Calcium (atomic number 20) and argon (atomic number 18) both have mass number 40. Such pairs of nuclides are called what?

Introduction / Context:
This question belongs to nuclear chemistry and the classification of nuclides. It involves comparing atomic numbers and mass numbers of different elements. Several terms like isotopes, isobars, and isotones are used to describe relationships between nuclides with regard to protons, neutrons, and total nucleons. Correct usage of these terms is very common in objective type exams.

Given Data / Assumptions:

• Calcium has atomic number 20 and mass number 40.
• Argon has atomic number 18 and mass number 40.
• Therefore, both nuclides have the same mass number but different atomic numbers.
• Options include isotones, isochores, isobars, and isotopes.

Concept / Approach:
The definitions are as follows:

• Isotopes: same atomic number, different mass numbers.
• Isobars: same mass number, different atomic numbers.
• Isotones: same number of neutrons, different atomic numbers.
• Isochores is not commonly used in this nuclear sense and may refer to constant volume lines in thermodynamics.

Since calcium 40 and argon 40 have the same mass number 40 but different atomic numbers 20 and 18, they clearly fit the definition of isobars.

Step-by-Step Solution:
Step 1: Note the mass numbers. Calcium nuclide given has mass number 40. Argon nuclide given also has mass number 40. Step 2: Note the atomic numbers. Calcium has atomic number 20, argon has 18, so they are different elements. Step 3: Match with definitions. Same mass number but different atomic numbers defines isobars. Therefore calcium 40 and argon 40 are isobars.

Verification / Alternative Check:
We can compute the number of neutrons to cross check. Calcium 40 has 40 minus 20 equals 20 neutrons. Argon 40 has 40 minus 18 equals 22 neutrons. Since the number of neutrons is not the same, they are not isotones. Because they have different atomic numbers, they cannot be isotopes of the same element either. The only correct classification based on same mass number is isobars.

Why Other Options Are Wrong:
Option A: Isotones would require equal number of neutrons, which is not true here. Option B: Isochores relate to constant volume processes in physical chemistry and do not describe a relationship between nuclides. Option D: Isotopes have the same atomic number but different mass numbers, which is not the case because calcium and argon are different elements.

Common Pitfalls:
Learners often confuse isobars and isotopes because both involve the word bar or tope and both relate to mass and atomic numbers. A good memory aid is that isotopes share the same atomic number (same element), while isobars share the same mass number but are different elements. Writing small tables of Z, A, and N for a few examples can help fix these concepts firmly.

Final Answer:
Calcium 40 and argon 40 are isobars of each other.