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💡 One number for many twins
Chlorine is a mix of two isotopes. So what single mass do we write for chlorine? We take a fair, abundance-weighted average.
What is average (weighted) atomic mass?
- Average mass of an element’s isotopes, weighted by natural abundance .
- Not a simple mean — more common isotopes count more.
- Chlorine: 75% \({}^{35}\text{Cl}\) + 25% \({}^{37}\text{Cl}\).
Weighted average
=
\( \left(35\times\dfrac{75}{100}\right)+\left(37\times\dfrac{25}{100}\right) \)
=
\( \dfrac{105}{4}+\dfrac{37}{4}=\dfrac{142}{4}=35.5\ \text{u} \)
👤 Meet a Scientist — Homi Jehangir Bhabha
- Indian physicist, known as the father of the Indian nuclear programme .
- Established key institutions like TIFR and BARC .
- Championed the peaceful use of atomic energy for electricity, agriculture and medicine.
🔬 Ready to Go Beyond
Electron microscopes such as Scanning Tunnelling Microscopes (STMs) and Transmission Electron Microscopes (TEMs) can image materials at atomic level — STMs mainly study surfaces, while TEMs reveal how atoms are arranged inside thin samples.
⏸▶ Pause and Ponder
- 17. Both atoms have 11 protons, so same atomic number (11) → same element (sodium). One has 12 neutrons (A = 23), the other 13 (A = 24) → different mass numbers . They are isotopes .
- 18. Bromine average \(= \left(79\times\dfrac{49.7}{100}\right)+\left(81\times\dfrac{50.3}{100}\right) \approx 80.0\ \text{u}\).
Important Definitions
- Average atomic mass — the average mass of all the naturally occurring isotopes of an element.
- Weighted average atomic mass — the average found by weighting each isotope mass by its percent natural abundance.
