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1) Molecular orbital energy level diagram
There are two types of orbital energy level diagrams for homonuclear diatomic molecules: one is suitable for O 2 , F 2 molecules or molecular ions [Figure 6-23(a)]; the other is suitable for B 2 , C 2 , and N 2 molecules or molecular ions [Figure 6-23(b)]
.
Figure 6-23 Orbital energy level diagram of homonuclear diatomic molecules
For O 2 and F 2 molecules, the energy of the 2s and 2p orbitals is quite different, and the combination of 2s and 2p orbitals is not considered, so the energy of p orbitals is lower than that of π 2p orbitals; for B 2 , C 2 , and N 2 molecules Since the 2s and 2p orbital energy of atoms are not much different, when combining into sub-orbitals, there are not only 2s-2s overlap and 2p-2p overlap, but also 2s-2p overlap, so the σ 2p orbital energy is higher than the π 2p orbital
.
2) Electronic arrangement and key level
The arrangement of electrons in molecular orbitals also follows the principle of least energy, Pauli's principle and Hund's rule
.
The arrangement of electrons in molecular orbitals can be represented by energy level diagrams or molecular orbital formulas.
The former is more intuitive and the latter is more convenient
.
Molecular orbital theory often uses bond levels to express the number of covalent bonds to describe the stability of molecules or molecular ions
.
The greater the bond level, the greater the bond energy, the shorter the bond length, and the more stable the molecule or molecular ion
.
H 2 molecule: The molecule has 2 electrons in total, and the electrons are arranged in the energy level diagram as shown in Figure 6-24(a)
.
Figure 6-24 Molecular orbital energy level diagram and electron arrangement of H 2 and He 2 +
The molecular orbital formula of H 2 is (σ 1s ) 2 and the bond level is 1, which is consistent with the valence bond theory
.
The electrons of H 2 molecules are filled into the bonding orbital, and the energy is lower than in the atomic orbital.
This energy difference is the essence of the chemical bond in the molecular orbital theory
.
He 2 + ion: This ion has 3 electrons in total, and the electrons are arranged in the energy level diagram as shown in Figure 6-24(b)
.
The He 2 + orbital formula is (σ 1s ) 2 (σ* 1s )1, and the bond level is 0.
5
.
N 2 molecule: The energy level diagram of molecular orbital is shown in Figure 6-23(b)
.
The molecule has 14 electrons in total, and the 6 2p orbital electrons are arranged in the energy level diagram as shown in Figure 6-25
.
The molecular orbital formula of N 2 is (σ 1s ) 2 (σ* 1s )2(σ 2s ) 2 (σ* 2s )2(π2 py ) 2 (π2 pz )2(σ2 px ) 2 , abbreviated as KK(σ 2s ) 2 (σ* 2s ) 2 (π2 py ) 2 (π2 pz ) 2 (σ2 px ) 2 , where KK represents the 1s orbital of 2 atoms full of electrons
.
The bond level of the N2 molecule is 3, 1 σ bond, and 2 π bonds, which are consistent with the results of valence bond theory
.
O 2 molecule: The energy level diagram of molecular orbital is shown in Figure 6-23(a)
.
The molecule has 16 electrons in total, and 8 2p orbital electrons are arranged in the energy level diagram as shown in Figure 6-26
.
Figure 6-25 Energy level diagram and electron arrangement of N 2 molecular orbital
Figure 6-26 O 2 molecular orbital energy level diagram and electron arrangement
O 2 molecular orbital formula is abbreviated as KK(σ 2s )2(σ* 2s )2(σ2 px ) 2 (π2 py ) 2 (π2 pz )2(π*2 py ) 1 (π*2 pz ) 1 , numerator The bond level of is 2, 1 σ bond, and 2 three-electron π bonds, which are consistent with the results of valence bond theory
.
The O 2s molecular orbital formula and the electronic arrangement of the molecular orbital energy level diagram show that the O2 molecule has 2 single electrons and the molecule is paramagnetic, which is consistent with the experimental results
.
According to the molecular orbital theory , the paramagnetism of O 2s molecules can be explained well , but the valence bond theory cannot explain it reasonably
.
Molecular orbital theory can explain O 2 + , O 2 - paramagnetic and O 2 2+ , O 2 2- diamagnetism
O 2 2+ > O 2 + > O 2 > O 2 - > O 2 2-