Q.Why are solids rigid?

In a solid the constituent particles are held together by strong intermolecular/interatomic forces in fixed positions. They can only oscillate about their mean positions and cannot move past one another, so the solid keeps its shape and resists deformation.

Step 1 – Nature of forces in a solid.

In the solid state the constituent particles (atoms, ions or molecules) are packed close together and held by very strong intermolecular (or interatomic/interionic) attractive forces.

Step 2 – Restricted motion.

Because these forces are strong and the particles occupy fixed positions in the lattice, they cannot translate; they only vibrate (oscillate) about their mean positions.

Step 3 – Consequence.

Since the particles cannot slide past one another, the solid maintains its shape and resists any change of form — this is what we call rigidity.

Argue from the kinetic-molecular picture: strong interparticle forces plus fixed lattice positions restrict motion to vibration only, which prevents deformation — hence rigidity.

• Saying particles are "completely motionless" — they still vibrate about mean positions.
• Confusing rigidity (fixed shape) with incompressibility (definite volume) and not mentioning the strong forces / fixed positions.
• Attributing rigidity to close packing alone without citing the strong attractive forces.