Problem 41 - General Chemistry: Principles and Modern Applications

Question

Which statement is probably true concerning the masses of individual chlorine atoms: All have, some have, or none has a mass of 35.4527 u? Explain.

Step-by-Step Solution

Answer

None of the chlorine atoms has a mass of 35.4527 u. The given mass is an average value, considering the natural abundance of different chlorine isotopes.

Step 1: Understand the Isotopes

Isotopes are variants of a particular chemical element with differing numbers of neutrons, hence different mass numbers. However, different isotopes of a single element will all have the same number of protons. Therefore, Chlorine has different isotopes with different masses but they all have the same atomic number.

Step 2: Identify the Chlorine Isotopes

There are two stable isotopes of chlorine: Chlorine-35 and Chlorine-37. Chlorine-35 has an atomic mass of approximately 34.96885 u and makes up about 75% of naturally occurring Chlorine. Chlorine-37, on the other hand, has a mass of approximately 36.96590 u and constitutes the remaining 25%.

Step 3: Evaluate the Given Mass

The given atomic mass, 35.4527 u, is the average atomic mass of chlorine taking into consideration the natural abundance of its isotopes. Therefore, it is not the exact mass of each chlorine atom, but an average value.

Step 4: Formulate the Conclusion

Since the atomic mass given is an average and does not match exactly with the individual isotopes, no single chlorine atom will have an atomic mass of 35.4527 u. The masses of individual chlorine atoms will vary depending on whether they are chlorine-35 or chlorine-37.

Key Concepts

Atomic Mass

Atomic mass is essentially the weighted average mass of an element's isotopes, considering their relative abundance. This value is what you often see on periodic tables, and it reflects an average rather than the precise weight of any single atom. In the context of chlorine, this average comes out to be 35.4527 u.

Why is it a weighted average? Simply because elements often exist in nature as a mix of isotopes, which are atoms with the same number of protons but different numbers of neutrons. For chlorine, the common isotopes are chlorine-35 and chlorine-37.

• Chlorine-35 has an approximate mass of 34.96885 u.
• Chlorine-37 is heavier, with a mass of about 36.96590 u.

Because chlorine-35 is far more common, making up about 75% of chlorine in nature, it pulls the average atomic mass closer to its value. This means the average mass you see, known as atomic mass or atomic weight, isn't exactly the same as any individual atom's mass.

Isotopic Abundance

Isotopic abundance refers to the relative proportion of each isotope present in a natural sample of an element. So, in our day-to-day experience with elements like chlorine, we interact with a mixture of their isotopes.

For chlorine, isotopic abundance is particularly interesting because it has a pronounced split between its isotopes.

• About 75% of naturally occurring chlorine is the isotope chlorine-35.
• The remaining 25% is chlorine-37.

This balance between the two isotopes greatly influences the average atomic mass because each isotope contributes to it according to its abundance. Hence, the concept of isotopic abundance is crucial when determining the atomic weight or average atomic mass of an element, as it affects how closely the average reflects each particular isotope's mass.

Neutrons in Isotopes

Neutrons are subatomic particles found in the nucleus of an atom, alongside protons. The number of neutrons, when combined with the number of protons, determines the isotope of an element. For chlorine, both common isotopes have 17 protons, but they differ in the number of neutrons.

• Chlorine-35 has 18 neutrons (35 - 17 = 18).
• Chlorine-37 has 20 neutrons (37 - 17 = 20).

The neutron difference doesn't affect the chemical properties drastically since these depend on the electron configuration. However, the differing number of neutrons does affect the mass of isotopes, which is why chlorine-35 and chlorine-37 have different atomic masses. This variance due to neutrons is fundamental in understanding why isotopes have different physical properties like mass but are chemically similar. Understanding the role of neutrons helps explain why no single chlorine atom actually has a mass of 35.4527 u — that value is simply an average influenced heavily by these neutronic differences.