SHAPES STRUCTURES GEOMETRY VALENCE SHELL ELECTRON PAIR REPULSION 7/25/2019 Dr Seemal Jelani

Valence shell electron pair repulsion, VSEPR, is a super- simple technique for predicting the geometry of atomic centres in small molecules and molecular ions: 7/25/2019 Dr Seemal Jelani

Model deals primarily with bond angles and molecular geometries KEY TOOLS 7/25/2019 Dr Seemal Jelani

Atomic centers with VSEPR determined geometry can be joined together into molecular bodies like Cyclohexane and glucose: 7/25/2019 Dr Seemal Jelani

Cyclohexane Twist-boat shape conformation α-Glucose Boat shape 7/25/2019 Dr Seemal Jelani

This molecular building-block logic can be extended, enabling large biomolecular structures like DNA to be modelled and understood: 7/25/2019 Dr Seemal Jelani

Generate the VSEPR geometry of an atomic centre such as: Carbon in methane, CH4 Nitrogen in ammonia, NH3 The VSEPR Technique 7/25/2019 Dr Seemal Jelani

The overall geometry of the atomic centre is determined by the mutual repulsion between the electron pairs of the total coordination number. The effect can be replicated by holding 2, 3, 4, 5 or 6 balloons together: 7/25/2019 Dr Seemal Jelani

7/25/2019 Dr Seemal Jelani

Methane, CH4, has a perfect tetrahedral bond angle of 109°. 28' (109 Methane, CH4, has a perfect tetrahedral bond angle of 109°.28' (109.47°), while the H-N-H bond angle of ammonia, NH3 :, is slightly less at 107 7/25/2019 Dr Seemal Jelani

7/25/2019 Dr Seemal Jelani

The oxygen of water has two bonded electron pairs and two nonbonded "lone" electron pairs giving a total VSEPR coordination number of 4. But the geometry is defined by the relationship between the H-O-H atoms and water is said to be "bent" or "angular" shape of 104.5°. 7/25/2019 Dr Seemal Jelani

7/25/2019 Dr Seemal Jelani

2 Balloons give a linear geometry 3 Balloons give a trigonal planar geometry 4 Balloons give a tetrahedral geometry 5 Balloons give a trigonal bipyramidal geometry 6 Balloons give an octahedral geometry 7/25/2019 Dr Seemal Jelani

Valence-shell electron-pair repulsion (VSEPR) theory Assumes that each atom in a molecule will be positioned so that there is minimal repulsion between the valence electrons of that atom. Valence-shell electron-pair repulsion (VSEPR) theory 7/25/2019 Dr Seemal Jelani

In simple molecules in which there are no nonbonding electrons, there are five basic shapes: 7/25/2019 Dr Seemal Jelani

All diatomic molecules are linear. Molecules with two atoms around a central atom such as BF2 are linear because positioning the two attachments at opposite ends of the central atom minimizes electron repulsion. LINEAR - Bond angle = 180 7/25/2019 Dr Seemal Jelani

Linear shape 7/25/2019 Dr Seemal Jelani

MX2 (where M is the central atom and X is are the bonding atoms). Generic Formula:   MX2 (where M is the central atom and X is are the bonding atoms). Linear shape 7/25/2019 Dr Seemal Jelani

TRIGONAL PLANAR - Bond angle = 120 Molecules with three atoms around a central atom such as BF3 are Trigonal planar because electron repulsion is minimized by positioning the three attachments toward the corners of an equilateral triangle. TRIGONAL PLANAR - Bond angle = 120 7/25/2019 Dr Seemal Jelani

MX3 (where M is the central atom and X is are the bonding atoms). Generic Formula:   MX3 (where M is the central atom and X is are the bonding atoms). Trigonal planar 7/25/2019 Dr Seemal Jelani

7/25/2019 Dr Seemal Jelani

TETRAHEDRAL - Bond angle = 109.5 Molecules with four atoms around a central atom such as CH4 are tetrahedral because electron repulsion is minimized by position the four attachments toward the corners of a tetrahedron. TETRAHEDRAL - Bond angle = 109.5 7/25/2019 Dr Seemal Jelani

