Absolutely Small - Michael D. Fayer [93]
Be is two electrons past the He closed shell configuration. As an atom, these two electrons would be paired in the 2s orbital. These are Be’s valence electrons. In BeH2, each H has one electron in a 1s orbital. For Be to make two electron pair bonds, one to each H atom, it will promote one of the 2s electrons to a 2p orbital, which we will take to be the 2pz, as shown at the top of Figure 14.5. The second line of the figure shows schematics of the 2s and the 2pz orbitals separated. These actually have the same center, the nucleus of the Be. These orbitals are electron probability amplitude waves. Waves can be added and subtracted to give new waves. We start with two atomic orbitals, the 2s and the 2pz, and by addition and subtraction, we get two new atomic orbitals, called hybrid orbitals. When we add the waves together, we get regions of constructive interference and regions of destructive interference because the lobes of the probability amplitude waves have signs. The third line in Figure 14.5 shows the sum of the 2s and 2pz orbitals. This is called an sp hybrid, which we have labeled spz+ because it is an sp hybrid made from an s orbital and the 2pz orbital, and its big positive lobe points long the +z direction. The second to the bottom line of Figure 14.5 shows the 2s minus the 2pz. One way to think of this is just that we have flipped the 2pz orbital shown in the top line of the figure around so the positive lobe points to the left rather than to the right, and then we add. We have labeled this hybrid orbital spz- because its big positive lobe points in the—z direction. This orbital has the same shape as the spz+ orbital but it points in the opposite direction. We start with two atomic orbitals, 2s and 2pz, and we end with two hybrid atomic orbitals with their positive lobes pointing along the z axis in opposite directions. The bottom line of Figure 14.5 shows the schematic of both hybrid atomic orbitals as they would occur in the Be atom. There is one Be nucleus with the two hybrid orbitals pointing along the +z and -z directions.
FIGURE 14.5. Top: Be valence electrons with one promoted to the 2pzorbital. Next: The 2s and 2pzorbitals of Be shown separated. Next: The sum of 2s and 2pzto form the hybrid atomic orbital spz+. Next: 2s—2pzto form the hybrid orbital spz-. Bottom: The two hybrid orbitals of Be point in opposite directions along z.
To make BeH2, Be will use the two hybrid atomic orbitals to form electron pair bonding molecular orbitals with two hydrogens. The bonding is shown schematically in Figure 14.6. The top of the figure shows two hydrogen atoms, Ha and Hb, approaching a Be. Hydrogen atom electrons are in 1s orbitals, 1sa and 1sb. The beryllium has its two hybrid atomic orbitals, spz- and spz+, pointing at the hydrogen 1s orbitals. The middle portion of the figure shows a schematic illustration of the overlapping atomic orbitals. The hydrogen 1sa orbital on the left side of the figure will form a bonding MO with the Be spz- hybrid atomic orbital. This bonding MO will contain two electrons, one from the hydrogen and one of the two Be valence electrons. The hydrogen 1sb orbital on the right side of the figure will form a bonding MO with the spz+ hybrid orbital. The electron from Hb and the other Be valence electron will form another covalent bond. These are σ bonds since there is electron density along the line connecting the nuclei. The result is the linear BeH2 molecule shown at the bottom of the figure.
FIGURE 14.6. Top: Two H atoms approach a Be. Middle: The H 1s orbitals form electron pair bonds with the two Be sp hybrid orbitals to produce the linear BeH2molecular shown as