VSEPR Models Flashcards
Explain electrons filling the d-orbitals in cases like Cr and Cu
- One of the s-electrons moves into the d-shell, because of the additional stability obtained when the d-orbitals are exactly half filled or completely filled
- By using a single s-electron Cr can have an oxidation state of (I)
Explain the oxidation state of Sc
Sc can have an oxidation state of (II) when both s-electrons are used for bonding or (III) when two s-electrons and one d-electron are involved.
Explain the Complex ion geometry
CN = 2, linear
CN = 3, trigonal
CN = 4, square planar or tetrahedral
CN = 6, octahedral
Explain covalent bonds fully
- Covalently bonded molecules have characteristic shapes depending on the number of electron
domains surrounding the central atom. Why? - Covalent bonds are formed due to sharing of electron pairs between atoms.
- Since electrons are negatively charged, they repel other electron pairs nearby.
- These electron-electron cause the atoms bonded to the central atom to move as far apart as
possible. - When this happens, the molecule has a characteristic shape or geometry that depends on the
number of atoms or lone pairs surrounding the central atom.
What does VSEPR stand for
Valence Shell Electron-Pair Repulsion Theory
What are the 3 basic principles of VSEPR
- Electron pairs repel each other . Electron pairs may be in bonds to other atoms or in lone pair orbitals.
- Electron pairs will tend to stay as far apart as possible to minimize repulsion.
- Geometry is dictated by the need of each electron pair to have as great a distance as possible separating it from other electron pairs.
Explain electron pair repulsions briefly
- Lone pairs are more repulsive towards other electron pairs compared to bonding pairs
- Lone pairs require more space than bonding pairs
Explain lone pair repulsion in more detail
- Lone-pair to lone-pair repulsions are most severe
- Lone-pair to bonding-pair repulsions are less severe
- Bonding-pair to bonding-pair repulsions least significant
What is occupancy
It defines how many groups are needed to occupy the space around an atom.
Groups = atoms or lone pairs
Explain occupany being independent
- Occupancy independent of the presence of multiple bonds - a particular atom B can occupy only one position bonded to A, whether it is bonded by a single or double bond
Explain prototype geometry
- Can be obtained from the table when the occupancy (x + y) is known. The prototype geometry tells how atoms and lone pairs are arranged around the central atom.
If the occupancy is two, what will the prototype geometry and hybridization be
Prototype geometry : Linear
Hybridisation : sp
If the occupancy is three, what will the prototype geometry and hybridization be
Prototype geometry : Trigonal planar
Hybridisation : sp2
If the occupancy is four, what will the prototype geometry and hybridization be
Prototype geometry : Tetrahedral / Square planar
Hybridisation : sp3 / sd3
dsp2
If the occupancy is five, what will the prototype geometry and hybridization be
Prototype geometry : Trigonal bipyramidal / Square pyramidal
Hybridisation : TBP = dsp3
dsp3
If the occupancy is six, what will the prototype geometry and hybridization be
Prototype geometry : Octahedral
Hybridisation : d2sp3
What must be taken into consideration when determining the actual geometry of the molecule
- The actual geometry tells how bonded atoms are arranged around the central atom.
- The arrangement of bonded atoms around the central atom takes place along the framework provided by the prototype geometry.
- Lone pairs of electrons take up space around the central atom and help to determine the prototype geometry (and thus the framework), but are not included as such in the geometry
Explain the General formula; Prototype geometry; Geometry for an Occupancy two
General formula: AB2
Prototype geometry: Linear
Geometry: Linear
Explain the General formula; Prototype geometry; Geometry for an Occupancy three
General formula: AB3, AB2E
Prototype geometry: Trigonal planar
Geometry: Trigonal planar (AB3), Angular/bent (AB2E)
Explain the General formula; Prototype geometry; Geometry for an Occupancy four
General formula: AB4, AB3E, AB2E2
Prototype geometry: Tetrahedral
Geometry: Tetrahedral (AB4), Trigonal pyramidal (AB3E),
Angular/bent (AB2E2)
Explain the General formula; Prototype geometry; Geometry for an Occupancy five
General formula: AB5, AB4E, AB3E2, AB2E3
Prototype geometry: Trigonal bipyramidal (TBP)
Geometry: Trigonal bipyramidal (AB5), See-saw (AB4E), shaped (AB3E2), Linear (AB2E3)
Explain the General formula; Prototype geometry; Geometry for an Occupancy six
General formula: AB6, AB5E, AB4E2
Prototype geometry: Octahedral
Geometry: Octahedral (AB6), Square pyramidal (AB5E),
Square planar (AB4E2)
For The formyls – why do the angles differ?
- The electrons of the C=O double bond require more space than the single bonds. The HCH and XCX angles collapse from the expected 120° to 116°, 111° and 108°
- The repulsive effect of the double bonds is balanced by repulsions between electrons of the single bonds
- The more electronegative groups (X) allow the single bonds to get closer to each other. This is possible because the electron density in the C-X bonds is further away from C and closer to X. The further away from C the electron density are, the smaller the X-C-X angle will become
Explain The angles of the NO’s
NO2− : Occupancy of 3, trigonal planar (expect 120°) O-N-O angle forced smaller because lone pair electrons on N requires a larger volume.
NO2 : Removal of one electron from lone pair – Less repulsion exerted
O-N-O angle opens to 132°
NO2+ : Occupancy of 2, linear (expect 180°)
O-N-O angle opens to 180° – no restrictions.
