Isotopes
2.3 Isotopes
1. Definition of Isotopes
1.1 Core Definition
- Isotopes are different atoms of the same element that have:
- The same number of protons (proton number / atomic number, Z)
- Different numbers of neutrons in their nuclei
- Therefore, different mass numbers (A)
1.2 Notation
- Nuclear symbols are written as:
A
Z X
Where:- A = mass number (protons + neutrons)
- Z = proton number (number of protons)
- X = chemical symbol
Examples:
- ¹²₆C → Carbon with mass number 12, proton number 6
- ¹⁴₆C → Carbon with mass number 14, proton number 6
1.3 Examples of Isotopes
- Carbon: ¹²₆C, ¹⁴₆C
- Chlorine: ³⁵₁₇Cl, ³⁷₁₇Cl
- Hydrogen: ¹₁H, ²₁H, ³₁H
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
2. Same Chemical Properties of Isotopes
2.1 Reason
- Chemical properties depend on the number of outer shell electrons
- Isotopes have the same:
- Number of protons
- Number of electrons (in neutral atoms)
- Electronic configuration
- Therefore:
- Same bonding behaviour
- Same chemical reactivity
2.2 Example
- ³⁵₁₇Cl and ³⁷₁₇Cl both have:
- 17 protons
- 17 electrons
- Electron configuration: 2,8,7
3. Different Physical Properties of Isotopes
- Physical properties depend partly on mass
- Different numbers of neutrons → different masses
- This can affect:
- Density
- Melting and boiling points
- Diffusion rates (lighter isotopes diffuse faster)
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
4. Interpreting Atomic and Ionic Symbols
4.1 Atoms
Example: ²³₁₁Na
- A = 23
- Z = 11
- Neutrons = 23 − 11 = 12
- Electrons in neutral atom = 11
- Electron configuration: 2,8,1
4.2 Ions
Example: ³⁵₁₇Cl⁻
- A = 35
- Z = 17
- Neutrons = 35 − 17 = 18
- Electrons = 17 + 1 = 18
- Electron configuration: 2,8,8
4.3 Worked Example
Symbol: ⁴⁰₂₀Ca²⁺
- A = 40
- Z = 20
- Neutrons = 40 − 20 = 20
- Electrons = 20 − 2 = 18
- Electron configuration: 2,8,8
5. Calculating Relative Atomic Mass (Ar) from Isotopic Data
5.1 Definition
Relative atomic mass (Ar) = weighted average mass of all the isotopes of an element compared to 1/12 of the mass of a carbon-12 atom.
5.2 Formula
Ar = ((abundance₁ × mass₁) + (abundance₂ × mass₂) + …) / 100
5.3 Worked Example 1 – Chlorine
Given:
- ³⁵Cl: abundance = 75%, mass = 35
- ³⁷Cl: abundance = 25%, mass = 37
Step 1:
Ar = ((75 × 35) + (25 × 37)) / 100
Step 2:
Ar = (2625 + 925) / 100
Step 3:
Ar = 3550 / 100
Step 4:
Ar = 35.5
5.4 Worked Example 2 – Magnesium
Given:
- ²⁴Mg: abundance = 79%, mass = 24
- ²⁵Mg: abundance = 10%, mass = 25
- ²⁶Mg: abundance = 11%, mass = 26
Step 1:
Ar = ((79 × 24) + (10 × 25) + (11 × 26)) / 100
Step 2:
Ar = (1896 + 250 + 286) / 100
Step 3:
Ar = 2432 / 100
Step 4:
Ar = 24.32
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
6. Applications of Isotopes
6.1 Medical Uses
- Cobalt-60 → cancer treatment (radiotherapy)
- Iodine-131 → diagnosis and treatment of thyroid disorders
6.2 Industrial Uses
- Carbon-14 → radiocarbon dating of fossils and archaeological samples
- Uranium-235 → nuclear fuel in power generation
6.3 Scientific Research
- Isotopic tracers in chemical and biological processes
7. Summary Table
| Term | Definition | Example |
|---|---|---|
| Isotope | Same protons, different neutrons | ¹²₆C and ¹⁴₆C |
| Proton number (Z) | Number of protons in nucleus | Chlorine: Z = 17 |
| Mass number (A) | Total protons + neutrons | Chlorine-37: A = 37 |
| Ar formula | ((abundance₁ × mass₁) + (abundance₂ × mass₂)) / 100 | Cl: 35.5 |
| Same chemical props | Same electrons and configuration → same reactivity | ³⁵₁₇Cl & ³⁷₁₇Cl |