Journey Inside the Atom · Lesson 13 of 14
Isobars
“Different elements can share the same mass number while having different atomic numbers”
• Define isobars. • Explain why isobars are different elements. • Use calcium, potassium and argon as examples. • Distinguish isotopes from isobars. • Describe the modern electron-cloud transition. • Recognise atomic-scale imaging mentioned in the chapter.
Isotopes show that one element can have different mass numbers. The reverse question is also possible: atoms of different elements can have the same mass number. These are called isobars.
Isobars
Atoms of different elements that have the same mass number but different atomic numbers.
Calcium, Potassium and Argon
The chapter compares calcium, potassium and argon atoms with mass number forty. Their atomic numbers are twenty, nineteen and eighteen. They therefore have the same nucleon total but different proton counts, so they are different elements.
| Atom | Atomic number | Mass number | Identity clue |
|---|---|---|---|
| Calcium-40 | 20 | 40 | Twenty protons |
| Potassium-40 | 19 | 40 | Nineteen protons |
| Argon-40 | 18 | 40 | Eighteen protons |
Isotopes and Isobars Compared
| Feature | Isotopes | Isobars |
|---|---|---|
| Atomic number | Same | Different |
| Mass number | Different | Same |
| Element identity | Same element | Different elements |
| Direct difference | Neutron number | Proton number and element identity |
Problem
P has Z = 17, A = 35. Q has Z = 17, A = 37. R has Z = 18, A = 37.
- 1.P and Q have the same Z but different A, so they are isotopes.
- 2.Q and R have the same A but different Z, so they are isobars.
- 3.P and R have different Z and different A, so they are neither an isotope pair nor an isobar pair.
The Atomic Story Continues
The chapter emphasises that Bohr's model is not the final picture. In the modern quantum mechanical description, electrons are not treated as particles moving on sharply defined circular tracks. Instead, regions of likely electron presence are described, often visualised as electron clouds.
Seeing Atomic-Scale Detail
Scanning tunnelling microscopes can investigate surfaces at atomic-scale detail, while transmission electron microscopes can reveal how atoms are arranged in very thin samples. These instruments provide evidence about tiny structures rather than literal photographs of Bohr-style circular orbits.
The chapter highlights Homi Jehangir Bhabha and institutions such as TIFR and BARC in the development of atomic-energy research for peaceful applications including electricity, agriculture and medicine.
Use atomic number to decide element identity. Same Z with different A means isotopes; same A with different Z means isobars.
Quiz
Which description best matches Isobars?
Which term matches this description: Atoms of different elements that have the same mass number but different atomic numbers.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- Why are calcium-40 and argon-40 isobars rather than isotopes?
- Atoms with Z = 12 and mass numbers 24 and 25 are what relation?
- Atoms with A = 40 and atomic numbers 18 and 19 are what relation?
- State one way the modern electron picture differs from Bohr's fixed orbits.
Key Takeaways
• Isobars are atoms of different elements that have the same mass number. • They have different atomic numbers and therefore different numbers of protons. • Isotopes have the same atomic number but different mass numbers, while isobars show the opposite pattern. • Atomic number, not mass number, determines the identity of an element.