Question

a. What nuclide is used as the standard in the relative scale for atomic masses? b. What is its assigned atomic mass?

Short answer

a. Carbon-12 (\(^12C\)) b. 12 amu

Step-by-step solution

Identify the Standard Nuclide

The standard nuclide used as the reference in the relative scale for atomic masses is carbon-12. It is denoted as \(^12C\).

Determine the Assigned Atomic Mass

The assigned atomic mass of carbon-12 is exactly 12 atomic mass units (amu).

Key concepts

Carbon-12

In the world of chemistry and atomic science, certain elements and their isotopes play special roles.
One such isotope is carbon-12, often abbreviated as \(^12C\). Carbon-12 is a stable isotope of carbon, possessing 6 protons and 6 neutrons in its nucleus. This makes its atomic mass number 12.
Carbon-12 is used as a standard reference point in atomic mass measurement.
This means that when scientists talk about atomic masses of different elements, they often compare them to the mass of a carbon-12 atom. This isotope is extremely stable, which is crucial for accurate and consistent measurements.
The use of carbon-12 as a reference helps create a universal standard in atomic weights, allowing scientists across the globe to have a common point of understanding in their calculations.

Atomic Mass Units

To measure atomic mass, we need a unit of measurement. This is where atomic mass units (amu) come into play.
One atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom.
This definition links directly back to the use of carbon-12 as a standard nuclide.
The purpose of this unit is to simplify the numbers we get when measuring atoms. Since atoms are extremely small and their masses are incredibly tiny, using amu allows us to express these small numbers more conveniently.
In scientific calculations, using amu helps keep decimal places manageable and makes it easier to compare the masses of different atoms.
For instance, if an element's atom has a mass approximately 1.5 times that of a carbon-12 atom, it would be recorded as having an atomic mass of about 18 amu.

Relative Atomic Mass

Now that we understand carbon-12 and atomic mass units, let's talk about relative atomic mass.
Relative atomic mass (often simply called atomic weight) of an element is the ratio of the average mass of atoms of the element to one twelfth the mass of a carbon-12 atom.
It's a dimensionless quantity, meaning it doesn’t have a unit despite being a ratio involving masses.
This concept is particularly useful because it provides a way to compare the masses of different atoms using a common baseline.
It’s important to note that unlike the atomic mass of a specific isotope, the relative atomic mass takes into account the natural isotopic composition of an element.
For example, chlorine has two main isotopes: chlorine-35 and chlorine-37.
Its relative atomic mass is a weighted average of these isotopes, reflecting the proportions in which they occur in nature. This gives us a value useful for chemical calculations and reactions involving the element.
This number can be found on the periodic table, making it an essential reference for students and professionals alike.