The carbon-12 is the most abundant isotope of the element carbon, with 98.93% abundance. In addition, it is the main responsible for all the properties and applications attributed to the element carbon.
The 12 C with an atomic mass of 12 dalton exactly, is a reference for the establishment of the atomic masses of the other nuclides. The 12 C atom consists of six protons, neutrons, and electrons; however, the mass of the electrons is considered negligible.
Nuclear notation for the carbon-12 isotope. Source: Gabriel Bolívar
Elements usually have atomic masses expressed as whole numbers, followed by decimals, because elements can have several stable isotopes.
Therefore, the atomic masses of the elements are expressed as the weighted average of the atomic masses of their different isotopes. Taking into account that 12 C has an abundance of 98.93%, and 13 C an abundance of 1.15%, the atomic mass of carbon is 12.011 daltons.
The atomic mass of an element is expressed as the average of the atomic masses of its different atoms in relation to one twelfth of the mass of 12 C, known as a unified atomic mass unit; previously abbreviated as "uma", and currently as "u".
Nuclear notation
The upper image shows the notation or nuclear symbol for the carbon-12 isotope.
Describing it is the same as establishing the characteristics of the carbon atom par excellence. Its atomic number 6 indicates the number of protons in its nucleus. The number 12 is equal to the sum of the protons and neutrons, and is therefore the atomic mass; which is proportional to the nuclear mass.
And although this notation does not show it, there are 6 electrons that counteract the positive charge of the protons in the nucleus. From the point of view of chemical bonds, four of these six electrons are the ones that establish the foundations of life as we know it.
History
Dalton's research (1810) indicated that water contained a percentage of oxygen of 87.7% and hydrogen of 12.5%. But, Dalton pointed out that the formula for the water was OH. Davy and Berzelius (1812) corrected the formula for water to H 2 O.
Subsequently, Berzelius found the following water composition: oxygen 88.8% and hydrogen 11.2%. He considered that the atomic weight of hydrogen is 1 g, and that of oxygen 16 g.
Then they realized that using the atomic weight of 1 g for hydrogen, the atomic weight of oxygen would be 15.9 g. This reason, among others, led to establish the atomic mass of oxygen of 16 g as a reference standard for the atomic masses of the different chemical elements.
Oxygen was used as a reference standard from the end of the 19th century until 1961, when it was decided to use carbon as a reference standard for establishing the atomic masses of the different elements.
Before 1959, IUPAP and IUPAC used the element oxygen to define the mole as follows: 1 mole is the number of oxygen atoms present in 16 g.
General comments
To speak of 12 C is to refer to carbon as a whole; or at least 98% of its essence, which is enough for such an approximation. This is why this isotope itself has no use, while the element as such, and the solids it integrates, cover hundreds of applications.
However, it should be noted that living organisms have a special preference for this isotope than for 13 C; that is, the percentage of 13 C or the 12 C / 13 C ratio varies depending on the ecosystems, regions, or animal species.
This may be due perhaps to the fact that molecules with too many 13 C atoms, which are heavier, hinder or impair metabolic processes and the functioning of the body's cells; even if the percentage of 13 C in living beings were little higher than 1%.
Therefore, 12 C is the isotope of carbon responsible for life. And 14 C the "time meter" of its remains, thanks to its radioactive decay.
Another indirect utility of 12 C is to create "contrast" for the 13 C nuclear magnetic resonance spectroscopy technique, with which the carbon structure of organic compounds can be elucidated (discovered and constructed).
References
- Phillips, Basil. (July 08, 2019). What Is the Most Common Isotope of Carbon? sciencing.com. Recovered from: sciencing.com
- César Tomé López. (2019). Of the atomic weights. Recovered from: culturacientifica.com
- ElSevier. (2019). Carbon-12. ScienceDirect. Recovered from: sciencedirect.com
- R. Ship. (sf). Nuclear notation. Recovered from: hyperphysics.phy-astr.gsu.edu
- Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.