3.5.2 Isotopes of Carbon

This topic explains the three isotopes of carbon — carbon-12, carbon-13, and carbon-14 — including their structure, natural abundance, and properties.

Definition

Isotopes of carbon are atoms of carbon that have the same atomic number (6 protons) but different mass numbers due to different numbers of neutrons.

Simple explanation: All carbon atoms have 6 protons. But they can have 6, 7, or 8 neutrons — giving three different isotopes.

Main Idea

Carbon has three isotopes:

• Carbon-12 (¹²C) — 6 protons, 6 neutrons
• Carbon-13 (¹³C) — 6 protons, 7 neutrons
• Carbon-14 (¹⁴C) — 6 protons, 8 neutrons

All three have 6 protons (that's why they are all carbon), but they differ in the number of neutrons.

Core Concept

Natural abundance:

Simple explanation: Almost all carbon in nature is carbon-12. Carbon-13 is a small fraction. Carbon-14 is present in tiny amounts and is radioactive.

Detailed Explanation

Carbon-12 (¹²C)

Symbol: ¹²C

• 6 protons
• 6 neutrons

• Almost all carbon is carbon-12.
• It is the standard for atomic masses — assigned a mass of exactly 12 amu.
• It is stable (non-radioactive).
• Natural abundance: 98.8%.

Simple explanation: Carbon-12 is the most common form of carbon. It is the standard against which all atomic masses are measured.

Carbon-13 (¹³C)

Symbol: ¹³C

• 6 protons
• 7 neutrons

• It has one extra neutron compared to carbon-12.
• It is stable (non-radioactive).
• Natural abundance: 1.1%.
• Used in NMR spectroscopy and scientific research.

Simple explanation: Carbon-13 is a heavier, stable form of carbon. It is used in research to study carbon-containing compounds.

Carbon-14 (¹⁴C)

Symbol: ¹⁴C

• 6 protons
• 8 neutrons

• It has two extra neutrons compared to carbon-12.
• It is radioactive.
• Natural abundance: 0.009%.
• Used in carbon dating to estimate the age of once-living things.
• Used to trace the path of carbon in photosynthesis.

Simple explanation: Carbon-14 is the radioactive isotope of carbon. It is used by archaeologists and scientists to find the age of ancient objects.

Physical Properties of Carbon Isotopes

The book states:

• Different forms of carbon are black or greyish-black solids except diamond.
• They are odourless and tasteless.
• They have high melting and boiling points.
• They are insoluble in water.

Note: These properties apply to carbon as an element. Diamond is a colourless, transparent form of carbon.

Natural Abundance of Carbon Isotopes

The book gives these values:

¹²C = 98.8\%, ¹³C = 1.1\%, ¹⁴C = 0.009\%

Simple explanation: Out of every 1000 carbon atoms, about 988 are carbon-12, about 11 are carbon-13, and less than 1 is carbon-14.

Determining Relative Atomic Mass from Isotopes

The relative atomic mass of carbon can be calculated from its isotopes and their abundances:

Relative atomic mass = (98.8 × 12) + (1.1 × 13) + (0.009 × 14)100

= 1185.6 + 14.3 + 0.126100

= 1200.026100

= 12.00 amu

Simple explanation: Even though carbon has three isotopes, its relative atomic mass is very close to 12 because carbon-12 is so abundant.

Intermediate Level

Why Carbon-14 Is Radioactive

Carbon-14 has too many neutrons relative to protons in its nucleus. This makes the nucleus unstable, so it undergoes radioactive decay.

Decay process: ¹⁴₆C → ¹⁴₇N + ⁰_-1e

Carbon-14 decays into nitrogen-14 by emitting a beta particle.

Carbon Dating

Carbon-14 is used to estimate the age of carbon-containing substances.

• Carbon atoms circulate between oceans and living organisms at a rate much faster than they decay.
• The concentration of C-14 in all living things keeps on increasing.
• After death, organisms no longer pick up C-14.
• By comparing the activity of a sample with the activity of living tissues, we can estimate how long it has been since the organism died.

This process is called dating.

Why Carbon-14 Is Useful in Dating

• Carbon-14 has a half-life of about 5,730 years.
• This makes it useful for dating objects up to about 50,000 years old.
• It is especially useful for dating once-living things (wood, bone, cloth, etc.).

Comparison of Carbon Isotopes

Important Points

• - Carbon has three isotopes: ¹²C, ¹³C, ¹⁴C.
• - Carbon-12: 6 protons, 6 neutrons; most abundant (98.8%); standard for atomic masses.
• - Carbon-13: 6 protons, 7 neutrons; 1.1% abundance; stable.
• - Carbon-14: 6 protons, 8 neutrons; 0.009% abundance; radioactive.
• - Physical properties of carbon: black or greyish-black solids (except diamond); odourless, tasteless; high melting and boiling points; insoluble in water.
• - Carbon-14 is used in carbon dating and as a tracer in photosynthesis.
• - Relative atomic mass of carbon = 12.00 amu (calculated from isotope abundances).

Remember

• - Carbon isotopes: ¹²C, ¹³C, ¹⁴C.
• - All have 6 protons (same element).
• - ¹²C: 6 neutrons; 98.8%; stable; standard for atomic masses.
• - ¹³C: 7 neutrons; 1.1%; stable; used in NMR.
• - ¹⁴C: 8 neutrons; 0.009%; radioactive; used in carbon dating.
• - Relative atomic mass of carbon = 12.00 amu.
• - Carbon-14 decays into nitrogen-14.

Common Mistakes

• 1. Thinking all carbon atoms have the same mass. Carbon has three isotopes with different masses.
• 2. Thinking carbon-14 is the most abundant isotope. Carbon-12 is the most abundant (98.8%).
• 3. Thinking carbon-14 is stable. It is radioactive.
• 4. Confusing carbon-13 and carbon-14. Carbon-13 is stable (7 neutrons); carbon-14 is radioactive (8 neutrons).
• 5. Forgetting that the relative atomic mass of carbon is 12.00 amu. Even with three isotopes, it is very close to 12 because carbon-12 dominates.
• 6. Thinking carbon-14 dating works for all ages. It is useful up to about 50,000 years.

Quick Revision

• Carbon has three isotopes: carbon-12, carbon-13, carbon-14.
• All have 6 protons (same element).
• Carbon-12: 6 protons, 6 neutrons; 98.8% abundant; stable; standard for atomic masses.
• Carbon-13: 6 protons, 7 neutrons; 1.1% abundant; stable; used in NMR.
• Carbon-14: 6 protons, 8 neutrons; 0.009% abundant; radioactive; used in carbon dating and photosynthesis tracer.
• Physical properties of carbon: black/greyish-black solids (except diamond); odourless, tasteless; high melting/boiling points; insoluble in water.
• Relative atomic mass of carbon: 12.00 amu.
• Carbon-14 decays into nitrogen-14.

60-Second Revision

• Carbon isotopes: ¹²C, ¹³C, ¹⁴C.
• ¹²C: 6p, 6n; 98.8%; stable; standard.
• ¹³C: 6p, 7n; 1.1%; stable.
• ¹⁴C: 6p, 8n; 0.009%; radioactive; carbon dating.
• Relative atomic mass of C = 12.00 amu.
• Warning: carbon-14 is radioactive; carbon-12 is the most abundant.