Problem 115 - Chemistry Matter and Change

Question

Infer Sulfur's average atomic mass is close to the whole number \(32 .\) Chlorine's average atomic mass is 35.453 , which is not a whole number. Suggest a possible reason for this difference.

Step-by-Step Solution

Answer

Sulfur's mass is close to a whole number due to a dominant isotope; chlorine's varied isotopes result in a non-whole average.

Step 1: Understanding Atomic Mass

The average atomic mass of an element is the weighted average of the masses of the isotopes of the element. This average reflects both the mass and the relative abundance of each isotope.

Step 2: Analyzing Sulfur's Atomic Mass

Sulfur's average atomic mass is close to 32, indicating that it likely has one dominant isotope, specifically an isotope with a mass number of 32, which is common for elements with a whole or nearly whole number atomic mass.

Step 3: Analyzing Chlorine's Atomic Mass

Chlorine's average atomic mass is 35.453, not a whole number, suggesting that chlorine has significant contributions from more than one isotope, likely isotopes with mass numbers that differ by several units. This non-integer result means that the isotopes do not equally contribute equivalently or that their contribution is not from isotopes with mass numbers close to each other.

Step 4: Concluding Reason for the Difference

The difference in atomic masses likely arises from the number and relative abundance of isotopes. Sulfur likely has one isotope that is much more abundant, while chlorine has two or more isotopes with similar abundances leading to a weighted average that falls between whole numbers.

Key Concepts

Understanding Sulfur Isotopes

Sulfur is an element known for its simplicity when it comes to isotopes. Isotopes are versions of the same element that have the same number of protons but different numbers of neutrons. Most sulfur in nature exists as the isotope sulfur-32, which is why the average atomic mass of sulfur is close to 32.

The reason sulfur is predominantly found as sulfur-32 is due to its abundance and stability, making it the overwhelmingly dominant isotope. While sulfur does have other isotopes like sulfur-33, sulfur-34, and sulfur-36, their presence is much less common and does not significantly impact the average atomic mass.

• Sulfur-32: Most common and stable.
• Minor isotopes include S-33, S-34, and S-36.

Exploring Chlorine Isotopes

Chlorine, on the other hand, shows a more complex isotopic picture. Unlike sulfur, chlorine's atomic mass isn't a whole number, reflecting a more diverse isotopic composition.

Chlorine mainly exists in two isotopic forms, chlorine-35 and chlorine-37. Both isotopes have a considerable presence, contributing to the non-whole number atomic mass of 35.453 for chlorine. This happens because the weighted average takes into account not only the mass of each isotope but also its abundance.

• Chlorine-35 is the lighter and more prevalent isotope.
• Chlorine-37 is heavier but still significantly abundant.

The result is the fractional average atomic mass, indicating that neither isotope completely dominates and both isotopes significantly contribute to the overall atomic mass of chlorine.

The Concept of Isotopic Abundance

Isotopic abundance is key to understanding why elements like sulfur and chlorine have different average atomic masses. It refers to how frequently a particular isotope appears relative to other isotopes of the same element.

Isotopes contribute to the average atomic mass based on their abundance.
For sulfur, the abundance of sulfur-32 is so high that other isotopes have a minimal effect on the average atomic mass. This results in an average that is very close to the mass number 32.
In contrast, chlorine showcases a balance between chlorine-35 and chlorine-37, leading to an average atomic mass that reveals this isotopic variety.

• Isotopic abundance varies greatly between elements.
• The more predominant an isotope, the closer the average atomic mass is to that isotope's mass number.

These variations are crucial in chemical identification and the study of elements across different scientific fields.