Sodium chloride is a typical ionic compound. Compounds like Sodium Chloride are made from a giant, endlessly repeating lattice of ions. Ionic compounds are described as having a giant ionic structure. This doesn’t mean that Sodium Chloride is a very large molecule, just that it is impossible to state exactly how many ions are present.
The sodium ions and chloride ions in Sodium Chloride alternate with each other in each of the three dimensions. The sodium ion forms the centre of the molecule and is bonded to 6 chloride ions. This is why Sodium Chloride is said to be 6:6 co-ordinated.
Metals
Metals are created by ions held together by metallic bonds to form a giant structure. This means that large but variable numbers of ions are involved. This giant structure is a three dimensional lattice where the metal ions are closely packed and are surrounded by delocalised electrons that are free to move through the structure. A metal bond is the electrostatic attraction that neighbouring cations have for the delocalised electrons between them.
These delocalised electrons are the reason why metals conduct electricity. The electrons are able to carry charge without disrupting the metal bonding between metal atoms. This is why Sodium is a conductor of electricity whereas Sodium Chloride cannot as there are no free electrons.
Metals have high melting and boiling points because of how strong the metallic bonds are. The strength of the bond is different for different metals and depends on the number of delocalised electrons between them and on how the atoms are structured.
For example Group 1 metals such as Sodium have quite low melting and boiling points because each atom can only offer one outer electron to become delocalised and for the metallic bond. Furthermore Group 1 atoms are relatively large atoms meaning that their nucleus is further away from the delocalised electron meaning the electrostatic attraction isn’t as great. Also Group 1 elements are not structured efficiently ( they have 8 co-ordinate bonds compared to 12 in most metals) this means that they are not able to form as many bonds meaning the heat energy doesn’t have to break as many bonds.
When a large stress is applied the layers of atoms “roll” over one another. Grain boundaries hinder this as the rows of atoms now do not line up properly meaning it is harder for the layers to “roll” over one another. Therefore the more grain boundaries a structure has the harder the metal is. However at grain boundaries the atoms are not in contact properly meaning the metallic bonds are not as strong. This means that if you make the metal stronger by increasing the number of grain boundaries you also make it more brittle.
Diamond
Diamonds are created by carbon atoms sharing electrons with 4 other carbon atoms forming 4 single covalent bonds. This structure is a giant covalent structure and continues on in the three dimensions. It is a symmetrical tetrahedral lattice.
However it is not a molecule as the number of atoms joined up in a real diamonds is completely random and depends on the size of the crystal.
Diamond has a high melting point because the very strong carbon-carbon covalent bonds have to be broken throughout the structure before melting can occur. The strong carbon-carbon covalent bonds are also the reason that diamonds are extremely hard.
These bonds use all the electrons and hold them tightly between the atoms meaning none are free to carry charge meaning that diamonds are not electrical conductors
Graphite
Graphite has a layer structure which is formed due to 3 carbon-carbon covalent bonds forming between close carbon atoms. Because carbon has four outer electrons the 4th spare electron becomes delocalised over the whole sheet of the atoms that for a single layer. The electrons form temporary dipoles as they move throughout the layers. These dipoles cause opposite dipoles in the sheets above and below and carry on through the rest of the structure.
The carbon atoms are arranged in a trigonal planar structure. This causes the layers to form regular hexagons. The distance between carbon atoms within a layer is less than the distance between layers. Between the layers are weak van der Waal’s forces this means that the layer structure is easy to break, which is why Graphite is used in pencils as it is easy to sharpen and marks the paper like lead.
Graphite has a high melting point and boiling point as the covalent bonds have to be broken throughout the structure as breaking the bonds between layers is not enough. The delocalised electrons within the layers mean that electricity can flow through Graphite. If a piece of graphite is attached into a circuit electrons can “fall off” on end of the sheet and be replaced with new ones at the other end.
