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
Hybridization
-the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the pairing of electrons to form chemical bonds
Bonding Orbital for H₂(First-row Homonuclear Diatomic Molecule)
-constructive interference, amplitude of each atom is added
-square of amplitude results in higher probability density at nuclei and middle
-most likely to find electrons in bond around nuclei
-second most likely to find electrons in between the two nuclei
-occurs in ground state
Antibonding Orbital for H₂ (First-row Homonuclear Diatomic Molecule)
-destructive interference
-square of amplitude results in higher probability near individual nuclei but node at middle
-occurs in excited state
MOT bond order equation
1/2[(# of electrons in bonding MO) - (number of electrons in antibonding MO)]
Bonding orbital
-a molecular orbital formed when atomic orbitals from different atoms overlap constructively, leading to increased electron density between the nuclei and thus promoting the formation of a chemical bond
-lower in energy
-higher volume
-more stable, bond
Antibonding orbital
-type of molecular orbital that occurs when atomic orbitals overlap destructively (out of phase), creating an antibonding orbital with decreased electron density between the nuclei, which weakens the bond and increases the molecule’s energy
-higher in energy
-lower in volume
-less stable, no bond
Why are resonance structures unnecessary for MOT?
-electron pairs may be delocalized (not connected to single atom) over an entire molecule
Sketching σ framework
-orient preferred bond on axis
-identify hybridization required of each element in molecular cation
-determine steric number (SN) of each atom
-use VSEPR and SN to determine predicted bond angles of each atom/ion
-placing all nuclei in plane of paper allows molecule to have a planar structure
Sketching π framework
-sketch in same orientation use for σ framework
-use separate framework for each component of π framework and label all nuclei
-y-axis is view when nuclei are going out of paper
-x-axis is view of looking at nuclei from top view
