Structure and Bonding

Allotropes of carbon 

Allotropes are each of two or more different forms in which an element can exist.

Carbon has four main allotropes:  diamond, graphite, graphene and buckminsterfullerene. 

Diamond has a giant structure of carbon atoms. The carbon atoms are connected by strong covalent bonds. (A covalent bond is a chemical bon that involves the sharing of electrons pairs between atoms.) Each carbon atom is covalently bonded to four others. Diamond has a very high melting point as it takes a lot of energy to break the covalent bonds; it is also very hard for the same reason. It is a poor conductor of electricity as the outer electrons are all fixed in the covalent bonds.

Graphite is also made of carbon atoms, but they are arranged in a giant structure containing layers of carbon atoms arranged in hexagons. Each carbon atom is covalently bonded to three others.  One electron from each carbon atom is delocalised. The delocalised electrons are free to move and can carry a current. Graphite has a high melting point as within the layers the atoms are connected by strong covalent bonds but it  is soft as the layers have only weak van der Waals forces to hold them together. The layers can slide over one another, which is why graphite in pencils leaves a mark on paper.

Graphene is a one-atom thick sheet of carbon atoms arranged in hexagons.

Buckminsterfullerene is made of  spherical molecules containing 60 carbon atoms. These molecules have strong covalent bonds between the carbon atoms, but there are only weak van der Waals forces between the molecules. The melting point of buckminsterfullerene is therefore much lower than that of diamond or graphite, and it is also much softer than diamond. It is a worse conductor of electricity than graphite. 

Ionic Bonding 

In ionic bonding, there is a strong electrostatic attraction between positive ions and negative ions. These forces of attraction act in all directions. Ionic bonding occurs in compounds formed of non-metals bonding with metals. When a metal atom reacts with a non-metal, electrons in the outer shell of the metal atom are transferred. Metal atoms lose electrons and become positively charged ions whilst non-metals gain electrons and become negatively charged ions.

An ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions in the lattice and this is called ionic bonding. These compounds have a high melting point and high boiling points because of the large amounts of energy needed to break the many strong bonds. When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and so charge can flow.  

Covalent Bonding

In covalent bonding there is a strong electrostatic attraction between a shared pair of electrons and the nuclei of the two atoms in the bond. Covalent bonding occurs in most non-metallic elements and in compounds of non-metals.

Giant covalent structures are solids with very high melting point. All of the atoms in these structures are linked to other atoms by strong covalent bonds. These bonds must be overcome to melt or boil these substances. Diamond and graphite are giant covalent bonds.

In metallic bonding there is a strong electrostatic attraction between the sea of mobile, delocalised outer electrons and the metal cations.  Metallic bonding occurs metallic substances and alloys. The electrons in the outer shell of metal atom are delocalised and so are free to move through the whole structure. The sharing of delocalized electrons gives rise to strong metallic bonds.

Van der Waals (intermolecular) forces are weak forces of attraction between molecules.

Metals have giant structures of atoms with strong metallic bonding. This means that most metals have high melting and boiling points.  In pure metals, atoms are arranged in layers, which allow metals to be bent and shaped. Pure metals are too soft for many uses and so are mixed with other metals to make alloys which are harder.

Structures and properties 

Substances made of a giant structure or lattice of ions such as sodium chloride tend to have high melting points as a lot of energy is needed to pull the ions apart. They tend to be brittle, crystalline solids at room temperature.   They are poor conductors of electricity as solids as the ions are fixed in the lattice and cannot move about. If they are melted or dissolved in water they become good conductors  of electricity although they are also decomposed by the process known as electrolysis.   Ionic compounds are often but not always soluble in water. 

Substances made of simple molecules tend to have low melting points as the weak van der Waals (or intermolecular) forces between the molecules mean that not so much energy is needed to pull them apart. The covalent bonds between the atoms are not broken when the substance melts. They are poor conductors of electricity because the molecules do not have an overall charge, and when solid are usually soft substances. 

Metals tend to have high melting points because a lot of energy is needed to overcome the strong electrostatic attraction between the metal cations and the sea of electrons. Metals are very good conductors of electricity because the sea of mobile, delocalised outer electrons can move when a potential difference is applied.  Metals are malleable because the metal cations can slide over one another and the sea of electrons can flow to re-form the bonds that are broken.