Calcium has atomic number 20 and mass number 40. Which one of the following hypothetical nuclides would be an isobar of calcium, that is, would have the same mass number 40 but a different atomic number?

Introduction / Context:
This nuclear chemistry question examines the concept of isobars. Atomic structure terms such as atomic number, mass number, isotopes and isobars are frequently tested in school exams. Isobars are nuclei that have the same mass number but different atomic numbers. Understanding how to count protons and neutrons and relate them to these definitions is essential to answer such questions correctly.

Given Data / Assumptions:

• Calcium is given with atomic number 20 and mass number 40.
• Atomic number is the number of protons in the nucleus.
• Mass number is the total number of protons plus neutrons.
• We are asked to identify a nuclide that has mass number 40 but a different atomic number from calcium.

Concept / Approach:
Isobars are defined as atoms or nuclei of different elements that have the same mass number but different atomic numbers. This means the sum of protons and neutrons must be equal to 40 for calcium and for the isobar, but the proton count must be different. For calcium with atomic number 20 and mass number 40, the number of neutrons is 40 minus 20, which is 20. To find a correct isobar, we need any other combination of protons and neutrons that adds up to 40 but has a proton number different from 20.

Step-by-Step Solution:

Verification / Alternative check:
A quick way to check is to write A equals Z plus N for each nuclide. Any combination that gives A equals 40 and Z not equal to 20 is a valid isobar of calcium 40. For example, argon 40 has atomic number 18 and mass number 40, and it is a well known isobar of calcium 40. Option A corresponds exactly to Z equals 18 and N equals 22, which matches argon 40. This agreement confirms that option A is the correct isobar, while the other options give mass numbers different from 40 and so cannot be isobars of calcium 40.

Why Other Options Are Wrong:

• 18 protons and 19 neutrons: Mass number is 37, not 40, so it is not an isobar of calcium 40.
• 20 protons and 19 neutrons: Mass number is 39 and atomic number equals that of calcium, so it would be an isotope of calcium, not an isobar.
• 22 protons and 16 neutrons: Mass number is 38, so it does not match the mass number of calcium 40.
• 21 protons and 18 neutrons: Mass number is 39 and so cannot qualify as an isobar of mass 40.

Common Pitfalls:
Many learners confuse isotopes and isobars. Isotopes have the same atomic number but different mass numbers, while isobars have the same mass number but different atomic numbers. Another frequent mistake is to look only at the number of neutrons rather than the sum of protons and neutrons. Carefully computing A equals Z plus N for each option prevents such errors and helps you apply the correct definition of an isobar in exam questions.

Final Answer:
The nuclide that is an isobar of calcium 40 is the Nuclide with 18 protons and 22 neutrons.