Which Of The Following Is A Molecular Solid

You're staring at a multiple-choice question on a chemistry exam. Because of that, " The options stare back: CO₂, SiO₂, NaCl, Fe. But only one fits the actual definition. Now, your mind races. They're all solids at some temperature, right? "Which of the following is a molecular solid?This is the moment where memorizing definitions falls apart and understanding structure pays off.

Let's walk through what a molecular solid actually is, why the distinction matters, and how to spot the right answer every time — without guessing.

What Is a Molecular Solid

A molecular solid is a solid composed of discrete molecules held together by intermolecular forces — not covalent bonds stretching through the lattice, not ionic attractions between cations and anions, not a sea of delocalized electrons. The molecules themselves stay intact. Practically speaking, they keep their identity. The forces between them are weaker: London dispersion forces, dipole-dipole interactions, hydrogen bonds.

Think of it like a crate of oranges. Each orange is a molecule. The crate holds them together, but the oranges don't fuse into one giant orange. In a covalent network solid (like diamond or quartz), the "oranges" are covalently bonded into one massive structure. Plus, in an ionic solid (like salt), you don't have discrete molecules at all — you have a repeating pattern of Na⁺ and Cl⁻ ions. In a metallic solid, you have cations swimming in an electron sea.

Molecular solids are different. The building blocks are neutral molecules. The lattice energy comes entirely from intermolecular forces.

The key structural signature

If you could zoom in with a magic microscope, you'd see distinct molecular units separated by empty space (relatively speaking). The intramolecular bonds — the covalent bonds inside* each molecule — are strong. And the intermolecular forces between* molecules are much weaker. That weakness dictates almost every physical property.

Why Molecular Solids Matter

This isn't just classification for classification's sake. The category predicts behavior.

Molecular solids tend to be soft. Consider this: they have low melting and boiling points compared to ionic, covalent network, or metallic solids. Dry ice (solid CO₂) does this at -78.5 °C at atmospheric pressure. Think about it: many sublime — go straight from solid to gas — because the intermolecular forces are weak enough to overcome without passing through a liquid phase. Iodine crystals sublime too, leaving purple vapor.

They're usually poor electrical conductors. In real terms, no free ions, no delocalized electrons. Here's the thing — they're often brittle — the layers of molecules slide past each other and the weak forces snap. Many are volatile. You can smell solid camphor or naphthalene (mothballs) because molecules escape the surface easily.

In pharmaceuticals, this matters enormously. Most drug molecules form molecular solids. Because of that, polymorphism (different packing arrangements of the same molecule) can make or break a drug product. Their solubility, stability, bioavailability — all trace back to how those molecules pack in the crystal lattice and how easily they break away into solution. Same molecule, different solid form, different properties.

In materials science, molecular solids show up as organic semiconductors, explosives (TNT, RDX), and components of OLEDs. The weak intermolecular forces allow solution processing — you can dissolve them, print them, spin-coat them — which you can't do with silicon or gallium arsenide.

How to Identify a Molecular Solid

When you see a formula or name, run through this mental checklist.

First: Is it composed of discrete, neutral molecules?

CO₂ — yes, individual CO₂ molecules.
Here's the thing — h₂O (ice) — yes, individual H₂O molecules. I₂ — yes, diatomic iodine molecules.
Because of that, c₁₂H₂₂O₁₁ (sucrose) — yes, distinct sugar molecules. P₄ — yes, tetrahedral phosphorus molecules.
S₈ — yes, crown-shaped sulfur molecules.

Second: Are the intermolecular forces the only thing holding the lattice together?

No covalent bonds between molecules. Now, no ionic bonds. No metallic bonding.

Third: Does it lack a continuous network?

SiO₂ (quartz) — each Si bonded to four O, each O to two Si, extending infinitely. In practice, not molecular. Day to day, diamond — each C bonded to four others in a giant covalent network. SiC — same story.
In practice, not molecular. BN (boron nitride) — network solid.

Fourth: Is it not a metal or alloy?

Fe, Cu, Al, brass, steel — metallic solids. Even so, delocalized electrons. Not molecular.

Want to learn more? We recommend which is not a cranial bone of the skull and single displacement reaction examples in real life for further reading.

Fifth: Is it not an ionic compound?