TETRAHEDRAL - Bond angle = 109.5 Generic Formula:   MX4 (where M is the central atom and X is are the bonding atoms). TETRAHEDRAL - Bond angle = 109.5 7/25/2019 Dr Seemal Jelani

TETRAHEDRAL - Bond angle = 109.5 7/25/2019 Dr Seemal Jelani

Molecules with Five atoms around a central atom such as PF5 are Trigonal bipyramidal.  Three of the attachments  are positioned in a Trigonal plane with 120 bond angles.   Trigonal bipyramidal 7/25/2019 Dr Seemal Jelani

7/25/2019 Dr Seemal Jelani

Bond angle within the equilateral plane= 120 Bond angle between equatorial and axial plane= 90 TRIGONAL BIPYRAMIDAL 7/25/2019 Dr Seemal Jelani

The remaining two attachments are positioned perpendicular (90) to the Trigonal plane at opposite ends of the central atom.  This arrangement of atoms minimizes electron repulsion. Trigonal bipyramidal 7/25/2019 Dr Seemal Jelani

MX5 (where M is the central atom and X is are the bonding atoms). Generic Formula:   MX5 (where M is the central atom and X is are the bonding atoms). TRIGONAL BIPYRAMIDAL 7/25/2019 Dr Seemal Jelani

TRIGONAL BIPYRAMIDAL 7/25/2019 Dr Seemal Jelani

OCTAHEDRAL - Bond angle = 90 Molecules with six atoms around a central atom such as SF6 are Octahedral.  Four of the attachments are positioned in a square plane with 90o bond angles.   OCTAHEDRAL - Bond angle = 90 7/25/2019 Dr Seemal Jelani

7/25/2019 Dr Seemal Jelani

OCTAHEDRAL - Bond angle = 90 The remaining two attachments are positioned perpendicular 90o to the square plane at opposite ends of the central atom.  This arrangement of atoms minimizes repulsion. OCTAHEDRAL - Bond angle = 90 7/25/2019 Dr Seemal Jelani

OCTAHEDRAL - Bond angle = 90 Generic Formula:   MX6 (where M is the central atom and X is are the bonding atoms). OCTAHEDRAL - Bond angle = 90 7/25/2019 Dr Seemal Jelani

OCTAHEDRAL - Bond angle = 90 7/25/2019 Dr Seemal Jelani

Three Regions of Electron Density 7/25/2019 Dr Seemal Jelani

7/25/2019 Dr Seemal Jelani

7/25/2019 Dr Seemal Jelani

SHAPES WITH LONE-PAIRS There are seven shapes for molecules with one or more pairs of nonbonding electrons. SHAPES WITH LONE-PAIRS 7/25/2019 Dr Seemal Jelani

BENT (ANGULAR or V-SHAPED) Molecules with two atoms and one or two pairs of nonbonding electrons around a central atom such as H2O are bent BENT (ANGULAR or V-SHAPED) 7/25/2019 Dr Seemal Jelani

BENT (ANGULAR or V-SHAPED) If there is one pair of nonbonding electrons angle is less than 109 if there are two pairs of nonbonding electrons the angle is less 107 BENT (ANGULAR or V-SHAPED) 7/25/2019 Dr Seemal Jelani

BENT (ANGULAR or V-SHAPED) Generic Formula:   MXE2 (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons). BENT (ANGULAR or V-SHAPED) 7/25/2019 Dr Seemal Jelani

BENT (ANGULAR or V-SHAPED) 7/25/2019 Dr Seemal Jelani

Molecules with three atoms and one pair of nonbonding electrons around a central atom such as NH3 are Trigonal pyramidal.   TRIGONAL PYRAMIDAL 7/25/2019 Dr Seemal Jelani

These molecules are basically tetrahedral molecules with one of the attached atoms replaced by a pair of nonbonding electrons.   TRIGONAL PYRAMIDAL 7/25/2019 Dr Seemal Jelani

