Unit 11 Flashcards
Localized Electron Model (LEM)
-use of superposition principle to mix atomic orbitals (add/subtract standing waves) on the same atom to create new hybrid orbital sets
-bonding pair or lone pair that are hybridized can be described by LEM
Failures of LEM
-needs use of resonance to describe the delocalization of electrons
-molecules are known to exist in excited states, and LEM cannot describe or account for these
-LEM fails to predict certain molecular properties, such as unpaired electrons
-cannot describe delocalized electrons (those spread over more than 2 atoms)
Hybridization
-atomic orbitals fuse to form newly hybridized orbitals, which in turn, influences molecular geometry and bonding properties
-used to determine valence atomic orbitals used in the bonding
-mixing of pure s and p atomic orbitals on an atom to form hybrid atomic orbitals on that same atom
sp hybrid atomic orbitals
-180° bond angle (linear arrangement)
-2 sp hybrid atomic orbitals (2 lobes)
-2 pure p orbitals remain
sp² hybrid atomic orbital
-120° bond angle (trigonal arrangement)
-3 sp² hybrid atomic orbitals (3 lobes)
-1 pure p orbital remains
sp³ hybrid atomic orbital
-109° bond angle (tetrahedral arrangement)
-4 sp³ hybrid atomic orbitals (4 lobes)
-0 pure p orbitals remain
Procedure for determining hybridization around an atom
- Draw Lewis structure
- Determine shape (basic geometry) using VSEPR and thereby bond angles
- Pick type of hybrid orbitals that produce these bond angles
σ bond
-a type of covalent bond formed by the direct, head-on overlap of atomic orbitals along the internuclear axis between two atoms
-have no nodes containing the bond axis
-hybrid sp-orbitals are used for σ bonds
-strongest type of covalent bond where the electron density is concentrated directly between the nuclei of the bonded atoms
-primary bond in any single covalent bond and is present as the first bond in double and triple bonds as well
π bond
-a covalent bond that occurs as result of lateral overlapping of two lobes of an atomic orbital with two lobes of another atomic orbital that belongs to a different atom
-weaker than sigma bonds, but when combined with a sigma bond, they create a stronger bond between the atoms
-the two orbitals that are bonded share a nodal plane, which is where the electron density is zero (have one planar node that completely contains the bond axis)
-use pure, unhybridized p-orbitals
single bond
-has 1 σ component
-no orbital overlap loss when rotating a σ bond, rotation occurs almost freely around σ bond
-weakest and longest bond
double bond
-have 1 σ component and 1 π component
-significant overlap loss occurs when rotating a π bond, so double bonds are essentially rigid
-have middle length and strength
triple bond
-have 1 σ component and 2 π components
-strongest and shortest bond
Bond strength and bond length
-as bond order increases, strength of bond increases and length decreases
-length: triple<double<single
-strength: single<double<triple
Molecular Orbital Theory (MOT)
-method for describing the electronic structure of molecules using quantum mechanics
-molecular orbitals (MOs) can extend over an entire molecule
-MOs formed by interfering (SP principle) with atomic orbitals on different atoms
-describes both ground state and excited states of molecules an molecule ions
-explains the paramagnetic nature of O₂, which valence bond theory cannot explain
molecular orbital (MO)
-a mathematical function describing the location and wave-like behavior of an electron in a molecule
-qm electron wavefunctions that may be delocalized over an entire molecule
-result of hybridization of atomic orbitals
-used to make covalent bonds
