Structure and bonding II

QUICK POINTS

Diamond and graphite are allotropes of carbon, as they are different forms of the same compound in the same state. They show different physical properties.

Graphite consists of a hexagonal arrangement of carbon atoms in layers.

Graphite is soft, as weak intermolecular forces exist between the layers, which can slide over each other. Graphite conducts electricity as only three carbon atoms are bonded, leaving one mobile electron in each layer which can carry an electric charge. Diamond forms a tetrahedral arrangement of carbon atoms. Diamond is hard, as each carbon atom forms four strong bonds. Since all four electrons are involved in bonding, diamond has no free electrons and does not conduct electricity.

Giant molecular structures have a network of strong covalent bonds. This results in high melting and boiling points which require a large amount of energy to separate the atoms.

Graphite and diamond are composed of carbon atoms but their structures are different, hence, these solids are allotropes. They show the same chemical properties since they have the same element, carbon, but the difference in their structure causes them to have different physical properties.

Covalent compounds can have a simple molecular structure or giant covalent (macromolecular) structure.

Simple molecular compounds have very low melting and boiling points because of the weak intermolecular forces between the molecules. Little energy is, therefore, needed to separate them.

Giant ionic crystals have high melting and boiling temperatures due to the strength of the attractive forces between the ions which holds the crystal together in a regular threedimensional framework.

QUESTION 1

Sodium chloride and iodine have very different properties.

1. Suggest some of the properties of sodium chloride and iodine.

2. Explain these properties based on the particle arrangement and the forces between them.

ANSWERS

1. Sodium chloride is an ionic substance.

It is a crystalline solid at room temperature.

It has a high melting point.

It conducts electricity when molten or in aqueous solution. 2. Iodine is a covalent substance.

It is a crystalline solid at room temperature. It has a low melting point.

It does not conduct electricity.

COMMENT: Note that although both sodium chloride and iodine are solids, the differences in their structures lead to different melting points. Sodium chloride has a giant ionic structure, while iodine has a simple molecular structure.

Sodium chloride is composed of ions held together by strong ionic bonds in a crystal lattice structure. The strong attractive forces hold the ions together in the solid structure and are very hard to break. This requires a high melting point to break the bonds. The ions are mobile when molten, causing sodium chloride to conduct electricity.

Iodine is a covalent molecule with a simple molecular structure. Iodine molecules are held together by weak intermolecular forces that are easy to break. This causes iodine to have a low melting point. Iodine has no free electrons in its covalent structure, hence, iodine does not conduct electricity.

COMMENT: The bonds between iodine molecules are so weak that it quickly sublimes when heated. Remember: The bonding and structure of any substance influences its properties. The arrangement of carbon atoms in diamond and graphite is shown in the diagram.

1. Explain why diamond and graphite have very high melting points.

2. State the maximum number of covalent bonds formed by a carbon atom in a diamond crystal.

3. Explain, in terms of its structure, why graphite is able to be used as a lubricant.

4. Explain why graphite conducts electricity but diamond does not.

ANSWERS

1. Both diamond and graphite have a giant molecular structure. The carbon atoms are held together by strong covalent bonds which require high temperatures to break. This causes diamond and graphite to have high melting points.

2. In a diamond crystal, each carbon atom is bonded to four other carbon atoms, forming four covalent bonds.

COMMENT: Carbon has four electrons in its outer shell and can share with four other atoms to achieve stability. All four electrons are bonded in diamond but in graphite, only three electrons from each carbon are involved in bonding.

3. Graphite consists of carbon atoms arranged hexagonally in layers, with weak forces of attraction between layers. This makes it soft, as the layers can slide over each other.

4. Graphite conducts electricity because only three of the outer electrons on carbon are involved in bonding, leaving the fourth electron mobile to carry an electric current. Diamond has all four electrons from carbon involved in bonding, thus, there are no ‘free’ electrons to carry an electric current. Diamond does not conduct electricity.

COMMENT: Diamond and graphite are allotropes and both have giant molecular structures. The differences in their properties are due to the way the carbon atoms are arranged in their structures, with the tetrahedral structure of diamond forming a hard substance, while the hexagonal structure of graphite forms a softer substance.

Francine Taylor-Campbell is an independent contributor. Send feedback to kerry-ann.hepburn@gleanerjm.com.