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The formula is Atomic Mass (rounded to the nearest whole number) minus the Atomic Number.

This works because Protons and Neutrons have virtually the same mass, and electrons virtually none. As well as the Atomic Number being equal to the amount of Protons.

Essentially:

Protons + Neutrons - Protons = Neutrons. Basic Prealgebra.

So to find the amount of Neutrons in Erbium:

Erbium's Atomic Mass rounded (167) - Erbium's Atomic Number (68) = approx. 99 Neutrons

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14y ago

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