Answer to Question #77434 in General Chemistry for Sakshi

Question

Among LiF,NaF,and KF which one is thermally the most stable? Give reason for your answer?

Answer 1

Solution:
LiF > NaF> KF
Molecules are generally more stable when the indivdual atoms are about the same size. Aside
from the common ionic forces present in these compounds which result from a +1/-1 bond,
there are also Vander Waal/London Dispersion forces present. Using LiF as an example - The
electrons in the Lithium atom are attracted to the positive nucleus of the F atom, and vice
versa. Since the atoms are of relatively similar size, the attraction and repulsion between each
atom is similar, and more or less cancel one another out.
On the other end, KF is a larger cation bonded with a small anion. K + has a larger amount of
electrons in between its nucleus and Fluorines, therefore K electrons are much attracted to the
nucleus of F, but the electrons in F are not as attracted to the nucleus of K because there are so
many electrons in between repelling the already negative atom. The bigger the cation, the
greater this effect, hence the trend.