Question

Atomic Mass Unit (u): A unit of mass, abbreviated u, that is exactly 1/12 the mass of a carbon-12 atom. 1 u equals 1.66x 10-24 grams.

Answer 1

why did they take carbon as reference and not hydrogen ?

Answer 2

Why not? What's hydrogen got going for it?

Answer 3

atomic mass unit, refers to # of protons/neutrons ?, if so then why they refer to carbon ?

Answer 4

You are confusing atomic mass number, which is indeed the number of protons and neutrons in a nucleus, with the atomic mass unit -- which is a unit of mass, like the gram or pound.

You may be interested to know that the mass of nuclei are NOT integer multiples of each other. So you can pick, at most, one nucleus to have an integer mass. For example, you can pick carbon-12 to have a mass of exactly 12 amu (which is the current standard), and then the mass of hydrogen-1 (the most common isotope of hydrogen) is 1.0078 amu. Or, you could pick the mass of H-1 to be exactly 1 amu, and then the mass of C-12 would be 11.907 amu.

So...again, you've got to pick one nucleus to be the standard, and you can't pick more than one, and whichever you pick, the mass of all others is going to come out to be messy nonintegers -- so why H instead of C?

Answer 5

i said *refers*, i didnt say amu it is :)

Answer 6

i see atomic mass - they're not integer multiples of amu in periodic table, but i thought they should be integers, becoz all atoms have integral number of protons & neutrons, and amu refers to mass of one proton i think ? but this is a different question... il think about this and get back ltr, im good when i take things one at a time :)

still my present confusion is why C ? you're asking the same question i asked

Answer 7

The mass of 1 proton is 1.00728 amu. Quite close to 1 amu, but not exactly. I know what you're trying to say. You're just laboring under misconceptions, that's all. It does seem as if somehow the masses of the nuclei should all be integers, if you only defined the mass of 1 proton as exactly 1 amu.

But it doesn't work out that way. For one thing, neutrons are very slightly heavier than protons. For another, when the protons and neutrons fall together into the nuclei, they lose a heck of a lot of energy (that energy, when released, is what makes the big BANG of a hydrogen bomb). Einstein tells us that is exactly equivalent to losing mass, and in the case of protons and neutrons fusing to form a nucleus, that change in mass is quite measureable. Unfortunately, it's not something neat and easy like a loss of 0.01 amu per nucleon. It's something challenging to predict, and depends on the stability of the nucleus (some nuclei are more stable than others).

So EVEN IF we defined 1 amu to be exactly the mass of 1 proton, the masses of all elements other than hydrogen would NOT come out to be integers. So why pick hydrogen? It's rather a pain to handle. So instead we pick a nice solid elements, something easy to prepare: carbon. And we say 1 amu = exactly 1/12 the mass of a carbon-12 atom.

Answer 8

oh makes sense thank you. i get why atomic masses in periodic table are not integer multiples of C mass. i think ive understood everything u said except the reason why hydrogen is pain to handle and C is easy to prepare. since therez no relation between different atom's masses, and since hydrogen is the lowest mass element, i would have still inclined toward hydrogen

Answer 9

Because it's a reactive gas, that's all. Carbon is a solid, and fairly unreactive. You can keep it in a beaker, you don't need to keep it in some gas line carefully purged of air.

Answer 10

thnks