Before carbon, oxygen was considered as a base to calculate the relative masses. It was rejected later. Explain why.Before carbon, oxygen was considered as a base to calculate the relative masses. It was rejected later. Explain why.
Answers
The reason is because cabon in nature, C12 accounts for 98.89% .2 remaining isotope is negligible
After a long period of research In 1961, IUPAC adopted the isotope carbon-12 as a basis for measuring atomic mass.
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The standard atomic weight (or atomic weight) scale has traditionally been a relative value, that is without a unit, with the first relative atomic mass basis suggested by John Dalton in 1803 as 1H. Despite the initial mass of 1H being used as the natural unit for relative atomic mass, it was suggested by Wilhelm Ostwald that relative atomic mass would be best expressed in terms of units of 1/16 mass of oxygen (1903). This evaluation was made prior to the discovery of the existence of elemental isotopes, which occurred in 1912.[8]
The discovery of isotopic oxygen in 1929 led to a divergence in relative atomic mass representation, with isotopically weighted oxygen (i.e., naturally occurring oxygen relative atomic mass) given a value of exactly 16 atomic mass units (amu) in chemistry, while pure 16O (oxygen-16) was given the mass value of exactly 16 amu in physics.
The divergence of these values could result in errors in computations, and was unwieldy. The chemistry amu, based on the relative atomic mass (atomic weight) of natural oxygen (including the heavy naturally-occurring isotopes 17O and 18O), was about 1.000282 as massive as the physics amu, based on pure isotopic 16O.
For these and other reasons, the reference standard for both physics and chemistry was changed to carbon-12 in 1961.[9] The choice of carbon-12 was made to minimise further divergence with prior literature.[8] The new and current unit was referred to as the unified atomic mass unit, u.[10] and given a new symbol, "u", which replaced the now deprecated "amu" that had been connected to the old oxygen-based system. The dalton (Da) is another name for the unified atomic mass unit.[11]
1u=1/12 m C-12
Despite this change, modern sources often still use the old term "amu" but define it as u ( 1/12 of the mass of a carbon-12 atom), as mentioned in the article's introduction. Therefore, in general, "amu" likely does not refer to the old oxygen standard unit, unless the source material originates from the 1960s or before.