Problem 46 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is the standard for atomic mass? (a) \({ }_{1} \mathrm{H}^{1}\) (b) \(\mathrm{C}^{12}\) (c) \({ }_{6} \mathrm{C}^{4}\) (d) \({ }_{8} \mathrm{O}^{16}\)

Step-by-Step Solution

Answer

The standard for atomic mass is \(^{12}\text{C}\); option (b) is correct.

Step 1: Understand the question

We need to determine which of the given options is the standard for atomic mass. The standard for atomic mass is the isotope that scientists use as a reference point for calculating the atomic mass of other elements.

Step 2: Identify common atomic standards

The most commonly used standard is the isotope of carbon, specifically the carbon-12 isotope, denoted as \(^{12}\text{C}\). This isotope is used as a standard because it is stable and has a whole number atomic mass, making it a convenient reference.

Step 3: Analyze the given options

We have four options: (a) \(^{1}_{1}\text{H}\) (hydrogen), (b) \(^{12}\text{C}\) (carbon-12), (c) \(^{4}_{6}\text{C}\), and (d) \(^{16}_{8}\text{O}\) (oxygen-16). We need to determine which one is used as the standard for atomic mass.

Step 4: Choose the correct answer

Based on the knowledge that \(^{12}\text{C}\) is the standard for atomic mass, option (b) is the correct choice. Other options do not represent the isotope used as the standard reference in atomic mass calculations.

Key Concepts

Standard Atomic Weight

The standard atomic weight is a crucial concept in chemistry. It refers to the average mass of atoms of an element, measured in atomic mass units (amu). This standard is not fixed at the weight of any single isotope but is an average that takes into account all naturally occurring isotopes of that element.
It is often close to the mass number of the most abundant isotope. To provide consistency and clarity in calculations, scientists use a standardized isotope to establish a baseline for comparability.
This is where the idea of using reference isotopes comes into play. The calculated value allows chemists to conduct precise calculations such as determining the quantity of an element in a sample. This standardization is essential not only for scientific study but also for industrial applications.

Carbon-12 Isotope

The carbon-12 isotope, denoted as \( ^{12} ext{C} \), plays a pivotal role as the standard for atomic mass. Each carbon-12 atom has exactly 12 atomic mass units, which provides a clean and simple reference for comparison.
Why choose carbon-12? There are several compelling reasons:

• Stability: Carbon-12 is stable, with zero radioactivity, ensuring that the standard remains constant over time.
• Natural Abundance: It is the most abundant carbon isotope found in nature, making it a logical choice.
• Convenience: Having 12 whole number atomic mass units simplifies calculations.

Thus, the carbon-12 isotope is integral in ensuring consistency when measuring and comparing the masses of different elements. Its establishment as the benchmark keeps atomic mass calculations straightforward and uniform across various scientific disciplines.

Reference Isotopes

In the realm of chemistry, reference isotopes are used to serve as a point of comparison when calculating atomic masses of various elements. These isotopes, through their consistent and known atomic masses, provide a universal benchmark.
The atomic mass of other elements is calculated relative to the mass of the reference isotope, which is carbon-12. When selecting a reference isotope, certain qualities are necessary:

• Stability: Ensures no unexpected changes in mass.
• Non-radioactivity: Avoids decay that could alter mass.
• Consistent abundance: Provides reliability in measurements.

With carbon-12's specific mass set precisely at 12 amu, it acts as an essential tool in making preparation and analysis easier in various applications. Reference isotopes provide the foundation needed for definitive and repeatable scientific inquiry into the world of atoms and their interactions.