It means that with total four orbitals combined, four new hybrid orbitals are generated, and they all named as sp 3 hybrid orbitals. Number of hybrid orbitals = the total number of atomic orbitals that are combined The symbol sp 3 here identify the numbers and types of orbitals involved in the hybridization: one s and three p orbitals. In the hybridization for CH 4, the 2s and three 2p orbitals are combined to give a new set of four identical orbitals, that are called sp 3 hybrid orbitals. Simply speaking, hybridization means the mathematical combination of several orbitals to generate a set of new hybrid orbitals. To explain the bonding of carbon and other atoms that cannot fit into the simple valence bond theory, a new theory called orbital hybridization will be introduced as a supplement to the valence bond theory.ġ.6.2 Hybridization and the Structure of CH 4 However, carbon always has four bonds in any stable organic compound. Figure 1.6g Orbital diagram of valence electrons in carbon atomīased on the valence bond theory, with two half-filled orbitals available, the carbon atom should be able to form two bonds. The valence electron configuration of carbon atom is 2s 22p 2 as shown in the orbital diagram. However, when the valence bond theory is applied to organic molecules, for instance CH 4, it does not work. The covalent bond in molecular fluorine, F 2, is a σ bond formed by the overlap of two half-filled 2 p orbitals, one from each fluorine atom as shown here. Figure 1.6e H-F σ bondĪ σ bond can also be formed through the overlap of two p orbitals. Therefore, the 1s orbital of the hydrogen atom overlaps head-to-head with the half-filled 2p orbital of the fluorine atom to form the H-F σ bond, as shown below. The filled orbital cannot form bonds, so only the half-filled 2p is available for overlap. Figure 1.6d Orbital diagram of valence electrons in fluorine atomįor the three 2p orbitals, two of them are filled and the other one is half-filled with one single electron. The fluorine atom has the valence electron configuration of 2s 22p 5 as shown in the orbital diagram. The valence bond theory works well to explain the bonding in HF as well, with the 2p orbital of fluorine atom involved in the overlapping. Figure 1.6c Cylindrical symmetry property of σ bond σ bonds are cylindrically symmetrical, meaning if a cross-sectional plane is taken of the bond at any point, it would form a circle. The bond formed by head-to-head overlap is called σ (sigma) bond. When the two atoms get closer than the optimal distance, the repulsion between the two nuclei become predominant, and the energy of the system becomes even higher.Īnother important character of the covalent bond in H 2 is that the two 1s orbitals overlap in a way that is referred to as head-to-head. The bond energy is 7.22×10 -19 J for one H-H bond, or 435kJ/mol. The energy difference between the most stable state (lowest energy state with optimum distance) and the state in which the two atoms are completely separated is called the bond (dissociation) energy. H 2 molecules have a bond length of 74 pm (often referred to as 0.74 Å, 1Å= 10 -10m). The optimal distance is also defined as the bond length. The energy lowers to its minimum level when the two atoms approach the optimal distance. As they are getting closer, orbitals start to overlap, and there is attraction between the nucleus of one atom and the electron of the other atom, so the total energy of the system lowers. When the two atoms are separate, there is no overlap and no interaction. Figure 1.6b Potential energy of the hydrogen molecule as a function of internuclear distance The overall energy changes of the system versus the distance between the two hydrogen nuclei can be summarized in the energy diagram below. The shared pair of electrons are under the attraction of both hydrogen nuclei simultaneously, resulting in them serving as a “glue” that holds the two nuclei together. When two hydrogen atoms are approaching each other, the two 1s orbitals overlap, allowing the two electrons (each H donates 1 electron) to pair up for the bonding with the overlapping orbitals. The atomic electron configuration of a hydrogen atom is 1s 1, meaning that there is one electron (which is also the valence electron) in the sphere-shaped 1s orbital. Let’s start with the simple molecule H 2. The valence bond theory describes the covalent bond formed from the overlap of two half-filled atomic orbitals on different atoms. We have talked about how covalent bonds are formed through the sharing of a pair of electrons here we will apply the valence bond theory to explain in more detail how the sharing happens.
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