Type: IONIC SOLID
Sodium fluoride and potassium fluoride are both ionic solids that form simple cubic body lattice structures with their respective ions occupying octahedral coordination sites.
BONDING
Ionic solids are held together by strong ionic bonds caused by the electrostatic forces of attraction between the oppositely charged ions. These ions are created as the metal loses electrons to become a cation while the non-metal gains those electrons to become an anion.
Bond Strength: FACTORS
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Ionic Charge
Both compounds have the same ionic charge as they are both composed of one 1+ ion and one 1- ion, so this does not play a major role in comparing their bond strengths.
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Interionic distance
This is the primary differentiating factor between the bond strengths of KF and NaF. While both compounds have F- as the anion, K has 3 shells as an ion while Na as an ion only has 2 shells and hence is smaller, leading to a reduced distance between the ions and a higher charge density, resulting in NaF having stronger bonds.
Properties: NaF
Properties: KF
Melting Point
993°C
Sodium fluoride has a high melting point of 993°C as it is held together by strong ionic bonds that require a lot of energy input to break apart, and consequently melting it.
Melting Point
858°C
Potassium fluoride also has a high melting point of 858°C, which is slightly lower than NaF's melting point as it has a lesser charge density, meaning it is held together by relatively weaker (but still strong) ionic bonds.
Solubility
Good
Sodium fluoride is composed of ions (charged particles) that the polar water molecules are attracted to and will pull apart, hence dissolving the solid into its dissociated ions in the water, leading to its excellent solubility.
Solubility
Good
Potassium fluoride is also composed of ions that will undergo dissociation induced by the polar water molecules, hence giving it good solubility in water.
Conductivity
Good*
Sodium fluoride has good conductivity when dissolved or melted* as its consitutent ions become free allowing them to move around and freely carry electric charge.
Conductivity
Good*
Potassium fluoride, is also similar to sodium fluoride as it too has ions that are locked in solid state and cannot conduct electricity but will get freed up when molten or dissolved* as their rigid bonds become broken.
