3.1专业英语选读.ppt

The nonmetals that are gases at room temperature are the low-molecular weight diatomic molecules and the noble gases that exert very small intermolecular forces. As the molecular weight increases, we encounter a liquid (Br2) and a solid (I2) whose vapor pressures also indicate small intermolecular forces. Certain properties of a few nonmetals are listed in Table 2. Table 2. Molecular weights and Melting Points of Certain Nonmetals Diatomic Molecules Molecular Weight Melting Point ℃ Color H2 2 -259.1 None N2 28 -210 None F2 38 -223 Pale yellow O2 32 -218 Pale blue Cl2 71 -102 Yellow-green Br2 160 -7.3 Red-brown I2 254 113 Gray-black Simple diatomic molecules are not formed by the heavier members of Groups V and VI and at ordinary conditions. This is in direct contrast to the first members of these groups, N2 and O2. The difference arises because of the lower stability of π bonds formed from p orbitals of the third and higher main energy levels as opposed to the second main energy level. The larger atomic radii and more dense electron clouds of elements of the third period and higher do not allow good parallel overlap of p orbitals necessary for a strong π bond. This is a general phenomenon—strong πbonds are formed only between elements of the second period. Thus, elemental nitrogen and oxygen form stable molecules with both σ and π bonds, but other members of their groups form more stable structures based on σbonds only at ordinary conditions. Note that Group VII elements form diatomic molecules, but πbonds are not required for saturation of valence. Sulfur exhibits allotropic forms. Solid sulfur exists in two crystalline forms and in an amorphous form. Rhombic sulfur is obtained by crystallization from a suitable solution, such