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) \({ }_{6} \mathrm{C}^{12}\) (c) \({ }_{6} \mathrm{C}^{4}\) (d) \({ }_{8} \mathrm{O}^{16}\)

Step-by-Step Solution

Answer

Option (b) \( { }_{6} \mathrm{C}^{12} \) is the standard for atomic mass.

Step 1: Understanding Atomic Mass Standard

The atomic mass is defined relative to the isotope of carbon, specifically the carbon-12 isotope. The atomic mass standard is not just a random choice but is based on historical convention and isotopic abundance, providing a common basis for scientists to compare atomic weights.

Step 2: Identifying Options

Look at each option provided in the question: (a) (b) (c) (d). We need to identify which isotope of an element is used as the standard for atomic mass. The correct choice will have widespread acceptance and usage in scientific calculations.

Step 3: Selecting the Correct Answer

Among the given options, only one isotope aligns with the standard definition. Option (b) \({ }_{6} \mathrm{C}^{12}\) represents the carbon-12 isotope, which is the standard for atomic mass. Since the atomic mass unit (amu) is defined as 1/12th of the mass of a carbon-12 atom, this is the correct answer.

Key Concepts

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but differ in atomic mass. Isotopes are fascinating in that they retain the chemical properties of their parent element, but their physical properties, such as mass and stability, can differ. Here’s what makes isotopes interesting:

• Their presence explains why some elements have non-integer atomic weights.
• They can be stable or radioactive, playing crucial roles in various applications, from dating archaeological finds to medical diagnostics.

For example, Carbon is an element with several isotopes, including Carbon-12 and Carbon-14, the latter being famous for its use in radiocarbon dating. Despite these differences, the chemical behavior of all of these carbon isotopes remains largely identical.

Carbon-12

Carbon-12 is one of the stable isotopes of carbon and it is particularly significant because it serves as a reference standard for atomic masses. Every element's atomic mass is measured in relation to Carbon-12. This choice was made because:

• Carbon-12 is both abundant and stable, making it a reliable benchmark.
• Dividing the mass of a Carbon-12 atom into 12 equal parts gives a convenient unit of mass, the Atomic Mass Unit (amu).

This isotope of carbon includes 6 protons and 6 neutrons, giving it a mass number of 12. The decision to use Carbon-12 as the gold standard allows chemists and physicists around the world to maintain consistency when measuring and comparing the atomic mass of different substances.

Atomic Mass Unit (amu)

The Atomic Mass Unit (amu) is a unit of mass used to express atomic and molecular weights. The definition of 1 amu is precisely 1/12th of the mass of a single atom of Carbon-12. Why is the amu important, you may ask?

• It provides a practical way for scientists to quantify atomic and molecular masses, given that atoms and molecules are extremely small.
• Using the amu, we could easily compare the relative weights of different atoms.

This unit simplifies the sometimes-complex calculations of atomic and molecular mass, making them more approachable for scientists and students alike.