Ionic structures and bonding
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Some exam "Dos and Don'ts": Ionic structures and bonding
Advice on answering questions on topics often highlighted by examiners as being poorly answered in the past. Make sure that the detail covered by the question is included in your syllabus specification.
The mistake of mentioning molecules when talking about ionic bonding has already been pointed out. Ionic bonding is not the transfer of electron to form ions, but the strong attraction between the oppositely charge ions that results.
When asked to compare melting points of two ionic compounds, you must talk about the strength of the ionic bonding, which depends on the size (radii) and the charges on the ions. Hence NaCl has a much lower melting point than MgO for two reasons. If you have access to a data book in your exam, you can support your argument by quoting values for the charges and radii.
Questions on electrolysis and the conduction by molten salts or their aqueous solutions are often very poorly answered. ONLY metals (and graphite) conduct electricity by means of a flow of mobile electrons.
Molten or dissolved ionic substances conduct because the ions, normally fixed in position, are free to move (+ ions to the - electrode, and - ions to the + electrode). There are NO electrons flowing in a molten ionic compound or an aqueous solution of it, though many students maintain there are!
Likewise molecular substances, even if liquid or dissolved, do not normally conduct electricity because there are no mobile electron or ions to carry the charge. All the electron are tied up in covalent bonds.
If the molecular susbtance happens to react react with water, like HCl, then the solution will conduct - by menas of ions.
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updated: 01 June 2007