The force of repulsion of these electrons makes the bond angle between the attached atoms less than 109.5.  For example, in NH3, the H-N-H bond is 107.5. 7/25/2019 Dr Seemal Jelani

Generic Formula:  MX3E (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons). TRIGONAL PYRAMIDAL 7/25/2019 Dr Seemal Jelani

TRIGONAL PYRAMIDAL 7/25/2019 Dr Seemal Jelani

Molecules with four atoms and one pair of nonbonding electrons around a central atom such as SF4 are seesaw- shaped.   SEESAW-SHAPED 7/25/2019 Dr Seemal Jelani

These molecules are basically Trigonal bipyramidal molecules with one of the Equatorial- positioned atoms (in the Trigonal plane) replaced by a pair of nonbonding electrons.   7/25/2019 Dr Seemal Jelani

SEESAW-SHAPED (DISTORTED TETRAHEDRAL) This leaves the two axial- positioned atoms and two of the equatorial-positioned atoms in the shape of a seesaw or a teeter- totter. SEESAW-SHAPED (DISTORTED TETRAHEDRAL) 7/25/2019 Dr Seemal Jelani

Generic Formula:   MX4E (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons) SEESAW-SHAPED 7/25/2019 Dr Seemal Jelani

SEESAW-SHAPED 7/25/2019 Dr Seemal Jelani

Molecules with three atoms and two pairs of nonbonding electrons around a central atom such as ClF3 are T-shaped.   T-SHAPED 7/25/2019 Dr Seemal Jelani

Molecules with three atoms and two pairs of nonbonding electrons around a central atom such as ClF3 are T-shaped.   T-SHAPED 7/25/2019 Dr Seemal Jelani

This leaves the two axial- positioned atoms and one of the equatorial-positioned atoms in a T-shape. T-SHAPED 7/25/2019 Dr Seemal Jelani

Generic Formula: T-SHAPED  MX3E2 (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons). T-SHAPED 7/25/2019 Dr Seemal Jelani

T-SHAPED 7/25/2019 Dr Seemal Jelani

Molecules with two atoms and three pairs of nonbonding electrons around a central atom such as XeF2 are linear   LINEAR 7/25/2019 Dr Seemal Jelani

These molecules are basically Trigonal bipyramidal molecules with all three of the equatorial- positioned atoms (in the Trigonal plane) each replaced by a pair of nonbonding electrons Linear 7/25/2019 Dr Seemal Jelani

This leaves only the two axial- positioned atoms which are still 180 from each other on opposite ends of the central atom. LINEAR 7/25/2019 Dr Seemal Jelani

Generic Formula:   MX2E3 (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons) LINEAR 7/25/2019 Dr Seemal Jelani

LINEAR 7/25/2019 Dr Seemal Jelani

Molecules with five atoms and one pair of nonbonding electrons around a central atom such as BrF5 are square pyramidal SQUARE PYRAMIDAL 7/25/2019 Dr Seemal Jelani

These molecules are basically octahedral molecules with one of the attached atoms replaced by a pair of nonbonding electrons   SQUARE PYRAMIDAL 7/25/2019 Dr Seemal Jelani

This leaves four atoms in a plane as a square base and one atom positioned perpendicular (90)  to this plane. SQUARE PYRAMIDAL 7/25/2019 Dr Seemal Jelani

Generic Formula:   MX5E (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons) SQUARE PYRAMIDAL 7/25/2019 Dr Seemal Jelani

SQUARE PYRAMIDAL 7/25/2019 Dr Seemal Jelani

Molecules with four atoms and two pairs of nonbonding electrons around a central atom such as XeF4 are square planar.   SQUARE PLANAR 7/25/2019 Dr Seemal Jelani

These molecules are basically octahedral molecules with two of the attached atoms opposite each other around the central atom each replaced by a pair of nonbonding electrons.  This leaves four atoms in a square plane. SQUARE PLANAR 7/25/2019 Dr Seemal Jelani

Generic Formula:   MX4E2 (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons) SQUARE PLANAR 7/25/2019 Dr Seemal Jelani

SQUARE PLANAR 7/25/2019 Dr Seemal Jelani