# Thermal stability order of NaF, MgF2 and AlF3

I came up with a question to arrange thermal stability order of $$\ce{NaF}$$, $$\ce{MgF2}$$ and $$\ce{AlF3}$$ and I think the answer is $$\ce{NaF>MgF2>AlF3}$$ because $$\ce{Na+}$$ has largest ionic radius among the cations(anion is same) and also NaF has the greatest ionic character. And as we know greater the ionic character, the greater is the thermal stability.

But the answer is exactly opposite that is $$\ce{NaF. Where am I wrong?

Do lattice enthalpy also have a role to play in determining the thermal stability of metal chlorides and fluoride? If yes, then how?

• I what you understand by thermal stability, I have a feeling it's not what you think it is. – Mithoron Apr 11 at 18:02
• @Mithoron According to the most widely prescribed Chemistry textbooks (published by the NCERT) in India, thermal stability is measured by the melting/boiling point of an element/compound relative to other members in a group/period/family of compounds (and whether something decomposes on heating or doesn't exist stably at room temperature). The terminology isn't very accurate and some books are known to contain errors. – Kartik Apr 12 at 7:45
• @Kartik you can't trust NCERT just because it is the most widely prescribed chemistry textbook. They are known to contain notorious errors. – Nilay Ghosh Apr 12 at 8:52
• @Nilay Ghosh. Who's trusting it? Just clarifying how the context in which the term thermal stability is used in the question came about. Your comment helps that explain even more! – Kartik Apr 12 at 8:56