UNIT ONE MODULE ONE

CHEMISTRY OF THE ELEMENTS

OBJECTIVES

• Explain the variations in physical properties of the elements in terms of strucrture and bonding.
• Explain the reactions of the tetrachlorides with water.
• Discuss the trends in:
  • bonding
  • acid/base character
  • thermal stability of the oxides of oxidation states II and IV.
• Discuss the relative stabilities of the oxides and aqueous cations of the elements in their higher and lower oxidation states.
• Discuss the uses of ceramics based on silicon (IV) oxide.

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Metals are good conductors of electricity with atomic conductance (atomic electrical conductivity) greater than 10^-3 ohm^-1 cm^-1. Their conductivity slowly falls as the temperature rises.

Metalloids are poor conductors of electricity with some atomic conductance usually less than 10^-3 but greater than 10^-5 ohm^-1 cm^-1. The conductivity of the metalloids increase as the temperature rises. It is also considerably affected by the presence of impurities.

Non metals are usually or virtually non conductors. Their atomic conductance is usually less than 10^-10 ohm^-1 cm^-1.

The element carbon exists in the form of allotropes. Graphite is a poor conductor of electricity and would be classified as a metalloid. Diamond, however, is an insulator and is therefore classified as a non metal.

Carbon and silicon are non metals and give acidic oxides. Those of germanium, tin and lead are amphoteric. Although some lead oxides are definitely basic, covalency dominates with carbon and silicon and then ionic with tin and lead.

The most striking feature of the compounds of the group IV elements is the existence of two oxidation states, +2 and +4. The relative stabilities of the +2 and +4 oxidation states vary. In carbon and silicon compounds, the +4 is very stable relative to +2. Germanium forms oxides in both +4 and +2 states. However, GeO₂ is rather more stable than GeO. GeO₂ does not act as an oxidizing agent and GeO is readily converted to GeO₂.

In tin compounds, the +4 is only slight more stable than the +2 state. Thus aqueous tin (II) ions are mild reducing agents. They will convert Mercury (II) ions to mercury and iodine to iodide.

CERAMICS

The word ceramics literally means heat resistant and traditional ceramic materials are ones such as porcelain which are fired during manufacture. Hardened by heat are the best known ceramics and are based on clay, such as pottery.

Clays are found naturally and contain a number of minerals such as kaolinite, Al₂Si₂O₅(OH)₄, an alumino – silicate. Clay contains crystals. When water is added it acts as a lubricant, allowing the crystals to slide over one another. This makes the clay easy to shape. If the water is gently dried, the shaped articles become hard, but if more water is added the clay becomes moldable again. If the dry clay is then heated to a temperature of around 1000 ^oC (fired), chemical changes occur and a glass is formed, which glues’ the clay crystals together. These chemical changes are not reversible and the fired ceramic article can never be remolded. It will neither melt nor will it react with oxygen in air, as it is already an oxide.

Ceramics are good insulators of both heat and electricity and are brittle. This last property is because they have giant structures which are either covalently or ionically bonded.