Ionic Bonding and Structures

Step 3: State the resulting ions.

• Sodium becomes a $Na^{+}$ cation with a stable $2.8$ arrangement.
• Chlorine becomes a $Cl^{-}$ anion with a stable $2.8.8$ arrangement.
• The opposite charges cause the ions to attract each other in a 1:1 ratio.

The Nature of the Ionic Bond

Once the ions are formed, they are pulled together by an ionic bond. This bond is defined as the strong electrostatic attraction between oppositely charged ions.

Crucially, this attraction is non-directional. The electrostatic forces act in all directions around every single ion, pulling surrounding ions toward it and creating a massive, interconnected network.

Chemists often use a dot-and-cross diagram to visualise this process, using dots for one atom's electrons and crosses for the other's. When drawing the final ions, they must be placed in square brackets with the charge written on the top-right outside the brackets (e.g., $[Mg]^{2+}$).

The Giant Ionic Lattice

Ionic compounds never exist as isolated molecules. Instead, they pack tightly together to form a giant ionic lattice.

This lattice is a regular, repeating three-dimensional pattern of alternating positive and negative ions. The ions pack as closely as possible to maximise the attractive forces between opposite charges and minimise repulsion between identical charges.

Scientists use different models to represent this structure, but each has limitations. A ball and stick model shows the 3D geometry clearly but incorrectly suggests physical "sticks" exist between ions, leaving too much empty space. A 3D space-filling model accurately shows the tight packing and relative ion sizes, but it completely hides the internal arrangement. Because the lattice is massive, the standard chemical formula we write (like $NaCl$) is actually an empirical formula showing only the simplest mathematical ratio of the ions present.

Comparing Bond Strengths: NaCl vs MgO

The strength of an ionic lattice depends heavily on the charges and sizes of the ions involved. Higher charges and smaller ionic radii create significantly stronger electrostatic attractions.

Both $NaCl$ and $MgO$ are similar because they exist as giant ionic lattices held together by strong electrostatic attraction. However, they are fundamentally different in their bond strength.

Because $MgO$ contains divalent ions ($2+$ and $2-$) compared to the monovalent ions ($1+$ and $1-$) in $NaCl$, the electrostatic attraction in $MgO$ is roughly four times stronger. Consequently, a much larger amount of thermal energy is required to break the many strong bonds in the magnesium oxide lattice, resulting in a drastically higher melting point.