Q.Classify each of the following solids as ionic, metallic, molecular, network (covalent) or amorphous.

$(i)\ P_4O_{10}$ – molecular; $(ii)\ (NH_4)_3PO_4$ – ionic; $(iii)\ SiC$ – network (covalent); $(iv)\ I_2$ – molecular; $(v)\ P_4$ – molecular; $(vi)$ Plastic – amorphous; $(vii)$ Graphite – network (covalent); $(viii)$ Brass – metallic; $(ix)\ Rb$ – metallic; $(x)\ LiBr$ – ionic; $(xi)\ Si$ – network (covalent).

Rule for classifying.

• Molecular — discrete covalent molecules held by weak van der Waals / dipole forces.
• Ionic — cation–anion lattice held by electrostatic forces.
• Network (covalent) — a continuous 3-D covalent framework (giant molecule).
• Metallic — metal kernels in a "sea" of delocalised electrons (includes alloys).
• Amorphous — no long-range order.

Classification.

Identify the constituent particles and the binding force for each: discrete covalent molecules $\to$ molecular; ions $\to$ ionic; giant covalent lattice $\to$ network; metal/alloy $\to$ metallic; disordered $\to$ amorphous.

• Calling graphite or $SiC$ "molecular" — they are giant covalent (network) solids.
• Classifying $(NH_4)_3PO_4$ as molecular; the ammonium and phosphate ions make it ionic.
• Forgetting brass is an alloy and therefore metallic.