![]() When the molecules pack in the solid, the F atoms touch other F atoms, not Si atoms, and are not attracted much at all. In SiF 4, however, each Si is surrounded by 4 "fluoride ions" (because radius decreases across the periodic table) which naturally makes a tetrahedron, and thus clearly separated molecules. All the ion-ion attractions must be loosened to melt the solid, which requires high temperature. Each cation is surrounded by 6 fluoride ions, and each fluoride ion is surrounded by 6, 3, or 2 cations (depending on the formula). ![]() Instead of clearly-defined molecules, there is an alternating lattice of positive and negative ions. ![]() In the first 3, the structure is typical of ionic compounds. We can explain the change better using molecular structure than bond type. You might take this as the change from an ionic compound to a covalent compound, but the difference in electronegativity between Al and Si isn't that large, and both are much less electronegative than F. Notice that there is an abrupt change in the melt point between AlF 3and SiF 4. Shapes of molecules can have other effects as well. In general, shapes of molecules will influence how they react, because they determine polarity (which can help pull molecules together to react) and fit (whether the reactive parts can get close to each other). If it were linear, it would not be a great solvent for polar compounds, and all life would be completely different. For instance, water has a dipole moment because it is bent, not linear. \)ģD structures are important because they can have big effects on the properties of molecules.
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