There are two elements - calcium (atomic number 20) and argon (atomic number 18). The mass number of both the elements is 40. They are therefore known as
Isobars
Understanding Atomic Structure and Element Classification
The question asks us to identify the relationship between two elements, calcium and argon, given their atomic numbers and mass numbers. Calcium has an atomic number of 20 and a mass number of 40. Argon has an atomic number of 18 and a mass number of 40. We need to determine if they are isotopes, isochors, isobars, or isotones.
Key Concepts: Atomic Number and Mass Number
- Atomic Number (Z): This is the number of protons in the nucleus of an atom. The atomic number uniquely identifies an element. For calcium, Z = 20, and for argon, Z = 18. Since their atomic numbers are different, they are different elements.
- Mass Number (A): This is the total number of protons and neutrons in the nucleus of an atom. It is the sum of the atomic number (Z) and the number of neutrons (N): \(A = Z + N\). Both calcium and argon in this specific case have a mass number of 40.
- Number of Neutrons (N): We can calculate the number of neutrons using the formula \(N = A - Z\).
- For Calcium-40: \(N = 40 - 20 = 20\) neutrons.
- For Argon-40: \(N = 40 - 18 = 22\) neutrons.
Let's summarize the properties of Calcium-40 and Argon-40:
| Element | Atomic Number (Z) | Mass Number (A) | Number of Neutrons (N) |
|---|---|---|---|
| Calcium (Ca) | 20 | 40 | 20 |
| Argon (Ar) | 18 | 40 | 22 |
Analyzing the Options
Now, let's look at the definitions of the given options to see which one fits the relationship between Calcium-40 and Argon-40.
- Isotopes: Atoms of the same element (same atomic number, Z) but different mass numbers (different number of neutrons, N). Calcium-40 and Argon-40 have different atomic numbers (20 and 18), so they are not isotopes of each other.
- Isochors: This term relates to processes occurring at constant volume in thermodynamics and is not applicable to the classification of atoms based on their nuclear composition.
- Isobars: Atoms of different elements (different atomic number, Z) that have the same mass number (A). In this case, Calcium-40 (Z=20, A=40) and Argon-40 (Z=18, A=40) are different elements with the same mass number (A=40). This definition perfectly matches their relationship.
- Isotones: Atoms of different elements (different atomic number, Z) that have the same number of neutrons (N). Calcium-40 has 20 neutrons, and Argon-40 has 22 neutrons. Since the number of neutrons is different, they are not isotones.
Based on the analysis, Calcium-40 and Argon-40 are atoms of different elements that share the same mass number (40). This characteristic defines them as isobars.
Conclusion
Calcium (atomic number 20, mass number 40) and Argon (atomic number 18, mass number 40) have different atomic numbers but the same mass number. Therefore, they are known as isobars.
Revision Table: Atomic Relationships
| Term | Defining Characteristic | Atomic Number (Z) | Mass Number (A) | Number of Neutrons (N) | Example |
|---|---|---|---|---|---|
| Isotopes | Same element | Same | Different | Different | \(^{12}\text{C}\) and \(^{14}\text{C}\) |
| Isobars | Different elements | Different | Same | Different | \(^{40}\text{Ca}\) and \(^{40}\text{Ar}\) |
| Isotones | Different elements | Different | Different | Same | \(^{39}\text{K}\) (Z=19, N=20) and \(^{40}\text{Ca}\) (Z=20, N=20) |
Additional Information: Exploring Atomic Structure
Understanding the subatomic particles - protons, neutrons, and electrons - is fundamental to grasping these concepts. The nucleus contains protons and neutrons, and its composition dictates the mass number and the element's identity. Electrons orbit the nucleus in a neutral atom, the number of electrons equals the number of protons (the atomic number). However, the terms isotopes, isobars, and isotones are based on the composition of the nucleus (protons and neutrons).
Isobars, like Calcium-40 and Argon-40, demonstrate that atoms with the same total number of nucleons (protons + neutrons) can belong to completely different elements. Their chemical properties are vastly different because they have different numbers of protons and thus different electron configurations. However, they share the same mass number, which relates to their approximate atomic mass.
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