Question

Ar\(^ {18}_{40}\) and Ca\(^ {40}_{20}\) are considered as:

• A. Isobars
• B. Isomers
• C. Isotopes
• D. Isotones

Hint:

Isotopes of an element have the same number of protons (atomic number) but differ in the number of neutrons, leading to different mass numbers.

Answer 1

The Correct Option is A

• Option 1: Isobars - This is the correct answer. Ar-40 and Ca-40 are isobars because they have the same mass number (40) but different atomic numbers (18 for Ar and 20 for Ca). Isobars are atoms of different elements that have the same mass number.
• Option 2: Isomers - This is incorrect. Isomers are molecules with the same molecular formula but different structural arrangements, which is not applicable to the case of Ar and Ca.
• Option 3: Isotopes - This is incorrect. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Ar and Ca are different elements, so they cannot be isotopes of each other.
• Option 4: Isotones - This is incorrect. Isotones are atoms of different elements that have the same number of neutrons but different atomic numbers. Ar and Ca do not fit this definition because their numbers of neutrons are different.

Explanation: 

Ar-40 and Ca-40 are considered isobars because they have the same mass number (40) but different atomic numbers (18 for Ar and 20 for Ca). Isobars are atoms of different elements that share the same mass number but have different atomic numbers.

Answer 2

Breaking Ar-40 and Ca-40 down into their actual proton and neutron counts shows exactly which category they belong to.

1. Isobars: Argon has 18 protons, so Ar-40 has 18 protons and 22 neutrons (18 + 22 = 40). Calcium has 20 protons, so Ca-40 has 20 protons and 20 neutrons (20 + 20 = 40). Different elements, different proton counts, but the same total mass number of 40, which is exactly the definition of isobars.
2. Isomers: This term is reserved for molecules with an identical formula but different structural arrangements. Ar-40 and Ca-40 are single atoms, not molecules, so this term cannot describe them.
3. Isotopes: Isotopes need to be the same element with the same number of protons. Argon, with 18 protons, and calcium, with 20 protons, are different elements, so they fail this requirement.
4. Isotones: Isotones need the same neutron count. Ar-40 has 22 neutrons while Ca-40 has 20 neutrons, so their neutron counts do not match, ruling this option out.

With 18 and 20 protons respectively but the same mass number of 40, Ar-40 and Ca-40 fit the isobar relationship exactly.

So the correct answer is Isobars.