4He has a mass number of 4. The mass number listed is the average mass of all of the element's isotopes. On the periodic table, the mass number is usually located below the element symbol. There may be a few more or a few less neutrons, and so the mass is increased or decreased. The two added together results in the mass number:Įlements can also have isotopes with the same atomic number, but different numbers of neutrons. Each proton and neutron's mass is approximately one atomic mass unit (AMU). Because an electron has negligible mass relative to that of a proton or a neutron, the mass number is calculated by the sum of the number of protons and neutrons. The mass of an atom is mostly localized to the nucleus. Examples as seen on Periodic Table Atomic NumberĪverage Atomic Mass of all Element's Isotopes Copper has the atomic number of 29 for its 29 protons. For example, hydrogen has one proton and one electron, so it has an atomic number of 1. ![]() The atomic number is usually located above the element symbol. On the periodic table, the elements are arranged in the order of atomic number across a period. ![]() Every element has a different atomic number, ranging from 1 to over 100. O 2- still has an atomic number of 8, corresponding to the 8 protons, but it has 10 electrons. In the element symbol, the charge goes on the right side of the element. If it is charged, there may be more protons than electrons or vice versa, but the atomic number remains the same. If an atom is neutrally charged, it has the same number of protons and electrons.
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