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Chemguide: Support for CIE A level Chemistry Learning outcome 4(e) This statement looks at the lattice structures of a number of solid crystals with various sorts of bonding. Before you go on, you should find and read the statement in your copy of the syllabus. The links below will lead you to more information than this statement asks for. You will find that as well as the actual structures, there is an explanation of why the structures lead on to some of the important physical properties of the substances. You will need this sort of information for a later statement in this section - so don't miss it out! Statement 4(e)(i) is about ionic structures such as sodium chloride and magnesium oxide. You will find this explained on the page about ionic structures. You can ignore the section about caesium chloride. You will find magnesium oxide mentioned in passing - all you need to know about it is that it has exactly the same crystal structure as sodium chloride, but that the forces holding the ions together are greater because 2+ is attracting 2-, rather than 1+ attracting 1- in the sodium chloride case. Statement 4(e)(ii) is about simple molecular crystal structures such as iodine. You will find this on the page about molecular structures. Read down as far as the structure of iodine, but leave the bit about ice for the moment. That comes in a later statement (4(e)(iv)). Fortunately, learning outcome 4(e) says that you don't need to know about the concept of the unit cell. I take that to mean that you will not be asked for the details of the way that the iodine molecules pack together. It is probably enough to know that the crystal of iodine consists of iodine molecules packed closely together, and held in place by relatively weak van der Waals forces. Statement 4(e)(iii) is about giant molecular crystal structures such as silicon(IV) oxide (silicon dioxide), diamond and graphite. The word "allotropes" which appears in this statement, but nowhere else in the syllabus, might confuse you. Allotropes are different forms of the same element in the same physical state. You will find all this on the page about giant covalent structures. I personally prefer the term "giant covalent structure" to "giant molecular structure", although both terms are in common use. Strictly speaking a molecule contains a definite number of atoms joined together. That isn't true of these giant structures, where the number of atoms depends on the size of the crystal. Statement 4(e)(iv) is about hydrogen bonded structures such as ice. You will find this towards the bottom of the page about molecular structures. Statement 4(e)(v) is about metallic structures. You will find this on the page about metallic structures. Copper (mentioned in the syllabus) isn't specifically discussed, but it is a 12-co-ordinated metal, and so you should concentrate on the parts of the page related to 12-co-ordination. If you haven't done it recently, it might pay you to follow the link back to metallic bonding that you will find near the top of that page before you read on.
© Jim Clark 2010 (modified August 2013) |