![]() The distances between the two chlorine atoms would be different, giving rise to differences in physical properties would allow the two isomers to be distinguished and separated. If the coordination around the carbon were square, then there would have to be two isomers of CH 2Cl 2, as shown in the pair of structures here. For example, it was noted that there is only one dichloromethane, CH 2Cl 2. It is interesting to note that the tetrahedral coordination of carbon in most of its organic compounds was worked out in the nineteenth century on purely geometrical grounds and chemical evidence, long before direct methods of determining molecular shapes were developed. This is the most important coordination geometry in Chemistry: it is imperative that you be able to sketch at least a crude perspective view of a tetrahedral molecule. The angle between any two bonds will be 109.5°. Consequently, the four equivalent bonds will point in four geometrically equivalent directions in three dimensions corresponding to the four corners of a tetrahedron centered on the carbon atom. The latter calculation would be correct if all the atoms were constrained to be in the same plane (we will see cases where this happens later), but here there is no such restriction. What bond angle would lead to the greatest possible separation between the electron clouds associated with these bonds? In analogy with the preceding two cases, where the bond angles were 360°/2=180° and 360°/3=120°, you might guess 360°/4=90° if so, you would be wrong. Methane, CH 4, contains a carbon atom bonded to four hydrogens. If the central atom possesses partially occupied d-orbitals, it may be able to accommodate five or six electron pairs, forming what is sometimes called an “ expanded octet”. The orbitals containing the various bonding and nonbonding pairs in the valence shell will extend out from the central atom in directions that minimize their mutual repulsions. If the central atom also contains one or more pairs of nonbonding electrons, these additional regions of negative charge will behave very much like those associated with the bonded atoms. ![]() We therefore expect the two chemical bonds to extend in opposite directions, producing a linear molecule. Thus the two electron clouds contained in a simple triatomic molecule AX 2 will extend out in opposite directions an angular separation of 180° places the two bonding orbitals as far away from each other they can get. The fundamental idea of VSEPR thoery is that these regions of negative electric charge will repel each other, causing them (and thus the chemical bonds that they form) to stay as far apart as possible. The covalent model of chemical bonding assumes that the electron pairs responsible for bonding are concentrated into the region of apace between the bonded atoms. Like all electrons, these occupy regions of space which we can visualize as electron clouds- regions of negative electric charge, also known as orbitals- whose precise character can be left to more detailed theories ![]() The valence shell electron pair repulsion ( VSEPR) model that we describe here focuses on the bonding and nonbonding electron pairs present in the outermost (“valence”) shell of an atom that connects with two or more other atoms. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |