Solid is the 1st state of matter and also the most studied and used. Anyone entering into materials science engineering the 1st stage of study starts with learning and understanding solids. In my previous article I was briefly introducing the concept of SOLID, what is it, how does it occur and what are it main properties. Let’s shortly recall the 2 main categories of structure in which this state of matter occur. These are:
CRYSTALLINE STRUCTURE: = particles are with a well-organized pattern and shape, such as a 3D structure in which all bonds between particles have equal strength. Therefore the resulted solid has a distinct melting point.
AMORPHOUS STRUCTURE:= particles that have random arrangements, so they lack an organized shape and/or pattern and melt over a range of temperatures.
That’s what happen in an ideal solid structure. In reallity a solid is always a mix of both. It can indeed reach a very high level of ordered pattern (crystalline) such as Diamond which is very much a crystalline structure, yet it is still not 100% crystalline. The same cam be said about dis-ordered structure (amorphous) such as glass (SiO2) which is still not 100% amorphous.
Now, to split into categories even more, we can have 4 other different types of solids, namelly:
- COVALENT NETWORK SOLID,
- IONIC SOLID,
- MOLECULAR SOLID
- METALLIC SOLID
All these can exist both in crystalline and in amorphous version. But because most of them occur with an ordered structure we can put these 4 as subcategories of Crystalline Structure. In this article I’m gonna tell you about the COVALENT NETWORK SOLID.
WHAT IS A COVALENT NETWORK SOLID?
Let’s start with a first example. Have you ever heard of fossilized lightning? Most probably you do. When lightning strikes sand, it rapidly heats it to up to 30.000° C. That’s hotter than the Sun’s surface! This causes the silicon dioxide (SiO2) within the sand to turn into a crude form of glass! This glass is called sand fulgurite or “fossilized lightning” (a much cooler name). So, why does this happen?
This process occurs like that because silicon dioxide is a covalent network solid, which can be either ordered (crystal) (like how it is in sand or quartz) or disordered (amorphous)(like how it is in glass).
A Covalent Network Solid is = a crystal (ordered) or amorphous (non-ordered) solid in which the atoms are held together by covalent bonds in a continuous network.
A covalent bond is = a type of chemical bond where the atoms equally share one or more pairs of valence electrons within the bond. This happens because the atoms are trying to fill their outermost energy level, or valence shell, with a full complement of electrons. The more equally they are shared, the more covalent character the bond has.
In order to understand how this is possible we need to recall what the term Electronegativity is.
Electronegativity is the measurement of how much an atom wants to bond to another atom sharing electrons in a covalent bond.
The concept of electronegativity was 1st introduced by the American chemist Linus Pauling in 1932; His scale known as The Pauling Scale measures the electronegativity of an element, within a scale from 0.7 to 4. Electronegativity increases from left to right and down each column on the periodic table. The higher the value of the electronegativity, the more strongly that element attracts the shared electrons. Fluorine (F) is the most electronegative element, with an electronegativity of 3.98. while Cesium (Cs) is the least electronegative element with an electronegativity of 0.79.
As result based on electronegativity there are 2 types of covalent bonding: polar and non-polar. Such bonds occur between 2 (similar or different) non-metal atoms or between a non-metal and a metalloid with the condition of not having a large difference in electronegativity.
THE POLAR BOND = occurs between 2 or more atoms, if the atoms have significantly different electronegativities (more than 0.4).
Polar bonds do not share electrons equally, meaning the negative charge from the electrons is not evenly distributed in the molecule. This causes a dipole moment. A dipole moment occurs when one end of the bond is positive, and the other end is negative. A classic example of a polar bond is the bond in water between hydrogen and oxygen. The bond is classified as a polar bond because it has a large electronegativity difference of 1.4. Any covalent bond between atoms of different elements is a polar bond, but the degree of polarity varies widely and from here we can have different type of solids as well, most resulting solid are either molecular solids or ionic solids, not covalent network solids.
THE NON-POLAR BOND = share electrons equally. A bond between 2 atoms or more atoms is non-polar if the atoms have the same electronegativity or a difference in electronegativities that is less than 0.4. Covalent Network Solids generally occur as solids with non-polar bonds. A clasic example is between carbon atoms resulting diamond.
Therefore when atoms with an electronegativity difference of less than 2 units are joined together, the bond that is formed is a covalent bond. To be specific, the electronegativity difference between atoms in covalent bonds does not exceed 1.7. As a result of the similar electronegativity between atoms, the atoms easily share electrons.
When the atoms share electrons in this way, they become more stable and less likely to react with other substances. Because of this, there are no individual molecules and each piece of the substance is essentially one huge molecule, as the covalent bonding in the crystal extends throughout the entire crystal. So the entire solid can be considered a macromolecule (fancy word for “big molecule”). If you think of a covalent network solid like a quilt, the unit cells are the patches that repeat across the pattern. Since the unit cell is a “patch” of the entire macromolecule, the entire “quilt” is actually this pattern repeated many times.
EXAMPLES OF COVALENT NETWORK SOLIDS.
