The mass number should also not be confused with the relative atomic mass ( also called atomic weight ) of an element, which is the ratio of the average atomic mass of the different isotopes of that element ( weighted by abundance ) to the unified atomic mass unit.
42.
The modern form of the whole number rule is that the atomic mass of a given elemental isotope is approximately the mass number ( number of protons plus neutrons ) times an atomic mass unit ( approximate mass of a proton, neutron, or hydrogen-1 atom ).
43.
The size of a synthesized protein can be measured by the number of amino acids it contains and by its total molecular mass, which is normally reported in units of " daltons " ( synonymous with atomic mass units ), or the derivative unit kilodalton ( kDa ).
44.
Since the atomic mass unit ( amu ) is defined as 1 / 12 of the mass of the 12 C atom, it follows that the molar mass of a substance, measured in grams per mole, is numerically equal to its mean atomic or molecular mass measured in amu.
45.
When compared to the average mass per nucleon in carbon-12, which is moderately strongly-bound compared with other atoms, the mass defect of binding for most atoms is an even smaller fraction of a dalton ( unified atomic mass unit, based on carbon-12 ).
46.
This fact is made inevitable by the two decay methods possible : alpha radiation, which reduces the weight by 4 atomic mass units ( AMU's ), and beta, which does not change the weight at all ( just the atomic number and the p / n ratio ).
47.
An atomic mass of 16 was assigned to oxygen prior to the definition of the unified atomic mass unit based upon 12 C . Since physicists referred to 16 O only, while chemists meant the naturally-abundant mixture of isotopes, this led to slightly different mass scales between the two disciplines.
48.
The rate of a reaction involving a C H bond is typically 6 10 times faster than the corresponding C D bond, whereas a 12 C reaction is only 4 percent faster than the corresponding 13 C reaction ( even though, in both cases, the isotope is one atomic mass unit heavier ).
49.
:It could also be that, since the electrons are ~ 1 / 1800 the mass of the neutron or proton, they have zero mass number, while neutrons and protons have a mass that's a bit more than one atomic mass unit, are considered to have a mass number of 1.
50.
Since 1961 the unified atomic mass unit " u " has been defined as 1 / 12 the mass of 12 C . Before that the atomic mass unit " amu " was defined as 1 / 16 the mass of 16 O ( physics ) and as 1 / 16 the mass of O ( chemistry ).
