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Carbon-12 is of particular importance in its use as the standard from which atomic masses of all nuclides are measured, thus, its atomic mass is exactly 12 daltons by definition. Carbon-12 is composed of 6 protons , 6 neutrons , and 6 electrons .
Isotopes of carbon are atomic nuclei that contain six protons plus a number of neutrons (varying from 2 to 16). Carbon has two stable, naturally occurring isotopes. [69] The isotope carbon-12 (12 C) forms 98.93% of the carbon on Earth, while carbon-13 (13 C) forms the remaining 1.07%. [69]
The chemical elements are distinguished from each other by the number of protons that are in their atoms. For example, any atom that contains 11 protons is sodium, and any atom that contains 29 protons is copper. Atoms with the same number of protons but a different number of neutrons are called isotopes of the same element.
Isotopes are nuclides with the same number of protons but differing numbers of neutrons; that is, they have the same atomic number and are therefore the same chemical element. Isotopes neighbor each other vertically. Examples include carbon-12, carbon-13, and carbon-14 in the table above.
Carbon-13 (13 C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes , it makes up about 1.1% of all natural carbon on Earth.
The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number. For example, carbon-12, carbon-13, and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13, and 14, respectively. The atomic number of carbon is 6, which means that ...
[27] [28] Carbon's most common isotope at 98.9% is 12 C, with six protons and six neutrons. [29] 13 C is also stable, with six protons and seven neutrons, at 1.1%. [29] Trace amounts of 14 C also occur naturally but this isotope is radioactive and decays with a half life of 5730 years; it is used for radiocarbon dating. [30]
For other isotopes, the isotopic mass is usually within 0.1 u of the mass number. For example, 35 Cl (17 protons and 18 neutrons) has a mass number of 35 and an isotopic mass of 34.96885. [7] The difference of the actual isotopic mass minus the mass number of an atom is known as the mass excess, [8] which for 35 Cl is –0.03115.