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Increasing Atomic Number Doesn't Always Increase Atomic Mass

Protons, Neutrons & Isotopes

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Since atomic number is the number of protons in an atom and atomic mass is the mass of protons, neutrons, and electrons in an atom, it seems intuitively obvious that increasing the number of protons would increase the atomic mass. However, if you look at the atomic masses on a periodic table you will see that cobalt (atomic number 27) is more massive than nickel (atomic number 28). Uranium (number 92) is more massive than neptunium (number 93). Different periodic tables even list different numbers for atomic masses. What's up with that, anyway?

The reason increasing atomic number doesn't always equate to increasing mass is because many atoms don't have a number of neutrons equal to the number of protons. In other words, several isotopes of an element may exist. If a sizeable portion of an element of lower atomic number exists in the form of heavy isotopes, then the mass of that element may (overall) be heavier than that of the next element. If there were no isotopes and all elements had a number of neutrons equal to the number of protons, then atomic mass would be approximately twice the atomic number (approximately because protons and neutrons don't have exactly the same mass... the mass of electrons is so small that it is negligible). Different periodic tables give differing atomic masses because the percentages of isotopes of an element may be considered changed from one publication to another.

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