Isotopes
Isotopes are atoms of the same element that possess an identical number of protons (atomic number) but a different number of neutrons (mass number).
Identical Chemistry, Different Physics
Because isotopes have identical nuclear charges and therefore identical electron configurations, they exhibit completely indistinguishable chemical reactivity. They form the exact same bonds and undergo the same reactions.
However, their varying mass numbers result in distinctly different physical properties:
- Density
- Rate of gaseous diffusion
- Melting and boiling points
Heavy Water (D₂O)
In heavy water (D₂O), the standard protium isotope (¹H) is replaced by deuterium (²H), which has an extra neutron. Heavy water molecules are substantially more massive, so:
- Solid heavy water ice has a higher density than normal liquid water, so it sinks rather than floats
- Heavy water ice has a slightly higher melting point (3.8 °C) than the 0 °C threshold
This is a favourite examiner example because it demonstrates that isotopes have different physical properties despite identical chemical reactivity.
Common Isotope Examples
| Element | Isotope | Protons | Neutrons | Use |
|---|---|---|---|---|
| Hydrogen | \(^1_1\text{H}\) (Protium) | 1 | 0 | Normal hydrogen |
| Hydrogen | \(^2_1\text{H}\) (Deuterium) | 1 | 1 | Heavy water, NMR |
| Carbon | \(^{12}_6\text{C}\) | 6 | 6 | Standard for Ar scale |
| Carbon | \(^{14}_6\text{C}\) | 6 | 8 | Radiometric dating |
| Chlorine | \(^{35}_{17}\text{Cl}\) and \(^{37}_{17}\text{Cl}\) | 17 | 18 / 20 | Mass spectrometry problems |
Relative Atomic Mass (\(A_r\))
The relative atomic mass is the weighted mean of all naturally occurring isotopes of an element, measured relative to ¹⁄₁₂ of the mass of carbon-12.
Relative Atomic Mass (Ar) Has No Units
Ar is a dimensionless ratio. It is a comparison to ¹⁄₁₂ the mass of a carbon-12 atom, not a measurement with units. Never write "g" or "amu" after Ar. Use the data booklet values and report your answer to 2 decimal places unless instructed otherwise.
Finding Percentage Abundance from Ar
If given the \(A_r\) and asked to find the percentage abundance of two isotopes, use algebra:
Problem: Chlorine has \(A_r = 35.45\), with isotopes ³⁵Cl and ³⁷Cl.
Let fraction of ³⁵Cl = \(x\), and fraction of ³⁷Cl = \(1 - x\):
\[35x + 37(1-x) = 35.45\]
\[-2x = -1.55 \implies x = 0.775\]
³⁵Cl = 77.5% and ³⁷Cl = 22.5%
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