Isotopes and average atomic mass
Same element, different neutrons
1. General properties: mass, radius and metallic property
2. Half a neutron?
The periodic table gives chlorine an atomic mass of 35.5. Can an atom hold half a neutron?
Pick the right answer
Predict: where does the .5 come from?
- A. It is an average of chlorine atoms with different masses
- B. Each chlorine atom has 18.5 neutrons
- C. It is a rounding mistake in the table
Explanation
Chlorine atoms do not all have the same mass: chlorine has isotopes. The 35.5 in the table is the average mass of its atoms, not the mass of one atom.
3. Isotopes
Isotopes
Atoms of the same element with the same atomic number but different mass numbers, due to different numbers of neutrons.
| Isotope | Protons | Neutrons | Mass number |
|---|---|---|---|
| Chlorine-35 | 17 | 18 | 35 |
| Chlorine-37 | 17 | 20 | 37 |
Same protons, so same element. Different neutrons, so different masses.
4. Mix the isotopes
Natural chlorine is a mix of both isotopes. Change the share of chlorine-35. Find the table's value.
Chlorine-35 in the mix 50 %
Average atomic mass 36
5. An average, not one atom
gatheredatomic mass = (35 × 75) + (37 × 25)/100\\ = 35.5gathered
The more common isotope pulls the average towards its own mass.
6. A new element
Pick the right answer
Boron has two isotopes: boron-10 (20%) and boron-11 (80%). Its atomic mass is closest to:
- A. 10.8
- B. 10.5
- C. 11
- D. 10.2
Explanation
The atomic mass is an average weighted by how common each isotope is: 0.20 × 10 + 0.80 × 11 = 2 + 8.8 = 10.8. Boron-11 is far more common, so the average lies close to 11.
Fill in the blanks
In the nucleus, isotopes of one element have the same number of _____ but different numbers of _____.
Explanation
Same protons (same atomic number), different neutrons (different mass numbers).
7. Summary
- Isotopes: same atomic number, different mass numbers, due to different numbers of neutrons.
- The atomic mass in the table is the average of the isotopes, weighted by how common each is.
- Chlorine: 75% Cl-35 + 25% Cl-37 → 35.5, so no atom needs half a neutron.
Can you explain why the atomic mass of boron is not a whole number?
Reveal
Boron is a mix of two isotopes: boron-10 (20%) and boron-11 (80%). Its atomic mass is their weighted average: 0.20 × 10 + 0.80 × 11 = 10.8. Boron-11 is much more common, so the average is close to 11.