Pure diamond is a colorless, transparent, Regular octahedron shaped solid; Graphite is a dark gray, metallic luster, and opaque fine scale like solid. Graphite is very soft and can leave traces when drawn on paper, giving it a slippery feeling. Diamond and graphite have vastly different "shapes" or "properties", but they share the same "quality". They are a pair of "twin" brothers, both composed of carbon elements and known as "allotropes".
Diamond and graphite are twin brothers with completely different personalities. The graphite has a dark surface and a soft texture that breaks easily. On the contrary, diamond has a brilliant and dazzling appearance, making it the hardest substance in the world. Diamond can be used to cut glass and make drill bits because of its high hardness; Graphite can be used as a pencil lead and lubricant because it is very soft and has a slippery feeling. According to the Mohs scale standard, diamond is the highest grade.
Why is the difference between diamond and graphite so significant?
The reason is that their own structures are different. In graphite and diamond, the binding and arrangement of carbon atoms vary. In the chemical world, if an element has several different crystal structures, this phenomenon is called "allotropy". Due to different crystal structures, its physical or chemical properties may also vary, such as having different colors, densities, hardness, solubility, and different abilities in chemical reactions.
The structure of graphite is arranged in layers, like a thousand layers of cake, stacked layer by layer. In terms of a single layer, carbon is connected in a hexagonal shape into a flat network, and each carbon atom is tightly bound to its surrounding three carbon atoms through strong interactions. The distance between the "tightly bound" two carbon atoms is about 0.142nm (the length of each side of the hexagonal shape). The binding force between carbon atoms is formed by pairs of electrons (shared electrons) that contribute to each other. The shared electron pair binds two carbon atoms firmly. We call the binding force formed by the shared electron pair "Covalent bond".
The distance between the parallel layers is 0.335nm, almost twice the distance between the two carbon atoms in each plane layer, which is why the binding force between layers is weak. The binding force between layers is formed by each carbon atom providing an electron to move freely on each plane layer. These moving electrons do not connect the carbon atoms firmly and are easy to disperse. Therefore, this binding force is also called "Metallic bonding". Therefore, it is easy to slide and crack in a direction parallel to the layer, making graphite very soft and slippery.
Due to the free movement of electrons between layers, graphite can conduct electricity. Therefore, the biggest feature of the structure of graphite itself is that it is composed of a double bond type of strong Covalent bond and weak Metallic bonding. In this way, it makes it easy for the layers of graphite to slide and even break, making the texture of graphite very soft. However, due to the strong binding force between carbon atoms on the same plane layer (due to Covalent bond), it is extremely difficult to destroy, so the melting point of graphite is higher than that of diamond, and its chemical properties are more stable. The crystal structure of graphite also determines its physical properties.
Structural Model of Graphite
For a single crystal, graphite can be seen as a two-dimensional metal. That is to say, there are two different binding forces in graphite crystals, which result in differences in the conductivity and thermal conductivity of the crystal layers in the parallel and vertical directions. This phenomenon is called "anisotropy". According to research, the numerical ratio of its performance in two directions is 3:1-4:1. However, due to the chaotic crystal distribution of general graphite, overall, it does not show significant directionality.
Diamond is different. There are four carbon atoms in the crystal cell of each diamond (the smallest unit of diamond, just like the cell of biological tissue). Each carbon atom is distributed on the four top corners of the Regular tetrahedron, and the carbon atoms are connected with each other by a solid Covalent bond. Many cells of the same size and shape are Tight junction connected together in a regular way. And in diamond crystals, each carbon atom is tightly bound to its neighboring four covalent carbon atoms through strong interactions. The distance between the two tightly bound carbon atoms is approximately 0.155nm, forming a dense three-dimensional structure. The carbon atoms arranged in this way have high Binding energy. Because the distance between the carbon atoms is equal, the diamond crystal has a structure without gaps to multiply, and has the highest mechanical strength, thus making the diamond the hardest material in nature.
Structural Model of Diamond
In addition, compared to graphite, diamond has a denser structure. The density of diamond is about 1.5 times that of graphite (the density of diamond is about 3.5-3.53g/cm) ³, The density of graphite is approximately 2.09-2.23g/cm ³), It can be seen that carbon atoms are much denser in diamond than in graphite. From the above introduction, we can clearly understand that the hardness of a substance depends on the bonding direction and energy intensity between its atoms.
Compared to the conductivity of graphite, diamond is not conductive because each carbon atom in diamond interacts strongly with its surrounding four carbon atoms through a "shared electron pair". Almost all four electrons in the outermost layer of carbon atoms participate in the formation of this "force", with almost no free electrons, so it is basically not conductive.
The melting point of graphite mentioned earlier is higher than that of diamond, but their boiling points are the same. Why is this? The changes in the state of matter are related to the distance between particles. After the melting of graphite and diamond, the original arrangement of carbon atoms is broken, and the distance between atoms is equal, and the intensity of interaction is also the same, so the boiling point is the same.
So, looking at the essence through phenomena, structure determines properties. Diamond and graphite are both composed of carbon atoms, but due to the different arrangement of atoms, many physical properties differ greatly.
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