Sodium Through Argon
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The Si-0 bond has a considerable ionic character, and most of the electrons in the Si-Obond are drawn away from Si toward 0. Ionic bond radii, with large O2- and small Si4+ ions (O2-, 1.40 A; Si4+, 0.41 A), gives a better picture of silicate structures than covalent radii with relative sizes reversed (O, 0.66 A; Si, 1.17 A). Pure quartz, with the overall composition Of SiO2, is an endless
framework of Si and O atoms. Each Si is surrounded by four O atoms
at the corners of a tetrahedron, and each O atom is shared between
two adjacent tetrahedra (top). |
One silicon atom has "half a share" in each of four oxygen atoms around it, so the number of O atoms per Si atom is 2 (4 x 1/2), thereby accounting for the overall composition of SiO2. In other types of silicates, one or more of the four oxygens around a silicon atom may not be shared with other silicons, and the unshared oxygen atoms each carry one negative charge. The smallest freestanding unit of silicon and oxygen is the silicate ion, SiO4- with none of the four oxygen atoms shared, and with each of them negatively charged. These silicate tetrahedra can be linked together by sharing oxygens to form one-dimensional chains, two-dimensional double chains (or ladders) and sheets, and three-dimensional frameworks such as quartz. These one-, two-, and three-dimensional structures are the basis for all silicate minerals. Any negative charges arising from unshared oxygen atoms are balanced by positive metal ions inserted alongside the chains or between the layers. |