NaCl, MgO, CaF₂, KBr — ionic lattices. And no discrete neutral molecules in the solid state. (In the gas phase, NaCl exists as ion pairs, but that's not the solid.

If it passes all five, it's a molecular solid.

The borderline cases that trip people up

Graphite — layers of covalently bonded carbon sheets held by dispersion forces. The sheets* are giant molecules. The solid* is often classified as a covalent network solid (layered), not a molecular solid. Don't pick it.

Fullerenes (C₆₀, C₇₀) — now these* are molecular solids. Discrete soccer-ball molecules held by van der Waals forces. They dissolve in organic solvents. They form molecular crystals. This is a classic "which of the following" trap — graphite vs. C₆₀.

Silicon — covalent network. Not molecular.

Silica gel — amorphous SiO₂·nH₂O. Still a network, not molecular.

Polymer solids (polyethylene, nylon) — long chains. The chains are giant molecules. The solid is held by intermolecular forces between chains. Some textbooks call these molecular solids (polymeric molecular solids). Others give them their own category. In a typical general chemistry multiple-choice question, they're usually not the intended answer unless the other options are clearly ionic, metallic, or network. But technically? Yes, they fit.

Ice — molecular solid. Hydrogen-bonded network of H₂O molecules. The hydrogen bonds are intermolecular forces (special, strong ones, but still intermolecular). This is a molecular solid.

Common Examples of Molecular Solids

Grouping them by the dominant intermolecular force helps predict properties.

Nonpolar molecular solids — London dispersion forces only

Noble gas solids (Ne, Ar, Kr, Xe) — only exist at very low temperatures.
Think about it: diatomic halogens: F₂, Cl₂, Br₂, I₂. Hydrogen, nitrogen, oxygen, CO, CO₂, CH₄, C₂H₆, other small hydrocarbons.
Larger hydrocarbons: naphthalene, anthracene, paraffin wax.
Fullerenes: C₆₀, C₇₀.

These are the soft

These are the solids that feel yielding to the touch, melt readily when warmed, and often dissolve in organic solvents. Their lattice energy is modest because the forces holding the discrete molecules together are comparatively weak — typically van der Waals attractions, dipole‑dipole interactions, or, in the case of water ice, directional hydrogen bonds that are still intermolecular in nature. This means molecular solids tend to exhibit low thermal conductivity, are electrical insulators, and fracture rather than flow plastically under stress.

A familiar illustration is table salt’s cousin, sodium chloride, which belongs to the ionic class and therefore does not share these traits; instead, consider solid carbon dioxide (dry ice). At atmospheric pressure it sublimates directly from a crystalline array of discrete CO₂ molecules, demonstrating the weak cohesion characteristic of a molecular lattice. In real terms, another everyday example is paraffin wax, a mixture of long‑chain alkanes whose individual molecules are held together by London dispersion forces, giving the material its characteristic softness and low melting point. Ice, too, fits the profile: each water molecule is a neutral entity linked by hydrogen bonds, forming a crystalline network that is soft enough to be reshaped by pressure yet rigid enough to retain its shape until melting.

Polymers such as polyethylene or nylon also qualify, because the repeating units are covalently bonded into very large molecules, and the solid’s integrity derives from the entanglement of those molecules and the weaker forces between them. In this sense, the polymer chain itself is a “giant molecule,” and the crystal packing of those chains follows the same principles that govern simpler molecular solids.

When evaluating a candidate material, the five diagnostic questions remain the most reliable filter. If the substance is built from neutral, discrete molecular units, lacks any continuous covalent, ionic, or metallic lattice, and is not a metal, alloy, or extended network, then it is unequivocally a molecular solid. The borderline cases — graphite’s layered sheets, polymeric chains, and even certain hydrated oxides — remind us that classification can be nuanced, but the core criteria still apply.

Simply put, molecular solids are defined by their composition (discrete, neutral molecules) and the relatively weak intermolecular forces that bind them together. Their physical behavior — softness, low melting points, poor electrical conductivity, and brittleness — stems directly from this bonding scheme. Recognizing these traits allows chemists and materials scientists to predict how a solid will perform in applications ranging from packaging films to cryogenic storage, and to differentiate it confidently from ionic, metallic, or network solids.