1. 1.3 The Importance of Chemical Bonding Covalent Networks National 5 Chemistry

2. Learning Intentions Covalent Networks Pupils should be able to: 1.State that covalent networks have an unlimited number of atoms held by covalent bonds. 2.State that the five most common covalent molecules are carbon, silicon, boron, silicon oxide and silicon carbide.

3. Covalent Networks A covalent bond is the electrostatic attraction between the shared pair of electrons to both nuclei. Covalent substances that we have previously described are known as discrete molecular or covalent molecular. A few covalent substances have a different structure where they bond to one another in an enormous molecule. This is known as a covalent network and is made up of a large number of atoms that is impossible to count. There are only 3 covalent network elements: Boron, Carbon and Silicon

4. Carbon has four electrons in its outer electron level. To achieve a stable electron arrangement each atom requires four more electrons. In reality the atoms are arranged in a tetrahedral around a central atom. In this way a giant network s built up whereby all the atoms have filled stable electron arrangements except those on the outside surface. Diamond is a Covalent Network

5. Covalent Networks A covalent network structure is a vast 3-dimensional arrangement of atoms strongly bonded together by sharing electrons From the previous slide showing the structure of diamond, all but the _________ carbon atoms _________ their outer ___________ with _______ other carbons such that all possess stable ________ gas electron _______________. The resulting network of strong ____________ bonds, represented by ___ in the diagram, shows regularly ____________ carbon tetrahedra and, accordingly is described as a ____________. outer shareelectrons4 noble arrangements covalent repeating network

6. Two covalent network compounds are silicon dioxide and silicon carbide Covalent Network Compounds Silica (Silicon Dioxide) For example, in the covalent network structure of silica the ratio of silicon atoms to oxygen atoms is 1:2, so the formula is written as SiO 2. In the case of diamond there is only one type of atom present, namely carbon, so its formula is just C. Silicon dioxide is very hard. It has a very high melting point (1,610 °C) and boiling point (2,230 °C), is insoluble in water, and does not conduct electricity.

7. Silicon Carbide Grains of silicon carbide can be bonded together by sintering to form very hard ceramics that are widely used in applications requiring high endurance, such as car brakes, car clutches and ceramic plates in bulletproof vests.

8. Properties of Covalent Substances Discrete Covalent MoleculesCovalent Molecular Networks Usually exist as liquids and gases at room temperature e.g. water, the diatomic elements Only exist as solids at room temperature e.g. diamond Low m.p and b.pHigh m.p Strong covalent bonds within the molecules Weak molecular attractions between the molecules Strong covalent bonds throughout