Bonding and Molecular structure?

Question

How would you explain the following statements using principles of bonding and molecular structure?

a) the boiling point of argon is -186 degrees C whereas the boiling point of neon is -246 degrees C

b) solid sodium melts at 98 degrees C but solid potassium melts at 64 degrees C

c) more energy is required to break up a CaO(s) crystal into ions than to break up a KF(s) crystal into ions

d) molten KF conducts electricity but solid KF does not

Answer 1

Whoa, Nellie.  I just noticed that someone said that argon has a higher boiling point because of its greater mass.  While this is coincidentally true, the mass has nothing to do with the boiling point.  Gravitational effects at the atomic scale are negligible.

The reason Ar has a higher boiling point does involve London dispersion forces, but LDF's are proportional to the polarizability of a molecule.  The polarizability depends on the number of electrons and the volume over which they are spread.  Since argon has more electrons than neon and is physically larger, then argon is more polarizable, and will exhibit greater London dispersion forces.

Answer 2

a) Argon has a higher boiling point because it has stronger London dispersion forces.

b) The alkali metals show a decrease in melting points and boiling points due to the weaker metallic bonds between atoms as their size increase down the group.  The bonds become weaker because as the atoms become larger, they hold their electrons more weakly.

c) The CaO bond is stronger because two electrons have been exchanged (in KF, only one electron has been exchanged).  In general, the more electrons exchanged to create an ionic bond, the stronger it is.

d) Solid KF is rigid and lacks the mobility that molten KF posseses.  This mobility allows electrons to be exchanged between ions.

Answer 3

A) argon is a bigger atom so van der wall forces greater