Ionic Crystals

Why are ionic crystals hard but brittle?

Ions are locked in place; shifting layers causes like charges to align and repel → crystal shatters.

Why do ionic crystals conduct electricity only when melted or dissolved?

Ions must be mobile to carry charge.

Why do ionic crystals dissolve in water?

Water is polar and can separate and surround ions.

Why are covalent network solids extremely hard?

Continuous network of covalent bonds throughout the structure.

Why do covalent networks not conduct electricity?

Electrons are not free to move.

Why are covalent networks generally insoluble?

Bonds are too strong to be broken by solvents.

What allows metals to conduct electricity?

Delocalized (mobile) electrons.

Why are metals malleable and ductile?

Metal ions can slide past each other without breaking bonds due to electron sea.

What is the “sea of electrons” model?

Positive ions in a lattice surrounded by mobile electrons.

Why do polar molecular solids have higher melting points than non-polar ones?

Dipole-dipole forces are stronger than London dispersion forces.

Why are molecular solids poor conductors?

No charged particles are free to move.

How does molecular size affect London forces?

Larger molecules → stronger London forces → higher melting/boiling points.