Atoms, elements and compounds - Giant covalent structures (3)
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1.
Graphite and diamond are allotropes of carbon with distinct structures and bonding. Explain how the structures and bonding of graphite contribute to its use as a lubricant and an electrode.
Graphite consists of layers of carbon atoms arranged in hexagonal rings. Within each layer, carbon atoms are covalently bonded to three other carbon atoms, leaving one electron delocalised within the layer. These delocalised electrons allow for easy movement of electrons within the layers, making graphite soft and slippery. This property makes it an excellent lubricant, reducing friction between moving parts.
As an electrode, graphite's ability to conduct electricity is due to the delocalised electrons. These electrons are free to move throughout the structure, facilitating the flow of electric current. This makes graphite a suitable material for electrodes in electrochemical processes and electric motors.
Key points:
- Lubricant: Layer structure allows layers to slide easily over each other.
- Electrode: Delocalised electrons facilitate electron flow.
2.
Draw a diagram to show the giant covalent structure of silicon(IV) oxide ($SiO_2$). Label the atoms and the covalent bonds.
Diagram: (A simple diagram would be inserted here. Since I can't draw, I'll describe it. Imagine a large network of silicon and oxygen atoms. Each silicon atom is at the centre of a tetrahedron, with four oxygen atoms surrounding it. Each oxygen atom is bonded to the silicon atom and also to two other oxygen atoms. This continues throughout the structure, forming a continuous network. Arrows should clearly indicate the covalent bonds between the atoms. Label the silicon atoms as 'Si' and the oxygen atoms as 'O'. Label the bonds as 'covalent bonds'.
The structure is a continuous network, extending throughout the entire crystal. This is a key feature of giant covalent structures.
3.
Describe the arrangement of atoms in diamond and silicon(IV) oxide. Include the type of bond present and the overall shape of the structure. Explain how the type of bond contributes to the properties of each material.
Diamond: Diamond consists of carbon atoms arranged in a tetrahedral structure. Each carbon atom is covalently bonded to four other carbon atoms. This forms a rigid, three-dimensional network. The bonds are covalent, meaning the atoms share electrons. The strong covalent bonds between the carbon atoms are responsible for diamond's exceptional hardness and high melting point. The rigidity of the tetrahedral structure also contributes to its transparency and high refractive index.
Silicon(IV) Oxide (SiO2): Silicon(IV) oxide has a more complex structure than diamond, but it is also a giant covalent network. Each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is covalently bonded to two silicon atoms. This results in a three-dimensional network structure. The bonds are again covalent. The strong covalent bonds between silicon and oxygen atoms contribute to SiO2's hardness and high melting point. However, the structure is not as rigidly defined as diamond's, which is why SiO2 is more brittle than diamond. The covalent bonds also contribute to its chemical inertness.
The covalent bonds in both materials are the key factor determining their properties. The strong, directional nature of covalent bonds leads to the formation of rigid, three-dimensional networks, resulting in high hardness and high melting points. The specific arrangement of atoms (tetrahedral in diamond, more complex in SiO2) influences the overall properties, such as transparency and brittleness.