Question

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

Answer 1

- Hint: As the word suggests ‘atomic mass unit’ is a unit which is used by scientists to do the measurement of mass of atomic sized particles. A unit is taken as a standard to define quantities universally to avoid confusions and to have uniformity. It is defined with respect to the 6th element in the periodic table.

Complete step-by-step solution -
One atomic mass unit is defined as a mass exactly equal to one – twelfth the mass of one carbon -$12$ atom.
It is used to express the mass of atomic and subatomic particles. Because the atomic mass of an atom is actually very-very small because atoms are extremely small.
Earlier scientists could determine the mass of one atom relative to another atom by experiments.
It is abbreviated as ‘amu’. Nowadays it has been replaced by ‘u’ which is known as unified mass.
For taking standard elements scientists take carbon- $12$ because no other nuclides other than carbon - $12$ have exactly whole no. masses in this scale.
One amu = $1.66056\times {{10} ^ {-24}}$ g
This unit is ordinarily utilized in material science and science to communicate the mass of nuclear scale objects, for example, iotas, atoms, and basic particles, both for discrete occasions and various kinds of gathering midpoints.
Nowadays, we use mass spectrograph (an instrument) to measure the masses and relative concentrations of atoms and molecules.
The principle on which the mass spectrograph depends is Newton's second law of motion and momentum.

Additional info: Nowadays, determination of atomic masses becomes easy with the help of mass spectrometry.

Note: One amu is also known as one Dalton. Remember that amu is used for extremely small atomic particles not for huge and bulky particles. Students should note that molar mass of an element is different from atomic mass and it has no units. It is actually the mass of 1 mole of the atoms of the molecule.
$1\,mole\,=\,6.023\times {{10}^{23}}$