Why mass numbers cannot be used to arrange
elements in the periodic table.

Because the number of protons, which is the same as the atomic number, is what distinguishes various elements, not the atomic mass.

An element's atomic mass is made up of two parts: protons and neutrons. While atoms of the same element, such as C-14 and C-12, might have various numbers of neutrons, the number of protons in a carbon atom is always 6.

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