Fig 1439 A Gouy Balance Louis Georges Gouy 1854 1926 Weaknesses of the

Fig 1439 a gouy balance louis georges gouy 1854 1926

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Fig. 14.39 A Gouy Balance Louis Georges Gouy 1854-1926
Weaknesses of the Localized Electron/Valence Bonding Model: assumes totally localized electrons concept of resonance must be added model does not deal with unpaired electrons does not necessarily account for all the physical and chemical properties of certain compounds – O 2 Methane (CH 4 )
Limitations of the theory: Energy Levels unreliable in accurately describing energy levels of the e - For example: CH 4 according to hybrid orbital theory C sp 3 hybridized predicts electrons occupy four energetically equivalent orbitals Reality! Spectroscopy shows one pair of bonding electrons is lower in energy than the other three Molecular Orbital Theory explains the energetics of molecules very well
Molecular Orbital Theory molecular orbital theory assumes that valence electrons occupy orbitals (molecular orbitals) that spread throughout the entire molecule (ie. they are ____________ ) molecular orbitals (MO’s) are built by adding together (or superimposing) valence atomic orbitals of ____________ and symmetry Each MO can hold a maximum of 2 electrons, with opposite spins. The square of the MO wavefunction represents the probability or electron density as a theory it successfully explains the energetics of many molecules Eg. Lewis structure predicts only paired e - MO theory (as we will see) correctly predicts paramagnetism
MO theory can be quite complex! We will simplify by: (a) presenting a non-mathematical approach (b) considering only the simplest of molecules: first and second row homonuclear and heteronuclear diatomics same atoms ie. H 2 , Li 2 , N 2 + etc . different atoms ie. CO, CN - , NO etc.
The H 2 Molecule (H A H B ) Recall that the wavefunction of an orbital describes the electron density as a function of the distance (R) from the nucleus for the 1s orbital Ψ (1s) ~ e - k R (spherical) Let’s construct the molecular orbitals for the H 2 molecule by combining the H ___________ atomic orbitals on the two atoms Ψ R H A Ψ R H B bring the two atoms together
when we superimpose the wavefunctions there are two possible atomic orbital combinations: (a) Ψ + = Ψ (1s) A + Ψ (1s) B Ψ R ___________ overlap Ψ + region of ___________ electron density between the two nuclei bonding MO H A H B resulting molecular orbital Ψ
(b) Ψ - = Ψ (1s) A - Ψ (1s) B H A H B resulting molecular orbital Ψ Ψ R ____________ overlap Ψ - nodal plane (zero electron density) between the two nuclei anti-bonding MO
Let’s do the same thing using easier-to-visualize orbitals… Molecular orbitals are formed by combining the H 1s atomic Orbitals in two possible ways: (a) Constructive interference- bonding MO σ 1s type of bond atomic orbitals H A + H B H 2 molecular orbital region of enhanced electron density between nuclei ( ________ ) atomic orbitals used Fig. 14.25
(b) Destructive interference – anti-bonding MO atomic orbitals H A - H B H 2 molecular orbital electron density is zero between nuclei ( __________ ) atomic orbitals used σ * 1s type of bond antibonding Fig. 14.25
What do we mean by bonding and anti-bonding?

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