Animated Solution for Chemistry - States of Matter: Which of the following exists as covalent crystals in the solid state?

Imagine you are holding a handful of different solid materials: a chunk of iodine, a piece of silicon, some yellow sulphur powder, and a block of phosphorus. To the naked eye, they are all just solids. But if we could shrink down to the atomic level, we would see entirely different worlds.

In chemistry, crystalline solids are broadly classified into four main categories based on the forces holding them together: Ionic, Metallic, Molecular, and Covalent (Network) solids. The question asks us to identify the covalent crystal among the given options. To do this, we must understand the fundamental difference between a covalent bond and a covalent crystal.

The Molecular Impostors

Let's first look at Iodine ($I_2$), Sulphur ($S_8$), and Phosphorus ($P_4$).

It is true that inside an $I_2$ molecule, the two iodine atoms are held together by a strong covalent bond. However, when millions of $I_2$ molecules pack together to form a solid crystal, they do not form continuous bonds with each other. Instead, they act like discrete, independent units—much like marbles in a jar. The forces holding these "marbles" together are weak intermolecular forces known as van der Waals forces (specifically, London dispersion forces).

Because the structural units of the crystal are discrete molecules, Iodine is classified as a Molecular Solid. The exact same logic applies to Sulphur, which forms puckered $S_8$ rings, and Phosphorus, which forms discrete $P_4$ tetrahedra. They are all molecular solids, not covalent crystals.

Silicon and the Power of the Network

Now, let's turn our attention to Silicon ($Si$).

Silicon is a Group 14 element, sitting right below Carbon in the periodic table. It possesses four valence electrons. Instead of forming small, discrete molecules, a silicon atom uses its four valence electrons to form four strong single covalent bonds with four neighboring silicon atoms, arranging them in a perfect tetrahedral geometry.

But it doesn't stop there. Each of those neighboring silicon atoms is bonded to three more, and those to three more, and so on. This creates a massive, continuous, three-dimensional web of covalent bonds. There are no discrete molecules here; the entire crystal is essentially one giant molecule!

This continuous 3D lattice is the defining characteristic of a Covalent Network Solid. Because every atom is locked in place by strong covalent bonds, these solids are typically very hard and have extremely high melting points.

Therefore, Silicon is the only element among the choices that exists as a covalent crystal in the solid state.

Pro Tip: For your exams, always memorize the classic examples of covalent network solids. The most frequently tested ones are: 1. Diamond (an allotrope of Carbon) 2. Silicon ($Si$) 3. Silicon Carbide ($SiC$, also known as Carborundum) 4. Quartz ($Si{O}_2$) 5. Aluminum Nitride ($AlN$)

Keep this list in your mental toolkit, and you will instantly recognize network solids the next time they appear in a question!