Mass Number and Isotopic Mass
The mass number gives an estimate of the isotopic mass measured in atomic mass units (u). For 12C the isotopic mass is exactly 12, since the atomic mass unit is defined as 1/12 of the mass of 12C. For other isotopes, the isotopic mass is usually within 0.1 u of the mass number. For example, 35Cl has a mass number of 35 and an isotopic mass of 34.96885.
There are two reasons for the difference between mass number and isotopic mass, known as the mass defect:
- The neutron is slightly heavier than the proton. This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on 12C with equal numbers of protons and neutrons.
- The nuclear binding energy varies between nuclei. A nucleus with greater binding energy has a lower total energy, and therefore a lower mass according to Einstein's mass-energy equivalence relation E = mc2. For 35Cl the isotopic mass is less than 35 so this must be the dominant factor.
Read more about this topic: Mass Number
Famous quotes containing the words mass and/or number:
“Self-determination, the autonomy of the individual, asserts itself in the right to race his automobile, to handle his power tools, to buy a gun, to communicate to mass audiences his opinion, no matter how ignorant, how aggressive, it may be.”
—Herbert Marcuse (1898–1979)
“The growing good of the world is partly dependent on unhistoric acts; and that things are not so ill with you and me as they might have been, is half owing to the number who lived faithfully a hidden life, and rest in unvisited tombs.”
—George Eliot [Mary Ann (or Marian)