In 1961, the reference for the atomic mass unit (amu or u) was changed from naturally occurring oxygen with a value of 16.000 u to one based on a single isotope of carbon (carbon-12). Why was it changed? - The atomic composition of oxygen varies depending upon its source.- Carbon-12 is more common than oxygen.- Carbon exists as only one isotope, carbon-12.- Carbon can combine with four atoms of hydrogen to form CH4, methane, while oxygen only combines with two atoms of hydrogen to form water, H2O

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