CBSE - IX - Chemistry - Atoms and Molecules
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The atomic mass unit (Also known as amu) (symbol: u) or dalton (symbol: Da) is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 and has a value of 1.66 x 10 -27 kg. One amu is approximately equal to the mass of one proton or one neutron i.e. the weight of an atom in atomic mass units is equal to the number of neutrons plus the number of protons in that atom (its atomic weight).
For example: An atom of oxygen contains 8 protons, 8 electrons, and 8 neutrons, so it weighs 16 amu. The size of an amu unit was defined such that one proton weighs 1 amu and one neutron weighs 1 amu (protons and neutrons are about the same size as each other). And one can also say that one amu is one-sixteenth of the mass of an atom of oxygen-16.
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