Knowledge of shapes of a molecule are important for
determining functions and activity of molecules. Example:
proteins enzymes of a particular shapes serve a particular
function and activity because an enzyme’s shape(key) is
will only fit one other substrate( lock).
General outcome: In this lesson, students will describe the
roles of modelling and use theory of modelling in
explaining and predicting the structure, bonding and
properties of ionic and molecular substances.
Knowledge of shapes of a molecule are important for
determining functions and activity of molecules.
Example: proteins enzymes of a particular shapes
serve a particular function and activity because an
enzyme’s shape(key) is will only fit one other
substrate( lock)
Molecular shapes cont’d
General learner outcomes
 20–A2.1k recall principles for
assigning names to molecular
substances.
 20–A2.3k relate electron
pairing to multiple and
covalent bonds
 20–A2.2k explain why
formulas for molecular
substances refer to the
number of atoms of each
Specific learner outcomes
 20–A2.4k draw electron dot
diagrams of atoms and
molecules, writing structural
formulas for
 molecular substances and using
Lewis structures to predict
bonding in simple molecules
 20–A2.5k apply VSEPR theory to
predict molecular shapes for
linear, angular (V-shaped, bent),
 tetrahedral, trigonal pyramidal
and trigonal planar molecules
Teaching Department

Professor: Dr.
Carbonaro
Lab
Instructor
Teaching
Assistants
Head marker
Jeff Chu,
Jackson
Warner
Marking
Assistnats
EDIT Course: Mark Weighting
Final
Labs
Midterm
Using VSEPR to predict shapes…
 Valence-shell-electron-pair-replusion theory was designed by Linus Pauling in the late
1930s. The theory was based on the electron repulsion created by lone electrons and
bonding electron that surround the central atom of a molecular structure.
 According to VSEPR theory:

 Only the valence e- of the central atom are important in determining the overall shape of
a molecule.

 Valence e- are paired in a molecule or polyatomic ion.

 Bonded pairs of electrons and lone pairs of e- are treated equally.

 Valence e- repel one another (very important)

 Finally, the molecular shape is determined by the positions of the electron pairs when
they are maximum distance apart
Drawing Lewis structures
 First, consider all electronegative atoms and place the
atom with lowest electro negativity in the center of
structure.
 Place atoms of high electro negativity around the
central atom.
 Count the total number of e- and subtract this from
the e- already present in structure.
 Finally, place all ‘left over’ electrons on the central
atom.
VSEPR theory continued…
 Key concepts to consider when drawing stuctures to
determine shape.
 1. First draw the Lewis dot diagram and consider ALL
arrangement.
 2. Draw any left over electron in a way that they are far
apart from one another so that repel each other.
 3. Before determining shape look at the central atom
(noted by letter A), then at other surrounding bonded
atoms (noted by letter X), then finally lone pairs
(noted by letter E)
VSEPR





AX2- denotes as having 1 central atom, 2 surrounding bonding
atoms and zero lone pairs. Shape: linear. Example: CO2
AX3- 1 central atom, 3 bonding atoms, zero lone pairs. Shape:
trigonal planar. Example: BH3
AX4: 1 central atom, 4 bonding atoms, zero lone pairs. Shape:
tetrahedral. Example: CH4, SiH4.
AX3E1: 1 central atom, 3 bonding atoms, 1 lone pair. Shape:
Trigonal pyramidal. Example: NH3
AX2E2- I central atom, 2 bonding electrons, 2 lone pairs. Shape:
angular or bent-shaped. Example: H20
AX3E1- I central atom, 1 bonding atom, 3 lone pairs. Shape:
linear. Example: HCl
Guided practice
 Example 1 – Draw sulfur dioxide. Include number of
bonding atoms, lone pairs and VSEPR shape.
 Example 2- Draw chloromethane. Include number of
bonding atoms, lone pairs and VSEPR shape.
 Example 3 – Draw oxygen. Include number of bonding
atoms, lone pairs and VSEPR shape.
 YouTube - Chemistry VSEPR Theory
 http://www.youtube.com/watch?v=i3FCHVlSZc4