It's the averaged weight of all of the known isotopes of an element. Elements have versions of itself that have the same number of protons and electrons, but different number of neutrons. Since protons and neutrons have a mass of 1 amu (Atomic Mass unit), you change the neutron number, you change the mass . . . even though the element is still the same. Ex: Carbon 14 and Carbon 12 are isotopes of carbon. Both carbon elements, but they each have a different number of neutrons (8 vs 6), so they have different masses.
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What is weighted average atomic number
The average atomic mass of an element is the average of the atomic masses of its isotopes (that is a weighted average). You have to take into account the abundance of each isotope when they do your averaging.
The atomic mass of an element is the weighted average of masses of the isotopes of the element, weighted in proportion to their abundance.
It is calculated as the ratio of the mass of one atom of an element to one twelfth of the mass of an atom of carbon-12. In fact, the weighted average of the mass of an atom of an element - weighted according to its isotopic abundance.
The atomic mass of an element is the weighted average of the masses of all the stable isotopes of the element (if it has any), weighted by the natural occurrence levels of the isotopes in the elements as found on earth or in the atmosphere.