Question

(a) What isotope is used as the standard in establishing the atomic mass scale? (b) The atomic weight of boron is reported as 10.81 , yet no atom of boron has the mass of 10.81 amu. Explain.

Short answer

(a) Carbon-12 (\(^{12}\text{C}\)) isotope is used as the standard in establishing the atomic mass scale. (b) The atomic weight of boron is 10.81 amu because it is the weighted average of its two stable isotopes, boron-10 (\(^{10}\text{B}\)) and boron-11 (\(^{11}\text{B}\)), taking into account their natural abundance: (10 amu x 0.199) + (11 amu x 0.801) = 10.81 amu.

Step-by-step solution

Understand the concept of atomic mass, isotope, and atomic weight

Atomic mass is the mass of a single atom of an element, usually expressed in atomic mass units (amu). An isotope is a variant of a chemical element that has the same number of protons, but a different number of neutrons in the nucleus. Atomic weight is the average mass of all isotopes of an element, taking into account their natural abundance.

Identify the isotope used as the standard in establishing the atomic mass scale

The isotope that is used as the standard in establishing the atomic mass scale is carbon-12 (\(^{12}\text{C}\)). It is assigned a mass of exactly 12 atomic mass units (amu).

Explain why the atomic weight of boron reported as 10.81 amu

Boron has two stable isotopes: boron-10 (\(^{10}\text{B}\)) with a mass of 10 amu and boron-11 (\(^{11}\text{B}\)) with a mass of 11 amu. The atomic weight of boron is the weighted average of these isotopes based on their natural abundance. The natural abundance of boron-10 is about 19.9% while boron-11 has the natural abundance of about 80.1%. So, the atomic weight of boron is calculated as follows: Atomic weight of boron = (mass of boron-10 x abundance of boron-10) + (mass of boron-11 x abundance of boron-11) = (10 amu x 0.199) + (11 amu x 0.801) = 1.99 amu + 8.81 amu = 10.81 amu Although no individual atom of boron has the mass of 10.81 amu, the atomic weight represents the average mass of all boron atoms based on their natural abundance. Therefore, the atomic weight of boron is reported as 10.81 amu.

Key concepts

Isotopes

Isotopes are variations of the same chemical element, each having the same number of protons but differing in the number of neutrons within their nuclei. These variations result in different atomic masses for the isotopes of a single element. For instance, carbon normally has six neutrons, but isotopes of carbon can have either seven (carbon-13) or eight (carbon-14), while maintaining their identity as carbon because they still contain six protons.

Understanding isotopes is crucial because they can have vastly different physical or chemical properties. Some isotopes may be stable, while others are radioactive, meaning they can decay over time and emit radiation. This characteristic of isotopes has found applications in various fields such as medicine, where radioactive isotopes are used in diagnostic imaging and cancer treatment.

Atomic Weight

The atomic weight, also known as the relative atomic mass, is not the mass of a single atom, but rather a weighted average of the masses of all the isotopes of an element, as they occur naturally. This weighting reflects the relative natural abundance of each isotope. It's expressed in atomic mass units (amu), with the carbon-12 (\( ^{12}\text{C} \) ) isotope being used as the standard reference with an assigned exact mass of 12 amu.

For example, even though boron's isotopes possess whole-number atomic masses (boron-10 and boron-11 with 10 amu and 11 amu respectively), the atomic weight of boron is a non-whole number (10.81 amu) because it's a calculated average based on the isotopes' natural abundances and masses.

Calculation Example

To calculate the atomic weight, you multiply the mass of each isotope by its natural abundance percentage, then add these values together. For boron, with boron-10's abundance at 19.9% and boron-11's at 80.1%, the atomic weight can be calculated as:
\text{Atomic weight of Boron} = (10 amu \times 0.199) + (11 amu \times 0.801) = 10.81 amu.

This concept is crucial for understanding why the periodic table lists atomic weights as averages rather than specific atomic masses.

Natural Abundance of Elements

Natural abundance refers to the ratio of occurrence of an element's isotopes in nature. It's of significant importance when determining the atomic weight since these percentages help to compute the average mass of an element's isotopes. Factors such as nuclear stability and the history of the element on Earth can affect natural abundance.

For example, chlorine has two main isotopes, chlorine-35 and chlorine-37, with abundances of approximately 75% and 25% respectively. Thus, most chlorine atoms found naturally on Earth are chlorine-35. This variability in natural abundance has practical applications, like in isotope geochemistry, where isotopic ratios are used to trace the origin of samples or to understand geological processes.

Natural abundance values are determined through spectrometry or mass spectrometry, techniques that measure the mass of atoms and the ratios between different isotopes, providing insight into both the atomic weight calculations and dynamic processes such as radioactive decay chains.