One atomic mass unit (amu) is therefore, a mass unit equal to exactly one twelfth the mass of a carbon−12 atom.
Mass of 1 atom of C -12 isotope = 12 amu
1 amu = 1/12 x mass of 1 atom of C – 12 isotope
Mass of 1 atom of C-12 isotope (from mass spectrometer) = 2 x 10-23g
Notes:
• Atomic mass is not a mass but a number.
• Atomic mass is not absolute but relative to the mass of the standard reference element (C12).
• Gram atomic mass is atomic mass expressed in grams, but it has a special significance with reference to a mole (which is discussed later).
