AS Chemistry 9701: Ionic, Metallic and Dative Bonding Explained

Magnesium reacts with oxygen to form magnesium oxide, and magnesium metal itself is a good electrical conductor. Separately, ammonia gas reacts with hydrogen chloride gas to form solid ammonium chloride, $\text{NH}_4\text{Cl}$ , which contains the ammonium ion, $\text{NH}_4^+$ .

(a) Magnesium oxide has a melting point of over $2800\,^\circ\text{C}$ , far higher than that of sodium chloride ( $801\,^\circ\text{C}$ ). Explain, in terms of the ions present and the electrostatic forces between them, why magnesium oxide has such a high melting point. [2]

(b) Explain, in terms of structure and bonding, why solid magnesium metal conducts electricity, whereas solid magnesium oxide does not. [2]

(c) Use your knowledge of dative (coordinate) bonding to explain how the fourth N–H bond in $\text{NH}_4^+$ forms when $\text{NH}_3$ reacts with a hydrogen ion, $\text{H}^+$ . State which atom donates the bonding electron pair, and state how this dative bond compares in length and strength with the other three N–H bonds once it has formed. [3]

Part (a): Why magnesium oxide has such a high melting point

Magnesium oxide is a giant ionic lattice of $\text{Mg}^{2+}$ and $\text{O}^{2-}$ ions held together by strong electrostatic attractions between oppositely charged neighbours.

Two factors make these attractions much stronger than in sodium chloride:

Charge: $\text{Mg}^{2+}$ and $\text{O}^{2-}$ each carry a charge of magnitude $2$ , compared with $\text{Na}^+$ and $\text{Cl}^-$ which carry charge $1$ . Electrostatic force increases with the product of the charges, so MgO’s ionic attractions are considerably stronger.
Ionic radius: $\text{Mg}^{2+}$ and $\text{O}^{2-}$ are also smaller than $\text{Na}^+$ and $\text{Cl}^-$ , so the oppositely charged ions can approach more closely, further increasing the attractive force.

Melting a giant ionic lattice means overcoming (weakening) these strong electrostatic attractions throughout the whole structure, which requires much more thermal energy for MgO than for NaCl, consistent with MgO’s much higher melting point ( $2800\,^\circ\text{C}$ vs $801\,^\circ\text{C}$ ).

Part (b): Why magnesium conducts but magnesium oxide does not

Magnesium metal consists of a lattice of $\text{Mg}^{2+}$ cations surrounded by a “sea” of delocalised electrons (the outer-shell electrons that each magnesium atom has lost). These delocalised electrons are not attached to any one ion and are free to drift through the whole structure. When a voltage is applied, they flow through the metal, carrying charge, so solid magnesium conducts electricity.

Magnesium oxide, by contrast, has no delocalised electrons or mobile ions: the $\text{Mg}^{2+}$ and $\text{O}^{2-}$ ions are held in fixed positions by strong electrostatic forces throughout the rigid lattice. With no charge carriers free to move, solid MgO cannot conduct electricity (it only conducts once molten or dissolved, when the ions themselves become mobile).

Part (c): The dative bond in the ammonium ion

In $\text{NH}_3$ , nitrogen has three bonding pairs (shared with the three hydrogen atoms) and one lone pair of electrons, which is not shared with anything.

When $\text{NH}_3$ reacts with $\text{H}^+$ , the hydrogen ion has an empty $1s$ orbital and no electrons of its own. Nitrogen’s lone pair is donated into this empty orbital, forming a new covalent bond in which both electrons originated from nitrogen. This is a dative (coordinate) bond.

Donor atom: nitrogen supplies both electrons of the new bond; $\text{H}^+$ supplies none.
Once formed: although this fourth N–H bond was created differently from the other three (which each formed from one electron contributed by N and one by H), it becomes indistinguishable from them in length and strength. All four N–H bonds in $\text{NH}_4^+$ are identical, and the resulting ion is a regular tetrahedron. The origin of a bond’s electrons does not affect its properties once it has formed.

Final answers

(a) MgO’s higher melting point is due to the stronger electrostatic attraction between its doubly-charged, smaller $\text{Mg}^{2+}$ and $\text{O}^{2-}$ ions compared with the singly-charged ions in NaCl.
(b) Mg conducts via mobile delocalised electrons in the metallic lattice; MgO does not conduct as a solid because its ions are fixed in the ionic lattice.
(c) Nitrogen donates its lone pair into the empty orbital of $\text{H}^+$ to form the dative bond; the resulting bond is identical to the other three N–H bonds.