Solid Definition

Constitution in which one of the three states of matter occurs, which can be perceived by its rigidity, differentiating itself from liquid and gaseous. When the structure is ordered and the particles have determined positions, the solid is said to be crystalline, while if the atoms, ions or molecules that form it are randomly ordered, the solid will be amorphous (without shape).

With types of solids we refer to the structure that a solid adopts at the molecular level based on its macroscopic properties, and within this framework there is a general classification: crystalline and amorphous. According to this classification it is possible to characterize them. For example, crystalline solids have exact melting temperatures and flat faces and sharp angles on their edges are observed in their morphology. Thanks to this, it is possible to represent them in three-dimensional matrices called crystal lattices. As examples of crystalline solids we can name table salt (NaCl) and diamond.

Meanwhile, amorphous solids, therefore, do not have defined faces and their morphology is given by the molding or cutting that is carried out during their manufacture. Furthermore, amorphous solids do not possess precise melting points, however they soften within a range. Some castings of solids lead to the formation of liquid crystals whereby typical properties of both liquids and solids are present simultaneously and this is very useful in different industrial applications. The concept of an amorphous solid is often related to partially crystalline solids, some examples of which are plastic materials such as polypropylene, nylon and glass (SiO2). It should be noted that Silicon Oxide can also form crystalline networks, quartz.

Likewise, within crystalline solids we can make a subdivision based on the type of bond they present, which is why they are: molecular, covalent, ionic or metallic solids. Next, we will see each of them and their main properties.

Molecular Solids

Examples of molecular solids are water and carbon dioxide. In these cases, the binding force that predominates are intermolecular forces such as: Dispersion or London Forces, Dipole-Dipole Forces and/or Hydrogen Bonds.

Here the crystal lattice is made up of molecules and, therefore, they are soft, have low melting points (compared to other solids that have stronger bonding forces), and are poor conductors of heat and electricity.

In general, they are gases or liquids at room temperature and at low enough temperatures they form molecular solids.

Covalent Solids

In the case of covalent solids, networks of atoms are formed whose bond strength is the covalent bond. Typical examples we can mention Diamond, Quartz and Graphite.

In general, as main characteristics we can mention that they are very hard and poor thermal conductors (although some exceptions can be found).

Likewise, we find covalent solids with high melting points (which vary from 1200ºC to 1400ºC).

Ionic Solids

Ionic solids form lattices whose particles in the crystalline cell are anions and cations. The attractive force that holds the particles together in the lattice are electrostatic forces (between positive and negative ions). In general, it can be mentioned that they have high melting points thanks to their reticular binding energy while they are poor conductors of heat and electricity. However, when dissolved in water (they are soluble in it) or when melted, they are electrolytes capable of conducting electricity, since in the solid state they occupy fixed positions in the network and cannot move.

In addition, they are considered hard and brittle solids.

Examples of this type of solid are: table salt (NaCl) and other salts such as AgCl, K2SO4.

Metallic Solids

Metals form crystal structures where the lattice particles are cations held together by a cloud of electrons. Then, the three-dimensional network formed by positive ions of the metal in question forms a compact structure that is surrounded by the electronic sea.

All metals are in the solid state (except Mercury) and are known to be malleable, ductile, and to have high thermal conductivity (good conductors of both heat and electricity).

The predominant bonding force is metallic bonding, a type of electrostatic force between cations and electrons.

Examples of them are the metals that we usually know: Iron, Aluminum, Platinum, among others.

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