Question

There are three naturally occurring isotopes of argon, with mass numbers 36,38 , and 40 . a. Write the atomic symbol for each of these atoms. b. How are these isotopes alike? c. How are they different? d. Why is the atomic mass of argon listed on the periodic table not a whole number? e, Which isotope is the most prevalent in a sample of argon?

Short answer

a. \(^{36}_{18} \text{Ar}\), \(^{38}_{18} \text{Ar}\), and \(^{40}_{18} \text{Ar}\). b. Same number of protons. c. Different number of neutrons. d. Weighted average. e. Argon-40.

Step-by-step solution

Write the atomic symbol for each isotope

The atomic symbol is written with the mass number as the superscript and the atomic number (which is 18 for argon) as the subscript. The atomic symbols are: - \(^{36}_{18} \text{Ar}\)- \(^{38}_{18} \text{Ar}\)- \(^{40}_{18} \text{Ar}\)

Determine how these isotopes are alike

Isotopes of an element share the same number of protons but have different numbers of neutrons. Despite their mass differences, these isotopes have the same electron configuration and chemical properties since they all have 18 protons and 18 electrons.

Determine how these isotopes are different

The isotopes differ in the number of neutrons they contain. Argon-36 has 18 neutrons, Argon-38 has 20 neutrons, and Argon-40 has 22 neutrons. This results in differences in their mass numbers.

Explain why the atomic mass of argon is not a whole number

The atomic mass of argon listed on the periodic table is a weighted average of the masses of all its naturally occurring isotopes, taking into account their relative abundances. Hence, it is not a whole number.

Determine which isotope is the most prevalent

Argon-40 is the most prevalent isotope of argon in nature, making up about 99.6% of natural argon.

Key concepts

Atomic Symbol

The atomic symbol represents an element and its isotopes with specific notations. For argon, the atomic symbols for its three naturally occurring isotopes are essential to understand. Each symbol has a mass number as the superscript and the atomic number (18 for argon) as the subscript. This notation helps differentiate between the isotopes:
- \(^{36}_{18} \text{Ar}\) for Argon-36,
- \(^{38}_{18} \text{Ar}\) for Argon-38, and
- \(^{40}_{18} \text{Ar}\) for Argon-40.

Isotopes Similarities

Isotopes are variants of the same element that have identical numbers of protons. This core similarity means:
- Isotopes of argon all have 18 protons.
- They share the same electron configuration.
- Their chemical properties are identical because they have the same number of electrons.
So, despite having different masses, Argon-36, Argon-38, and Argon-40 behave the same in chemical reactions.

Isotopes Differences

While isotopes have the same number of protons, they differ in the number of neutrons they contain:
- Argon-36 has 18 neutrons.
- Argon-38 has 20 neutrons.
- Argon-40 has 22 neutrons.
This difference in neutron count leads to distinct mass numbers (36, 38, and 40). Thus, the mass of each isotope varies, even though their chemical properties remain the same.

Atomic Mass

The atomic mass of an element like argon is not a whole number because it is a weighted average of the masses of all its naturally occurring isotopes. This average considers the relative abundance of each isotope:
- If one isotope is more prevalent, it influences the average atomic mass more.
- The calculation involves multiplying each isotope’s mass by its relative abundance and summing these values.
Hence, Argon's atomic mass on the periodic table reflects this weighted average.

Isotope Prevalence

In natural samples of argon, certain isotopes are more common than others:
- Argon-40 is the most prevalent, making up about 99.6% of natural argon.
- The prevalence of an isotope affects its contribution to the average atomic mass.
Knowing the most common isotope helps in understanding the element's average atomic properties and its behavior in natural processes.