- An ionic solid is made up of positive ions (cations) and negative ions (anions) held together by electrostatic forces in a rigid array or lattice.
- Ionic bonding refers to the electrostatic attraction between cations and anions.
- The physical properties of ionic compounds are:
- High melting and boiling points
- Ionic solids do not conduct electricity (they are insulators).
- When molten (liquid) ionic compounds conduct electricity.
- When dissolved in water to form an aqueous solution ionic compounds conduct electricity.
Physical Properties of Ionic Compounds
Ionic compounds have high melting points.
The electrostatic attraction (ionic bond) between cations and anions is strong. It takes a lot of energy to overcome this attraction in order to allow the ions to move more freely and form a liquid.
The factors which affect the melting point of an ionic compound are:
- The charge on the ions.
In general, the greater the charge, the greater the electrostatic attraction, the stronger the ionic bond, the higher the melting point.
The table below compares the melting point and ion charges for sodium chloride and magnesium oxide.
||Melting Point (oC)
MgO has a higher melting point than NaCl because 2 electrons are transferred from magnesium to oxygen to form MgO while only 1 electron is transferred from sodium to chlorine to form NaCl.
- The size of the ions.
Smaller ions can pack closer together than larger ions so the electrostatic attraction is greater, the ionic bond is stronger, the melting point is higher.
The melting point of Group IA (alkali) metal fluorides is compared to the ionic radius of the cation in the table below.
||Melting Point (oC)
||Cation Radius (pm)
As the radius of the cations increases down Group I from Na+ to Cs+, the melting points of the fluorides decrease.
In order for a substance to conduct electricity it must contain mobile particles capable of carrying charge.
|Mobility of Ions
Solid ionic compounds do not conduct electricity because the ions (charged particles) are locked into a rigid lattice or array. The ions cannot move out of the lattice, so the solid cannot conduct electricity.
When molten, the ions are free to move out of the lattice structure.
- Cations (positive ions) move towards the negative electrode (cathode)
M+ + e -----> M
- Anions (negative ions) move towards the positive electrode (anode)
X- -----> X + e
When an ionic solid is dissolved in water to form an aqueous solution, the ions are released from the lattice structure and are free to move so the solution conducts electricity just like the molten (liquid) ionic compound.
Ionic solids are brittle.
When a stress is applied to the ionic lattice, the layers shift slightly.
The layers are arranged so that each cation is surrounded by anions in the lattice. If the layers shift then ions of the same charge will be brought closer together.
Ions of the same charge will repel each other, so the lattice structure breaks down into smaller pieces.