Three naturally occurring isotopes of carbon exist: The atomic mass of carbon is 12.01 amu. Which of the three carbon isotopes is most abundant in nature?
A) carbon-12
B) carbon-13
C) carbon-14
D) all three occur with equal abundance
The correct answer and explanation is :
The correct answer is A) carbon-12.
Explanation:
Carbon exists in nature in three isotopes: carbon-12 (¹²C), carbon-13 (¹³C), and carbon-14 (¹⁴C). These isotopes are forms of carbon that differ in the number of neutrons in their nuclei. While the atomic number of carbon is 6 (meaning it always has 6 protons), the number of neutrons in the nucleus varies, which gives rise to the different isotopes.
- Carbon-12 (¹²C):
- Carbon-12 is the most abundant isotope of carbon, making up about 98.9% of all naturally occurring carbon. Its nucleus contains 6 protons and 6 neutrons. This isotope is stable and is the standard by which atomic masses are measured, with its atomic mass assigned as exactly 12 atomic mass units (amu).
- Carbon-13 (¹³C):
- Carbon-13 makes up about 1.1% of naturally occurring carbon. It has 6 protons and 7 neutrons, and while it is stable, it is much less abundant than carbon-12. It is commonly used in various scientific applications, such as stable isotope labeling in molecular biology and metabolic studies.
- Carbon-14 (¹⁴C):
- Carbon-14 is a radioactive isotope that occurs in trace amounts in nature, approximately 0.0000000001% of carbon. It has 6 protons and 8 neutrons. This isotope is used in radiocarbon dating to estimate the age of organic materials, as it decays over time with a half-life of about 5,730 years. Despite its usefulness in dating, carbon-14 is extremely rare compared to carbon-12 and carbon-13.
The reason the atomic mass of carbon is 12.01 amu is because it is a weighted average of the masses of all the naturally occurring carbon isotopes. Since carbon-12 is by far the most abundant isotope, its contribution to the atomic mass is dominant, leading to an atomic mass close to 12 but slightly above due to the presence of carbon-13.
Thus, carbon-12 (¹²C) is the most abundant isotope in nature.
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