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

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

Step 1: Understanding Atomic Standards

To determine the standard for atomic mass, we need to identify the reference element or isotope used to define atomic mass units (amu). In chemistry, this standard is based on a specific isotope of carbon.

Step 2: Identifying the Reference Isotope

The standard for atomic mass is defined based on carbon-12 isotope, denoted as \( {}^6C^{12} \). This isotope is assigned an atomic mass of exactly 12 amu, and it serves as the reference from which all other atomic masses are derived.

Step 3: Comparing Options

Among the given options, we have: (a) \( {}_1H^1 \) (Hydrogen-1)(b) \( {}_6C^{12} \) (Carbon-12)(c) \( {}_6C^4 \) (d) \( {}_8O^{16} \) (Oxygen-16).The correct option, based on the identified reference isotope, is option (b), carbon-12.

Key Concepts

Atomic Mass Unit (amu)

The atomic mass unit (amu) is a convenient unit used in chemistry to represent the mass of atoms and subatomic particles. One atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom. This unit is abbreviated as amu, but you might also see it sometimes referred to as a dalton (Da), named after the famous chemist John Dalton.

The reason we use amu is that atoms are incredibly small, and measuring their mass in grams or kilograms is impractical. The atomic mass unit provides a scale that makes it easier to compare the masses of different atoms in a way that's relevant to chemistry.

When you're dealing with atomic masses, think of amu as a sort of measuring stick for atoms. It helps chemists understand relationships and make calculations that are much more accessible than trying to deal with their infinitesimally small actual masses in grams.

Reference Isotope

A reference isotope is an isotope of an element that serves as a benchmark for defining atomic masses. In the realm of chemistry, atomic masses need a consistent scale so that we can accurately compare one to another. This is where the role of a reference isotope becomes crucial.

The reference isotope is used to define how many atomic mass units an atom has. By international agreement, the carbon-12 isotope is used as the reference. This selection is not arbitrary; carbon-12 is chosen because it's naturally abundant, stable, and easy to isolate. With carbon-12 set as a reference, its mass is defined to be exactly 12 atomic mass units.

Using a reference isotope like carbon-12 ensures that all atomic masses are consistent and comparable across the scientific community, allowing researchers and students to align their understanding of atomic weights consistently.

Carbon-12

Carbon-12, written as \( {}^6C^{12} \), is a particular isotope of carbon that contains six protons and six neutrons, resulting in a mass number of 12. It's the most common isotope of carbon found in nature, which makes it ideal as a standard.

This isotope of carbon plays a vital part in the world of chemistry due to its role as the atomic mass standard. It's assigned an atomic mass of exactly 12 amu. All other atomic masses are calculated relative to this standard.

Choosing carbon-12 as the atomic mass standard means that when you encounter atomic mass values, they are essentially telling you how that mass compares to one-twelfth of a carbon-12 atom's mass. This approach allows scientists to establish a uniform scale for atomic weights, making scientific communication and calculations much simpler.