As mentioned in the example earlier, the lightning can form glass out of sand, and glasses (amorphous solids) in general are formed when the substance is rapidly heated then cooled. The atom’s initially orderly structure is disrupted, and the rapid cooling prevents atomic ordering from occurring. So the Silicon dioxide as SiO2 (as glass) is one of the most commonly known type of covalent network solid. Silicon dioxide (glass) is an amorphous covalent network solid. (Fig 3 b.)
In it’s crystalling form the silicon dioxide is called quartz, (Fig 3 a) which is the second most abundant material in the earth’s crust. The chemical formula for quartz is SiO2, but this formula only indicates the ratio of silicon to oxygen and is not meant to imply that there are distinct SiO2 molecules present. Since quartz is symmetrical and rigid, while glass is not, it can be subjected to greater temperatures and pressures (i.e. it is stronger). Even though the formula is SiO2, you’ll see that silicon is bonded to three oxygen. And as mentioned previously, there are no individual molecules in a covalent network solid. You can’t isolate a SiO2 molecule because there aren’t any.
Another most common example of CN solid is the 3 different arrangements of carbon, namely: Diamond, graphite and fullerene (Fig. 4 a, b & c). While these 3 materials are made of the same very simple component – just carbon atoms – their appearance and behavior are completely different because of the different types of bonding in the solids. Graphite for instance is an exceptional example, composed of planar sheets of covalent crystals that are held together in layers by noncovalent forces. Unlike typical covalent solids, graphite is very soft and electrically conductive.This ability of a single element to form multiple solids with different crystalline arrangements is called allotropy.
Diamond is a covalent network solid because each C atom from its structure makes 4 covalent bonds to 4 other C atoms. A diamond is essentially one huge molecule. Graphite may also be regarded as a network solid, even though there is no bonding in the z direction. Each layer, however, is an “endless” bonded network of carbon atoms. And the C60 molecule, called buckminsterfullerene, though the shorter name fullerene is often used, is a sphere composed of 6-member and 5-member carbon rings. These balls are sometimes fondly referred to as “Bucky balls”. Fullerenes are an entire class of pure carbon compounds rather than a single compound.
Covalent Network Solids include also crystals of some other nonmetals, and some covalent compounds such as silicon carbide (carborundum, the abrasive on sandpaper). Amethysts, rubies, sapphire and many other minerals have networks of covalent bonds.
PROPERTIES OF COVALENT NETWORK SOLIDS.
Covalent network solids have:
LOW ELECTRICAL CONDUCTIBILITY = generally CN solids are poor conductors of electricity, hence are excellent electrical insulators. However the conductivity of a network solid is dependent on the type of bonding. For instance, molecules that have sheets held together also by intermolecular forces (have delocalized electrons), like graphite or mica, have high electrical conductivity. This is because electricity can “flow” across these delocalized electrons. On the other hand, molecules that are only covalent bonded (do not have delocalized electrons), like diamond or quartz, have low conductivity. This is because all the electrons are held in place by the covalent bonds, so there is no “room” for the movement of electrons. Graphite is black because it contains an immense number of alternating double bonds, which results in a very small energy difference between the individual molecular orbitals. Thus light of virtually all wavelengths is absorbed. Diamond, on the other hand, is colorless when pure because it has no delocalized electrons.
LOW TERMAL CONDUCTIVITY = Most CN solids are poor heat conductors, although their ability to conduct heat is variable. Diamond is an exception and is one of the most thermally conductive substances known, while SiO2 (as Quartz) is about 100 times less thermally conductive.
HIGH HARDNESS = Most covalent network solids are very hard, as exemplified by diamond, which is the hardest known substance, it can easily withstand a pressure of 6 million atmospheres. Lonsdaleite as rare form of diamond, is the hardest solid known, almost 60% harder than normal diamonds.
Fine watches and, increasingly, other electronic devices use sapphire crystalls instead of glass because the strong network bonding makes sapphire incredibly hard (in fact saphire, it is the 3rd hardest substance known) and scratch-resistant.
BRITTLE = Covalent Network Solids are also known to be extremely difficult to break.This is because, all the electrons are engaged in covalent bonds between atoms, thus rendering them immobile and unable to move! All compounds with the diamond and related structures are hard and not easily deformed, instead, they tend to shatter when subjected to large stresses. However, deformations that don’t require breaking of these bonds are easier to make, such as sliding sheets of graphite (this disrupts only the intermolecular forces, not the bonds).
HIGH MELTING POINTS = by virtue of their strong network of covalent bonds through the sample which are very difficult to break and transform the solid into liquid, this category of solids are among the highest-melting substances known. The melting point of diamond is over 3.500°C, while the melting point of SiO2 is around 1,650°C. Silicon carbide (SiC) is also a CN solid with a melting point = 2986°C and used commercially as an abrasive in sandpaper and grinding wheels. It is difficult to deform or melt these and related compounds because strong covalent (C–C or Si–Si) or non-polar covalent (Si–C or Si–O) bonds must be broken, which requires a large input of energy.
INSOLUBLE = Also due to very strong covalent bonds CN solids are generally insoluble in any solvent. When species dissolve, the solute particles (dissolving species) must fit in between the solvent particles (species that does the dissolving). Because the macromolecules in CN solids are so large, this makes them difficult to dissolve. Diamond rings probably wouldn’t be as valuable if the band and the stone dissolved in the shower.
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