[IBM]

As I know 1 amu = 1/12 X mass of the c-12 atom.

What does it mean by 1/12 in the definition of amu? Could you please explain it?

[chenbeier]

A carbon core C-12 contain 6 Proton and 6 Neutron. The mass is 12 amu  . To get 1 amu then you have to multiply by 1/12.

[Traumatic Acid]

The atomic mass unit tells you how many subatomic particles make up the nucleus. One subatomic particle = 1 AMU. Carbon has 12 subatomic particles, so 1/12 of that would be one.

Scientists love standardising units like that because these things are so constant. Another example is a standard kilogram. A standard kilogram is equal to the mass of a sphere of pure silicone with a specific diameter ( So specific is measured to the atom).   

[Borek]

The atomic mass unit tells you how many subatomic particles make up the nucleus. One subatomic particle = 1 AMU.

No. Not exactly. Cs-133 has 133 protons and neutrons, yet its atomic mass is 132.9. Who stole the 0.1 particle?

[DrCMS]

A standard kilogram is equal to the mass of a sphere of pure silicone with a specific diameter ( So specific is measured to the atom).

NO it is not.

The International Prototype Kilogram is a cylinder of platinum-iridium alloy stored in Paris but there are copies elsewhere.

The future standard will be based on Plancks constant after a vote in November.

The Avogadro project proposed a high purity silicon sphere ~93.6mm diameter as an alternative approach to a standard kilogram based on Avogadros number.

Silicone is the name for polymers with Si-O-Si links.

[Enthalpy]

The atomic mass unit tells you how many subatomic particles make up the nucleus. [...]

You should check what is called a subatomic particle.

[wildfyr]

Traumatic, electrons are subatomic particles. They may not weigh much, but if youre counting particles, they are in there and ruin your definition.

The definition uses the number of neutrons+protons in an atom of a specific isotope. There is a caveat even to this though, as a fellow on reddit blew my mind with the explanation for the actual atomic weight of ¹H:

The value of H = 1 is just a rough measure. Nucleons change mass a bit when they combine in a nucleus, the amount lost is called the mass defect. The mass of a proton is 1.672623E-27 kg, mass of a neutron is 1.674929E-27. So two protons and two neutrons should be 6.695104E-27 kg, however the actual mass of a helium nucleus is 6.644657230E−27 kg, which is lower than expected by 0.050447E-27 kg, which is off by about 1%. You can convert this mass defect to an energy using E = mc2 to get the nuclear binding energy.

So while the mass of one C-12 is almost 12 times the mass of hydrogen, it isn't exactly 12 times. 1 mole of hydrogen atoms is actually 1.007825 grams (about 1% off)., due to the mass defect lowering the mass of carbon.

Deuterium only accounts for 0.0115% abundance, and tritium is even more negligible, so that extra weight isn't much due to isotopic abundance. I never caught this before.

[Traumatic Acid]

Hi Wildfyre
I did specify that it was the number of subatomic particles which make up the nucleus, which electrons do not.

I was of the belief that electrons were disregarded when defining the AMUs of an element as they don't define what the element is like the nucleus does. Also the number of electrons present can vary from atom to atom. Correct me if I'm wrong. 

[Borek]

You are mistaking atoms and ions, atoms are neutral (what does it tell you about number of electrons?).

[wildfyr]

You're right, you did specify nucleus. But its still an inelegant definition in light of what I mentioned about how the nuclear binding energy, which can surprisingly change the mass by a significant amount.

[Enthalpy]

Quarks and gluons are subatomic particles too.
https://en.wikipedia.org/wiki/Subatomic_particle

[KemistryWebsites]

The atomic mass unit tells you how many subatomic particles make up the nucleus. One subatomic particle = 1 AMU. Carbon has 12 subatomic particles, so 1/12 of that would be one.

Scientists love standardising units like that because these things are so constant. Another example is a standard kilogram. A standard kilogram is equal to the mass of a sphere of pure silicone with a specific diameter ( So specific is measured to the atom).

It's not directly related to the number of 'sub atomic particles' (however we might define that!). 

The accurate reason is that by convention the mass of a carbon 12 atom is exactly 12 amu.  Therefore 1 amu is 1/12th of that mass.