A solid is a state of matter in which atoms or molecules are tightly bound together by powerful forces thereby creating a rigid body (with a defined geometric shape and volume).

The formation of a solid occurs with the establishment of bond forces between the atoms, of such intensity as to overcome the energy of thermal agitation. These bonding forces are five, of which three are strong (ionic, covalent, and metallic bond) and two weak (hydrogen bond and Van der Waals molecular forces). All these forces are attractive until the distance between the atoms is slightly less than the diameter of the atoms themselves, and they become violently repulsive for distances between the lower atoms; for this reason, the solids are difficult to compress.

Categories and classification of solids

In most cases, the atoms bind themselves according to a crystal lattice or amorphously, without the possibility of moving in space, except for vibration motions (caused by thermal energy). Most of the substances existing in nature are found in the solid crystalline state. Solids that do not have a crystalline structure are called amorphous solids.

The differences between the types of solid derive from the differences between their bonds; consequently, we can classify the solids into two main categories:

  1. crystalline solids (ionic solid, molecular solid, covalent network solids, and metallic solid);
  2. amorphous solids (such as common glass) based on how the particles are arranged.

Physical properties of solids


Mechanical properties of solids include:

  • elasticity;
  • plasticity;
  • viscoelasticity;
  • tensile strength;
  • compression strength;
  • shear strength;
  • fracture toughness;
  • ductility (low in brittle materials);
  • indentation hardness.


Thermal properties of solids include thermal conductivity, which is the property of a material that indicates its ability to conduct heat. Solids also have a specific heat capacity, which is the capacity of a material to store energy in the form of heat (or thermal lattice vibrations).

Thermal volume expansion of solids. When a certain amount of heat is administered to a solid, this effect affects a volume expansion (i.e., three-dimensional), due to the thermal effect of the temperature increase. Vice versa, if heat is removed, the solid will shrink due to the decrease in temperature.

The expansion is not always uniform in all dimensions, but some may be more evident than others depending on the geometric shape of the solid. For example, a beam will undergo a more evident longitudinal expansion (along with its prevailing dimension: i.e., the length) rather than transversely (i.e., in thickness).

The physical law that regulates this thermal expansion is the following:

where: V is the final volume, V0 is the initial volume, α is the coefficient of volume expansion of the body, ΔT is the temperature variation during the expansion process.


Electrical properties of solids include:

Electrical conductors such as metals and alloys are contrasted with electrical insulators such as glasses and ceramics. Semiconductors behave somewhere in between. Whereas electrons cause conductivity in metals, both electrons and holes contribute to the current in semiconductors. Alternatively, ions support electric current in ionic conductors.


Solid materials can transmit or reflect visible light (many materials can transmit some wavelengths while blocking others). This property is used for frequency-selective optical filters, which can alter the color of the incident light.

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