Question

Which one of the following is the standard for atomic mass? (a) \(\mathrm{H}^{1}\) (b) \({ }_{6} \mathrm{C}^{12}\) (c) \({ }_{6} \mathrm{C}^{14}\) (d) \({ }_{8}^{6} \mathrm{O}^{16}\)

Short answer

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

Step-by-step solution

Understand Atomic Mass Standard

The standard for atomic mass is a specific isotope that scientists have agreed to use as a reference point for measuring atomic masses. This standard is used to define the atomic mass unit (amu).

Identify the Standard Isotope

For many years, the standard for atomic mass was based on the isotope carbon-12, denoted as \(^{12}\text{C}\). Carbon-12 is a naturally occurring isotope of carbon.

Review the Options

The options given in the question are different isotopes: (a) \(\text{H}^1\), (b) \(^{12}\text{C}\), (c) \(^{14}\text{C}\), (d) \(^{16}\text{O}\). We need to identify which one is used as the standard.

Select the Correct Answer

Considering our knowledge from Step 2, the standard for atomic mass is \(^{12}\text{C}\) or option (b). This isotope is used to define the atomic mass scale.

Key concepts

Understanding Isotopes

An isotope is a variation of an element that has the same number of protons but a different number of neutrons. This results in the same atomic number but different atomic mass. Isotopes have identical chemical properties due to having the same number of electrons and protons, but they can have different physical properties due to their mass differences.

Each element can have multiple isotopes, which are usually denoted by the element's name followed by the mass number. For example, carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. While they all include 6 protons (as they are carbon), their number of neutrons and thus their mass numbers differ:

• Carbon-12 (\(^{12}\text{C}\)) has 6 neutrons.
• Carbon-13 (\(^{13}\text{C}\)) has 7 neutrons.
• Carbon-14 (\(^{14}\text{C}\)) has 8 neutrons.

Understanding isotopes is crucial for fields such as radiocarbon dating and nuclear medicine since isotopes can behave differently in nuclear reactions.

The Significance of Carbon-12

Carbon-12 (\(^{12}\text{C}\)) is an isotope that serves as the international standard for atomic mass. It is composed of 6 protons and 6 neutrons, making it particularly stable and abundant in nature. Scientists have agreed to set the atomic mass of carbon-12 exactly at 12 atomic mass units (amu).

This choice of standard is important because it allows for the uniform calibration of atomic masses of all elements. Since carbon is a common element in organic compounds, using its stable isotope, carbon-12, helps ensure precise and consistent measurements across the scientific community. Its role as a reference is essential in chemistry and physics for ensuring that atomic masses are correctly compared and calculated. By comparing other isotopes against carbon-12, scientists maintain consistency in measurements worldwide.

Defining the Atomic Mass Unit (amu)

The atomic mass unit (amu) is a standard unit of mass that quantifies the mass of atoms and subatomic particles. It is defined as one-twelfth of the mass of an atom of carbon-12. This means that one amu is equivalent to approximately 1.66 x 10^{-24} grams.

Using carbon-12 to define the atomic mass unit allows for a convenient and standardized way to measure and express atomic masses, which are fundamental to understanding chemical reactions and compounds. This standardization facilitates international scientific communication and research.

• The small size of the amu makes it ideal for dealing with the masses of individual atoms or molecules.
• The amu forms the basis of calculating molar masses, which are crucial in stoichiometry for converting between mass and moles in chemical equations.

Overall, the concept of amu and its relation to carbon-12 highlights the meticulous nature required for precision in the field of chemistry and physics.