Isotopes of carbon

Key takeaways

Isotopes of carbon belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Isotopes of carbon to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Isotopes of carbon from memory before moving on to harder problems.

Reference excerpt

Carbon (6C) has 14 known isotopes, from 8C to 20C as well as 22C, of which only 12C and 13C are stable. The longest-lived radioisotope is 14C, with a half-life of 5700 years. This is also the only carbon radioisotope found in nature, as trace quantities are formed cosmogenically by the reaction 14N + n → 14C + 1H. The most stable artificial radioisotope is 11C, which has a half-life of 20.34 minutes. All other radioisotopes have half-lives under 20 seconds, most less than 200 milliseconds. Lighter isotopes exhibit beta-plus decay into isotopes of boron and heavier ones beta-minus decay into isotopes of nitrogen, though at the limits particle emission occurs as well. The two lightest isotopes decay into helium via short-lived isotopes of lithium, beryllium and boron.

List of isotopes

Carbon-11 Carbon-11 or 11C is a radioactive isotope of carbon that decays to boron-11 with a half-life to 20.34 minutes. This decay mainly occurs due to positron emission, with around 0.19–0.23% of decays instead occurring by electron capture.

11C → 11B + e+ + νe + 0.96 MeV 11C + e− → 11B + νe + 1.98 MeV It is produced by hitting nitrogen with protons of around 16.5 MeV in a cyclotron. The causes the endothermic reaction

14N + p → 11C + 4He − 2.92 MeV It can also be produced by fragmentation of 12C by shooting high-energy 12C at a target. Carbon-11 is commonly used as a radioisotope for the radioactive labeling of molecules in positron emission tomography. Among the many molecules used in this context are the radioligands [11C]DASB and [11C]Cimbi-5. Due to the short half-life, the chemical reactions used to manipulate the radioisotope as generated and incorporate it into a biomolecule must be efficient. Popular key intermediates include [11C]phosgene, for carboxylation-related reactions, though other synthons, such as [11C]carbonyl fluoride and [11C]carbon dioxide, are also being explored. For methylation, [11C]iodomethane and related synthons are used.

Stable isotopes

Carbon-12 and carbon-13 account for approximately 98.9% and 1.1% (respectively) of the naturally occurring carbon on Earth. However, the ratio of stable 13C and 12C in a material can vary due to differences in precursor source and isotopic fractionation induced by a variety of biogeochemical processes. The quantities of the different isotopes are commonly measured via isotope-ratio mass spectrometry and expressed as parts per thousand (‰ or "per mille") divergence from the ratio of a standard:

δ C 13 = ( ( C 13 C 12 ) sample ( C 13 C 12 ) standard − 1 ) × 1000 {\displaystyle \delta {\ce {^{13}C}}=\left({\frac {\left({\frac {{\ce {^{13}C}}}{{\ce {^{12}C}}}}\right)_{\text{sample}}}{\left({\frac {{\ce {^{13}C}}}{{\ce {^{12}C}}}}\right)_{\text{standard}}}}-1\right)\times 1000} ‰ Peedee Belemnite ("PDB"), a fossil belemnite from the eponym Late Cretaceous geological formation in North and South Carolina (USA), was the original reference standard used for standardizing carbon isotope ratio values. Due to the depletion of the original PDB, an artificial "Vienna PDB" standard, or "VPDB", is generally used today.

Paleoclimate 12C and 13C are measured as the isotope ratio δ13C in benthic foraminifera and used as a proxy for nutrient cycling and the temperature dependent air–sea exchange of CO2 (ocean ventilation). Photosynthetic organisms, such as algae and plants find it easier to use the lighter carbon isotope (12C) when they convert carbon dioxide and water into glucose and oxygen using sunlight and photosynthesis:

6 CO2 + 6 H2O → C6H12O6 + 6 O2 For example, large blooms of plankton (free-floating organisms) absorb large amounts of 12C from the oceans. Originally, the 12C was mostly incorporated into the seawater from the atmosphere. If the oceans that the plankton live in are stratified (meaning that there are layers of warm water near the top, and colder water deeper down), then the surface water does not mix very much with the deeper waters, so that when the plankton dies, it sinks and takes away 12C from the surface, leaving the surface layers relatively rich in 13C. Where cold waters well up from the depths (such as in the North Atlantic), the water carries 12C back up with it; when the ocean was less stratified than today, there was much more 12C in the skeletons of surface-dwelling species. Other indicators of past climate include the presence of tropical species and coral growth rings.

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Worked examples

Example 1 — a first encounter with Isotopes of carbon

Start with the simplest possible case. Write down what Isotopes of carbon claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Isotopes of carbon before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Isotopes of carbon ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Isotopes of carbon

In research: Isotopes of carbon appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Isotopes of carbon in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Isotopes of carbon is common in secondary-school and first-year university syllabi. It links to neighbouring topics Isotopes of carbon, Lists of isotopes by element, so understanding it makes those chapters shorter.
In everyday life: Look for Isotopes of carbon outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Isotopes of carbon in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Isotopes of carbon means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Isotopes of carbon out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Isotopes of carbon in simple terms?

Carbon (6C) has 14 known isotopes, from 8C to 20C as well as 22C, of which only 12C and 13C are stable. The longest-lived radioisotope is 14C, with a half-life of 5700 years.

Why does Isotopes of carbon matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Isotopes of carbon?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Isotopes of carbon.