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đź’ˇ 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?
Weighted Average Atomic Mass
Average mass of the isotopes of an element, each weighted by its natural abundance in nature. Chlorine equals 35.5 u (75 percent Cl-35, 25 percent Cl-37)
- Average mass of an element’s isotopes, weighted by natural abundance .
- Not a simple mean — more common isotopes count more.
- Chlorine: 75% 35 Cl + 25% 37 Cl .
Weighted average
=
(35×
75
100
)+(37×
25
100
)
=
105
4
+
37
4
=
142
4
=
35.5
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 can image materials at atomic level.
- STM means Scanning Tunnelling Microscope.
- TEM means Transmission Electron Microscope.
- STMs mainly study the surfaces of materials.
- 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 = (79× 49.7 100 )+(81× 50.3 100 ) ≈ 80.0 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.
