Ionic Compounds and Their Properties

What are ionic compounds?

Ionic compounds are made of positive ions (cations) and negative ions (anions). An ionic bond is the strong electrostatic attraction between these oppositely charged ions.

How do they form?

Metals lose electrons to form cations, and non-metals gain electrons to form anions. For example, sodium loses one electron to form $\text{Na}^+$, and chlorine gains one electron to form $\text{Cl}^-$, making sodium chloride (NaCl). Magnesium and oxygen form $\text{Mg}^{2+}$ and $\text{O}^{2-}$, making magnesium oxide (MgO). The overall charge of the compound is zero.

Giant ionic lattice

Ionic compounds do not exist as small molecules. Their ions build a giant lattice: a regular 3D arrangement of alternating positive and negative ions. Think of a 3D checkerboard or a stack of tiny magnets with $+$ next to $-$ in all directions.

Properties explained

• High melting and boiling points: Many strong attractions must be overcome to separate ions, so lots of energy is needed.
• Electrical conductivity: Solid ionic compounds do not conduct because ions are fixed in place. When molten or aqueous (dissolved in water), ions are free to move and carry charge, so they conduct well.
• Brittleness: If layers shift, like charges line up and repel, causing the crystal to split.
• Often soluble in water: Water molecules attract ions and pull them apart, so many (not all) ionic solids dissolve.

Common misconceptions

• “Ionic compounds are molecules.” They are giant lattices, not small molecules.
• “If it conducts when molten, it conducts when solid.” Solid ionic compounds do not conduct.
• “Dissolving is the same as melting.” Dissolving uses water to separate ions; melting uses heat to make a liquid.