An isotope of which element is used as the standard for the relative mass scale for atoms?
A) carbon
B) oxygen
C) hydrogen
D) helium
The correct answer and explanation is :
Correct Answer:
A) Carbon
Explanation:
The isotope used as the standard for the relative mass scale of atoms is carbon-12 (¹²C). The atomic mass unit (amu), also known as the Dalton (Da), is defined based on this isotope.
Why Carbon-12?
- Definition of Atomic Mass Unit
- One atomic mass unit (1 amu) is defined as 1/12th the mass of a single carbon-12 atom. This means that the mass of a carbon-12 atom is exactly 12 amu.
- Historical Development
- Initially, hydrogen (the lightest element) and oxygen were considered as reference standards.
- In the early 20th century, oxygen-16 (¹⁶O) was widely used, but differences in measurement scales between physicists and chemists led to inconsistencies.
- In 1961, the International Union of Pure and Applied Chemistry (IUPAC) officially adopted carbon-12 as the standard due to its stable and abundant nature.
- Advantages of Carbon-12 as a Standard
- Stability: Carbon-12 is a stable isotope, meaning it does not undergo radioactive decay.
- Abundance: It is the most common isotope of carbon, making up about 98.9% of natural carbon.
- Precise Measurement: Its mass can be accurately measured using mass spectrometry, ensuring consistency in atomic mass calculations.
Use in Chemistry and Physics
- Carbon-12 serves as the basis for determining the atomic masses of all other elements.
- It is essential in molecular weight calculations, stoichiometry, and isotope studies.
Thus, carbon-12 is the internationally accepted reference isotope for atomic mass measurements.
