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💡 Different elements, same weight
Calcium, potassium and argon are three different elements — yet each can have a mass number of 40. Meet the isobars.
What are isobars?
- Atoms of different elements — different atomic numbers (Z) .
- But the same mass number (A) — same total nucleons.
- Example: Ca (Z=20), K (Z=19), Ar (Z=18) can each have A = 40.
As our journey ends, remember the story is not over: even Bohr’s model was later refined. Today we picture electrons as electron clouds — regions where an electron is most likely to be found. More exciting discoveries still lie ahead!
NCERT Question 3 — The composition of the nuclei
The composition of the nuclei of three atomic species X, Y, and Z are given as follows.
Explain the relation between the following: (i) Y and Z (ii) Z and X.
View Answer →
| Nucleus | X | Y | Z |
|---|---|---|---|
| Number of protons | 18 | 17 | 17 |
| Number of neutrons | 19 | 18 | 20 |
Important Definitions
- Isobars — atoms of different elements that have the same mass number but different atomic numbers.
- Electron cloud — the modern picture in which electrons exist in regions where they are most likely to be found.
Isotopes vs Isobars
Isotopes
Isobars
Same element
Different elements
Same atomic number (Z)
Different atomic numbers (Z)
Different mass numbers (A)
Same mass number (A)
Example: C-12, C-13, C-14
Example: Ca-40, K-40, Ar-40
