This video explains why the Relative Atomic Mass of the carbon-12 isotope is used as the standard against which all other atoms’ Relative Atomic Masses are measured. Carbon has 3 isotopes, all of which are interesting in themselves- Carbon-12, the most abundant by a long shot, Carbon-13, and the extremely rare and Carbon-14. The ratio of the isotopes varies slightly from place to place, and can be used as an identifier for where the carbon comes from. Carbon-12 and 13 are primordial, Carbon-14 is made in earth’s upper atmosphere as a result of cosmic radiation bombarding the atoms that make up the air. Being radioactive, Carbon-14 is useful in dating ancient fossils.

Transcript:
Remember, the atomic mass scale was originally based on the mass of a hydrogen atom being equal to 1 atomic mass unit. This was later changed to the mass of a carbon atom being 12 u. This made 1 u equal to 1/12th the mass of a carbon atom.

This updated the definition of 1 u as 1/12th the mass of a carbon atom.
An interesting coincidence is that an inch is a 1/12th of a foot, originally a real foot. The problem is that different people have different foot lengths. To solve this, society chose King Henry 1st’s foot as the fixed standard.
In the case of carbon, it has 3 Isotopes each with a different mass- carbon-12 with 6 neutrons, carbon-13 with 7 neutrons and carbon-14 with 8 neutrons. Do we average their masses and use that as the benchmark? The problem is the proportion of carbon’s isotopes changes from place to place on earth, and especially in outer space, so the benchmark would depend on where we get our carbon sample from.
As we need a fixed standard that’s the same throughout the universe, scientists picked only the carbon-12 isotope as being equal to exactly 12 u. All other atoms are then compared to a carbon-12 atom, making it the King Henry of atomic masses.
So updating, an element’s relative atomic mass is the average mass of its isotopes relative to the mass of a carbon-12 atom taken as exactly 12.
What about the element carbon itself? What’s its average, or relative atomic mass? Not just carbon-12 isotope, but taking into account all its isotopes found in nature? About 99% of carbon naturally occurring atoms are carbon-12, and only 1% are carbon-13. Less than one in one TRILLION is Carbon-14 so we can ignore it when averaging. So carbon’s relative atomic mass, is very close to 12. It’s actually 12.01, which leans 99% closer to 12 than to 13. As you might expect.
The proportion of the different isotopes varies at different places on earth, and across the universe. A famous meteor found in Antarctica in 1984 was identified as coming from Mars, because of its particular proportion of carbon-13 to carbon-12 isotopes in its carbon content.

Carbon-12 and carbon-13 and carbon-14 all look and feel like carbon, and all behave like carbon, because they all are carbon. For example, Their atoms all make 4 chemical bonds when joining to other atoms, so they all bond in the same way with oxygen to make carbon dioxide molecules. This is because atoms bond chemically by sharing the electrons on their outside shells, so the number of neutrons in their nuclei doesn’t matter. We can fade the protons and neutrons out here. To tell the CO2 molecules apart, We could write the bottom one C-12O2, the middle one C-13O2 and the top one C-14O2.
But why is only one in a trillion carbon atoms C14? Where do they come from? They’re made when a neutron is knocked out of atoms high in the atmosphere by cosmic rays from outer space, then bombards a nitrogen atom in the air, turning it into a C-14 atom. Like all carbon atoms, these Carbon 14s react with oxygen in the air to become CO2 molecules, then get absorbed by plants during photosynthesis. Animals that eat the plants, and animals that eat animals that eat the plants, will also have some carbon 14 atoms in their bodies.
Carbon-14 atoms are radioactive, which means that you’re radioactive. By measuring the level of a fossil’s radioactivity, scientists can work out how old it is, up to 50,000 years. We’ve made a whole video on how carbon 14 dating works. Check it out.

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