Do Now: Explain the following in terms of Structure & Bonding. Solid sodium chloride does not conduct electricity, but when it is melted, sodium chloride is a good conductor of electricity

Sodium Chloride is an ionic substance, composed of a 3D lattice of alternating positive Na + ions and negative Cl - ions held together by strong electrostatic attractions between the ions (ionic bonds). Sodium chloride conducts electricity when in the liquid state, because when molten, the charged particles (ions) are free to move and so carry a current. Whereas when it is in a solid state, the ions are not free to move so does not conduct electricity.

Kaupapa / Learning Outcome At the end of this lesson I will be able to explain the properties of molecular and covalent network solids in terms of their structure and bonding.

Molecular Solids Eg. Sulfur Low melting point Does not conduct electricity

Molecular solids Molecular solids are made of molecules There are strong bonds within the molecules (strong intra-molecular forces) Weak attractions between the molecules (weak inter – molecular forces)

Melting Point Molecular solids have low melting and boiling points – Because the molecules are held together by weak intermolecular forces – Which means little energy is required to break the bonds – The molecules themselves are not broken up – just moved apart from each other when molten

Conductivity Molecular solids do not contain charged particles – Therefore do not conduct electricity

Solubility Depends on the polarity of the molecular solid and polarity of solvent. Polar molecules dissolve in polar solvents and non-polar molecules dissolve in non- polar solvents. – ‘like’ dissolves ‘like’

Covalent Network Eg. Diamond, Silica and graphite Very high melting point Most do not conduct electricity

Giant covalent network solids Giant covalent network solids are made of large networks of atoms arranged in covalent networks with strong covalent bonds between them – Linear chains – 2D layers – 3D networks

Linear chains ‘infinitely’ long one-dimensional chains e.g. polyethene

2-dimentional layers Graphite an allotrope of carbon made up of sheets of covalently bonded carbon atoms in a sheet, each carbon atom is bonded to three others in linked six-carbon rings weak intermolecular forces hold the sheets together each carbon atom uses three of its four valence electrons to form covalent bonds the remaining electron is in another type of covalent bond and is free to move (delocalised) between the layers

Melting high melting point (3600 o C) due to the strong covalent bonds within the layers

Electrical conductivity the only non-metallic element, and the only covalent substance that conducts electricity – due to the delocalised electrons which are free to move through the network

Solubility does not dissolve in water due to the strong covalent bonds

3-dimensional networks Diamond allotrope of carbon which exists as a 3-dimensional network in diamond, each carbon atom is bonded covalently to four other carbon atoms these four carbon atoms are in a tetrahedral arrangement

Silica SiO 2 has a similar tetrahedral structure to diamond each silicon atom is covalently bonded to four oxygen atoms

Other Properties 3-dimensioanl network solids have very high melting points due to the strong covalent bonds – crystals of diamond vaporize at 5000 o C do not conduct electricity as there are no charged particles do not dissolve in water due to the strong covalent bonds

Buckminsterfullerenes fullerenes (‘bucky balls’) are a third form(allotrope) of carbon (discovered in 1985) consist of molecules containing covalently bonded carbon atoms arranged in a closed cage shape (named after Richard Buckminster-Fuller, the architect who first designed a geodesic dome) C60 is round and the bonds form a pattern like a soccer ball (others are oval shaped) they are not giant covalent structures and are classed as simple molecules moderate melting points do dissolve in solvents do not conduct electricity (however act as superconductors if metal atoms are added to them)