Define one Atomic Mass Unit (a.m.u.).

Q. Define one atomic mass unit (a.m.u.).

Answer: The mass of individual atoms is extremely small. Mass spectrometers are modern devices that can measure such tiny masses. Atomic mass means the mass of an atom. One-twelfth (1/12th) the mass of one atom of carbon-12 of the mass of one carbon atom is the number of times an atom of an element is heavier than one atom of the same element. Electrons and nuclear binding energy also contribute to the total atomic mass. Atomic mass is primarily made up of protons and neutrons. 

The atomic mass unit (U or AMU) serves as a standard unit of measure for atomic mass. This unit represents a twelfth (1/12) of the mass of one unbonded carbon-12 atom. This is equivalent to 1.992646547 × 10−23 gram of the mass of one atom or molecule. It is important to note that atomic mass and molar mass have the same number, but are two entirely different quantities.

It is always expressed as a fraction of carbon -12 if any isotopic of any element is expressed as a mass. The mass of an atom of helium -4 is 4.0026 amu, for example. Approximately 31.972 amu is the mass of an atom of sulphur -32.  

There are six protons and six neutrons in the nucleus of a carbon -12 atom, giving it a mass number of 12. The nucleus contributes nearly all of the mass to the atom, so each proton and neutron has a mass of about 1 amu. It is evident from the helium and sulphur examples that the mass of individual atoms is not a whole number. Atomic masses are slightly affected by the interactions within the nucleus, and the electron’s very small mass is taken into account when determining an atom’s mass.