Iodine I 2 , sugar C 12 H 22 O 11 , and polyethylene are examples of compounds that are molecular solids at room temperature. Water and bromine are liquids that form molecular solids when cooled slightly; H 2 O freezes at 0 o C and Br 2 freezes at -7 o C. Molecular solids are characterized by relatively strong intramolecular bonds between the atoms that form the molecules and much weaker intermolecular bonds between these molecules. Because the intermolecular bonds are relatively weak, molecular solids are often soft substances with low melting points.
Dry ice, or solid carbon dioxide, is a perfect example of a molecular solid. The van der Waals forces holding the CO 2 molecules together are weak enough that dry ice sublimes it passes directly from the solid to the gas phase at o C. Covalent solids , such as diamond, form crystals that can be viewed as a single giant molecule made up of an almost endless number of covalent bonds. Each carbon atom in diamond is covalently bound to four other carbon atoms oriented toward the corners of a tetrahedron, as shown in the figure below.
Because all of the bonds in this structure are equally strong, covalent solids are often very hard and they are notoriously difficult to melt. Diamond is the hardest natural substance and it melts at C. Ionic solids are salts, such as NaCl, that are held together by the strong force of attraction between ions of opposite charge. Biochemistry Video Lessons. GOB Video Lessons. Microbiology Video Lessons. Calculus Video Lessons. Statistics Video Lessons.
Microeconomics Video Lessons. Macroeconomics Video Lessons. Accounting Video Lessons. Chem Question. Molecular solids do not conduct electric current. One form of silicon carbide, SiC, is an extremely hard solid that melts As seen in the table above, the melting points of metallic crystals span a wide range. Covalent network crystals - A covalent network crystal consists of atoms at the lattice points of the crystal, with each atom being covalently bonded to its nearest neighbor atoms see figure below.
The covalently bonded network is three-dimensional and contains a very large number of atoms. Network solids include diamond, quartz, many metalloids, and oxides of transition metals and metalloids. Network solids are hard and brittle, with extremely high melting and boiling points. Being composed of atoms rather than ions, they do not conduct electricity in any state. Molecular crystals - Molecular crystals typically consist of molecules at the lattice points of the crystal, held together by relatively weak intermolecular forces see figure below.
The intermolecular forces may be dispersion forces in the case of nonpolar crystals, or dipole-dipole forces in the case of polar crystals. Some molecular crystals, such as ice, have molecules held together by hydrogen bonds.
When one of the noble gases is cooled and solidified, the lattice points are individual atoms rather than molecules. In all cases, the intermolecular forces holding the particles together are far weaker than either ionic or covalent bonds. As a result, the melting and boiling points of molecular crystals are much lower. Lacking ions or free electrons, molecular crystals are poor electrical conductors.
Germanium lies in the p block just under Si, along the diagonal line of semi-metallic elements, which suggests that elemental Ge is likely to have the same structure as Si the diamond structure.
Thus Ge is probably a covalent solid. Zn is a d-block element, so it is a metallic solid. Arranging these substances in order of increasing melting points is straightforward, with one exception. We expect C 6 CH 3 6 to have the lowest melting point and Ge to have the highest melting point, with RbI somewhere in between.
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