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

Step 1: Find protons and neutrons.
Ar-40 has 18 protons and 22 neutrons, while Ca-40 has 20 protons and 20 neutrons.

Step 2: Match the definition.
Different proton counts, meaning different elements, but the same total mass number of 40, is exactly what defines isobars, ruling out isotopes, which need same protons, and isotones, which need same neutrons.

Step 3: Final Answer.
Ar-40 and Ca-40 are Isobars.

Answer 2

The notation in the question itself, with the mass number written on top and atomic number on the bottom, already hands us the key numbers, so reading it carefully is enough to answer.

1. Isotopes: The bottom numbers, the atomic numbers, are 18 for argon and 20 for calcium, which are different, so isotopes, which require the same atomic number, is ruled out right away.
2. Isomers: The notation describes single atoms, each with one top number and one bottom number, not a molecular formula with a structural arrangement, so isomers does not apply.
3. Isotones: Subtracting the bottom number from the top number gives the neutron count, 40 minus 18 is 22 for argon, and 40 minus 20 is 20 for calcium. These neutron counts differ, so isotones is ruled out.
4. Isobars: The top numbers, the mass numbers, are both 40 for argon and calcium, while the bottom numbers, the atomic numbers, differ. Same top number, different bottom number, is exactly the isobar pattern.

Reading the mass numbers directly off the notation shows both atoms share 40 while their atomic numbers differ, confirming the isobar relationship.

So the correct answer is Isobars.