Why is 1 AMU equal to 1/12th of the carbon-12 isotope?

It's a convention set by the scientists of the last 2 centuries. Carbon - 12, an isotope of carbon was chosen to be a standard for this convention because of several reasons - Abundance, ease of availability, the ease of analysing its lattice and stability. read more

Also he hydrogen was the lightest element known, so hydrogen was assigned 1 AMU. In this was 1 AMU is equal to the 1/12 of Carbon-12 Isotope. read more

Originally atomic hydrogen was defined as the weight of 1 AMU because it contains only 1 nucleon, a proton. For various reasons it was decided that carbon 12 would be used as a standard. The actual weight of 1 AMU is only slightly different when using the carbon 1 standard. read more

According to HowStuffWorks, an atomic mass unit is defined as the mass of a single proton or neutron. Hydrogen, the lightest element, has an average mass of 1.00794 amu. A carbon-12 atom is made up of six protons and six neutrons, hence 12 atomic mass units. read more