What is atomic mass unit(a.m.u.) ?

What is atomic mass unit :- The unit in which atomic and nuclear masses are measured is called atomic mass unit (a.m.u.).

For the study of nuclear phenomena, it is very essential to measure the masses of elements accurately. Now the masses of atoms are very small, for example, the mass of a carbon atom, ¹²C₆, is 1.992647 × 10^–26 kg. This value is very small, so Kilogram is not a very convenient unit to measure such small quantities.

Definition (What is atomic mass unit ?) :- 

One atomic mass unit is defined as 1/12 th of the mass of an atom of ₆C¹² isotope.

Relation between a.m.u and Kilogram

Mass of 1 mole(i.e. 6.023 × 10²³ atoms) of ₆C¹² = 12 g

⇒ Mass of 1 atom of ₆C¹² = (12/6.023 × 10²³) g

By definition, 1 a.m.u. = (1/12) × Mass of 1 atom of ₆C¹²

∴    1 a.m.u. = (1/12) × (12/6.023 × 10²³) g = 1.66 × 10^-24 g

1 a.m.u. = 1.66 × 10^-27 Kg

Note :- 

• a.m.u. is generally denoted by “u“.
• There are 12 nucleons in ₆C¹², therefore we can say that 1 a.m.u. represents the average mass of a nucleon.
• The device for accurate measurement of atomic masses is called “mass spectrometer“.

In terms of a.m.u.,

mass of electron (m_e) = 0.000548 u

mass of proton (m_p) = 1.007277 u

mass of neutron (m_n) = 1.008665 u

mass of hydrogen atom (m_H) = m_e + m_p = 1.007825 u

mass of helium atom [m (₂He⁴)] = 4.002603 u

 

Next Topic :- Electron Volt (eV)

Previous Topic :- Introduction to Nuclear Physics