It is said that solid things are made of atoms; legendary tiniest, most basic, physical things which can exist on their own, not just as a part of some other thing. It is said that atoms bind together in varying ways to form molecules. Molecules, in turn, it is said, bind together to form substances. In solid substances, molecules are said to often bind so tightly that they can barely move – though they may vibrate and rotate.
Solid things have a definite shape. Solids take up a specific amount of three-dimensional space – they have a definite volume.
According to legend, solids may be made up of 3d blocks made up of atoms, ions, or molecules bound together in a specific pattern; each block being, if pure, identical. These types of solids are said to be crystalline. (ions = atoms or molecules with a positive or negative charge)
According to lore, other solids are made up of atoms, ions, or molecules bound together without much of a particular pattern. These types of solids are said to be amorphous.
Those 3d blocks made up of atoms, ions, or molecules bound together in a specific pattern, of which crystalline solids are said to be formed, are called unit cells. Unit cells are said to come in different types.
According to legend, in 1850, Auguste Bravais created 7 categories of unit cell: cubic, tetragonal, monoclinic, orthorhombic, rhombohedral, hexagonal, and triclinic. According to lore, he created these categories based on differences in angles and in dimension.
It is said that Bravais also defined 14 different unit cell types: simple cubic, body-centered cubic, face-centered cubic / cubic closest packing, simple tetragonal, body-centered tetragonal, simple monoclinic, end-centered monoclinic, simple orthorhombic, body-centered orthorhombic, face-centered orthorhombic, end-centered orthorhombic, rhombohedral, hexagonal, and triclinic.
It seems that Bravais thought of a unit cell as the simplest repeating unit in a crystal and required that it’s opposite faces be parallel, and that it’s edges connect to equivalent points to be worthy of the name.
Unit cells are said to connect with each other by lattice points. In 2d / on a plane, lattice points seem thought to connect in either square arrays, or close-packed arrays.
According to legend, crystalline solids come in different types: ionic, molecular, covalent-network, and metallic.
Ionic solids are said to be bound by electrostatic attractions – bound by positive ions and negative ions attracted to each other. Ionic solids tend to be hard, to be brittle, and to have high to very high melting points. They are also said to be poor conductors of heat and electricity in solid form, yet they may conduct well as liquids. Salt is said to be an ionic solid.
According to lore, lattice points within ionic solids tend to be of larger ions; smaller ions are said to fit between. Spaces between lattice points in square array ionic compounds may be cubic or octahedral. Spaces between lattice points in close-packed array structures may be tetrahedral or octahedral.
Molecular solids are said to be bound by weaker forces such as hydrogen bonds, London dispersion forces, or dipole-dipole forces. Each are said to be based on attraction, perhaps temporary, perhaps only because molecules are touching, or almost touching, in just the right way.
Small, nonpolar (relatively neutral) molecules seem thought to provide the weakest attractive forces, lowering melting points. Larger nonpolar molecules seem thought to offer larger attractive forces, raising melting points. Polar molecules seem thought to provide a higher melting point than any of the nonpolar. Molecular solids seem to melt at relatively low temperatures, overall, regardless.
Molecular solids seem to vary in hardness. Molecular solids seem to vary in brittleness. They seem often thought to be flexible, however.
Molecular solids tend to be poor conductors.
Sucrose is said to be a molecular solid.
Covalent-network / Atomic solids are said to be bound by covalence; the sharing of electrons (negatively charged subatomic particles) between atoms. Covalent-network solids are said to formed by electronegative elements – elements which attract shared electrons in a covalent bond. Covalent-network solids are said to tend to be very hard. They are said to tend to have very high melting points. They are said to tend to be poor conductors. Diamond, and graphite, are said to be covalent-network solids.
Metallic solids are said to be metal atoms bound metallically. A metallic bond is said to form between positively charged ions / cations which share electrons throughout a lattice. According to legend, metallic solids consist of atoms of electropositive elements – elements which tend to lose electrons, and thus increase their positivity.
Metallic solids tend to be shiny. They tend to be malleable – they tend not to break or crack when being reshaped. They tend to be ductile – they may be drawn out to form wire. Though they vary in melting temperature, they tend to have high melting points. They vary in hardness. Metallic solids tend to be good conductors.
Crystalline solids may contain defects. Defects may alter a substance’s physical properties, such as electrical conductivity. Noted defects in crystalline solids include: vacancies, interstitial impurities, and substitution impurities.
Vacancies occur when a point in a pattern which should have atoms or ions doesn’t.
Interstitial impurities occur when atoms or ions are positioned at an off-pattern point.
Substitution impurities occur when off-pattern atoms or ions are included in the substance.
Substitution impurities, differing in size, tend to cause distortions.
Crystalline solids are said to likely melt all at once – to melt when they’ve been heated to a specific temperature.
As amorphous solids are said to be made up of atoms, ions, or molecules bound together without much of a particular pattern, it seems to be thought that this is why they tend not to melt all at once – they tend to melt, it seems, instead, slowly, over a range of temperatures.
Amorphous solids may derive from liquids which freeze before their molecules manage to arrange themselves in a thoroughly orderly pattern. They may often form from substances which consist of large molecules, or a mixture of molecules with more restricted movement.
Glass, plastics, and candle waxes are each said to be amorphous.