How are a mix of pre-1982 and post-1982 pennies a misleading or incorrect analogy for actual element isotopes?

Because the old pennies were 95% Cu and 5% Zn. The new pennies are mostly Zn with a small amount of Cu; therefore, the mass of the old and new pennies are different. That would prevent you from using the masses and averaging them. You can read more about the old and new compositions here.

http://en.wikipedia.org/wiki/Penny_%28United_States_coin%29

Using a mix of pre-1982 and post-1982 pennies as an analogy for element isotopes can be misleading or incorrect for a few reasons:

1. Isotopes are not different versions of the same element: While pre-1982 and post-1982 pennies are essentially the same currency, with only a change in composition, isotopes are different forms of an element with varying numbers of neutrons in their nuclei. Isotopes have distinct physical and chemical properties, thus going beyond simple composition variations like penny makeovers.

2. Isotopes have unique characteristics: Each isotope of an element behaves differently due to differences in their atomic mass. Isotopes may have different stability, radioactivity, and reactivity levels, among other properties. These variations are not comparable to the inconsistencies brought about by changes in the composition of pennies.

3. Isotope ratios are not determined by time: The pre-1982 and post-1982 analogy suggests that the ratio between isotopes can be determined based on when they were formed. But in reality, the ratio of isotopes within an element is determined by nuclear processes and can vary depending on the source, environment, or other factors, not merely the time of formation.

Therefore, while the penny analogy might help in understanding the concept of composition differences, it falls short in illustrating the complexity and uniqueness of isotopes, making it a misleading or incorrect comparison.

The analogy of using a mix of pre-1982 and post-1982 pennies to represent element isotopes can be misleading or incorrect for a few reasons.

First, let's understand what isotopes are. Isotopes are different forms of the same element that have the same number of protons but differ in the number of neutrons in their atomic nuclei. This difference in the number of neutrons gives each isotope a different atomic mass.

Now, let's compare this to the analogy of pennies. Pre-1982 pennies were made of pure copper, which means they have a different composition compared to post-1982 pennies. The pre-1982 pennies contain 95% copper and 5% zinc, while post-1982 pennies are made of zinc with a thin copper plating.

In the context of isotopes, using pre-1982 and post-1982 pennies to represent different isotopes of an element would imply that the isotopes have different elemental compositions, similar to the pennies. However, this is not the case for isotopes.

In reality, isotopes of an element have identical chemical properties since they have the same number of protons, which determines an element's chemical behavior. The only difference between isotopes is their atomic mass due to the varying number of neutrons. So, unlike the analogy of pennies, isotopes do not have different compositions or chemical properties within the same element.

To better illustrate isotopes, a more appropriate analogy could be different versions of the same car model, where the cars have identical features and performance characteristics but differ in color or weight. This analogy conveys the concept that isotopes share the same chemical behavior but have varying atomic masses.