Why are the numbers in an element’s atomic mass the same as the numbers in its molar mass ignoring the units?

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Why are the numbers in an element’s atomic mass the same as the numbers in its molar mass ignoring the units?

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Because we defined one mole of protons to be the number of particles that weighs 1 gram (basically…there’s some nuance to that). And protons and neutrons weight essentially the same thing, so the atomic mass (# of protons an neutrons) will be the mass of 1 mole of the thing.

Electrons do have some weight but it’s so tiny that it doesn’t really make a difference.

Because 1/12 of carbon 12 is approximately equal to the mass of a proton (very close to it) which is itself very close to the mass of a neutron.

Well, no. You cannot ignore the units. The mole is SPECIFICALLY defined to be 12 grams of Carbon-12 atoms. So it has to be in grams – the mole doesn’t work with any other unit of weight.

That is because of the simple reason that it was defined that way to make calculations easier. Creating a new mass unit (amu) which in amount is equal to the molar mass makes converting mass between a single atom/molecule and a mole of them way easier than if the mass of a single atom/molecule was measured in yoctograms (1amu=1.66yg)

This is just definitional, isn’t it?

This is like asking “why does ten thousand 1-gram weights weigh 10 kilograms?” Molar mass is the mass of 1 mol (a quantity) of an element, and the atomic mass is that divided by the quantity. Replace the word “mol” with “million” (it’s not 1 million, but it helps you see the tautology).

Rewritten this way the question asks: “why does an atom have the same mass as 1 millionth of a million atoms?”

The atomic mass is *defined* as “the molar mass, divided by the number of atoms in a mol”