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### Exercise 3: Total heat capacity of a diatomic material
-Naturally occurring lithium has [two stable isotopes](https://en.wikipedia.org/wiki/Isotopes_of_lithium): $^6$Li (7.5%) and $^7$Li (92.5%). Let us extend the Einstein model to take into account the different masses of different isotopes.
+We consider a crystal of lithium, which consists of the [two stable isotopes](https://en.wikipedia.org/wiki/Isotopes_of_lithium) $^6$Li (7.5%) and $^7$Li (92.5%) in their natural abundance. Let us extend the Einstein model to take into account the different masses of these different isotopes.
-1. Assume that the strength of the returning force $k$ experienced by each atom is the same. What is the difference in the oscillation frequencies of different isotopes of lithium in the lithium crystal?
-2. Write down the total energy of lithium assuming that all $^6$Li atoms are in $n=2$ vibrational state, and all $^7$Li atoms are in $n=4$ vibrational state.
-3. Write down the total energy of lithium at a temperature $T$ by modifying the Einstein model.
-4. Compute the heat capacity of lithium as a function of $T$.
+1. Assume that the strength of the returning force $k$ experienced by each atom is the same. What is the difference in the oscillation frequencies of the two different isotopes in the lithium crystal?
+2. Write down the total energy stored in the vibrations of the atoms of the lithium crystal, assuming that all $^6$Li atoms are in $n=2$ vibrational state and all $^7$Li atoms are in $n=4$ vibrational state.
+3. Write down the total energy stored in the vibrations of the atoms in the lithium crystal at a temperature $T$ by modifying the Einstein model.
+4. Compute the heat capacity of the lithium crystal as a function of $T$.
[^1]: Data source: [Wikipedia](https://en.wikipedia.org/wiki/Heat_capacities_of_the_elements_(data_page)), mainly the CRC Handbook of Chemistry and Physics.