Physics · nuclei · NEET
Yes, for NEET they mean the same unit of mass. The old name was atomic mass unit (amu). The modern SI-approved symbol is just u (sometimes called the unified atomic mass unit, Da). Both are defined as 1/12 the mass of a carbon-12 atom = 1.66 x 10^-27 kg. If a question says amu or u, treat them as identical.
A single proton has a mass of about 1.67 x 10^-27 kg. Writing such tiny numbers again and again is clumsy and error-prone. In u, a proton is about 1.007 u and a neutron about 1.009 u, which are easy numbers close to 1. So u is just a convenient unit for the very small masses of atoms and nuclei. 1 kg = about 6.022 x 10^26 u.
Take 1 u = 1.6605 x 10^-27 kg. Multiply by c^2 = (3 x 10^8)^2 = 9 x 10^16. Energy E = 1.6605 x 10^-27 x 9 x 10^16 = 1.4924 x 10^-10 J. Now divide by the charge on one electron 1.602 x 10^-19 to change joules into eV: E = 0.9315 x 10^9 eV = 931.5 x 10^6 eV = 931.5 MeV. So 1 u of mass is equivalent to 931.5 MeV of energy.
Strictly it is an energy value. That is why the correct way to write it is 1 u = 931.5 MeV/c^2. The MeV/c^2 tells you it is 'the mass whose energy equivalent is 931.5 MeV.' In numerical problems, when a mass difference is given in u, you multiply by 931.5 to directly get the released energy in MeV.
No, only carbon-12 is defined as exactly 12 u, so 1 u is exactly one-twelfth of it. A free proton is about 1.00728 u and a free neutron about 1.00866 u, both slightly more than 1 u. This small excess is important because it is the source of mass defect and binding energy in a nucleus.
1 u = 1.66 x 10^-27 kg = 1.66 x 10^-24 g. A quick check: since 1 mole of carbon-12 weighs 12 g and contains 6.022 x 10^23 atoms, one atom is 12 / (6.022 x 10^23) = 1.99 x 10^-23 g, and 1/12 of that is 1.66 x 10^-24 g.
1 atomic mass unit (u) is defined as 1/12 of the mass of one neutral atom of carbon-12. Its value is 1.66 x 10^-27 kg (more precisely 1.660539 x 10^-27 kg).
Carbon-12 is a stable, common, easily available atom. By international agreement its mass is fixed at exactly 12 u, which makes the atomic masses of most elements come out close to whole numbers.
1 u = 931.5 MeV/c^2. This comes from converting 1.66 x 10^-27 kg into energy using E = mc^2 and then expressing that energy in mega-electron-volts.
Find the mass defect or mass change in u, then multiply by 931.5. The result is the binding energy or the Q-value or energy released, directly in MeV. Example: 0.1 u x 931.5 = 93.15 MeV.
No. The kilogram is the SI unit of mass. The unit u is a non-SI unit that is accepted for use with SI because it is far more convenient for atomic and nuclear masses.
