Transcript Document
Valence Shell Electron Pair Repulsion Theory
What is the shape of BF
3
?
You do not need to take notes, this is just a review on what we did before our break --- just listen and watch.
1.
Predicting molecular structures: The Strategy
Draw the correct Lewis dot structure .
• Identify the central atom.
• Designate the bonding pairs and lone pairs of electrons on central atom.
2.
Count the regions of “high electron density” (HED) on the central atom.
Each LONE PAIR on the central atom is one HED.
Each ATOM bonded to the central atom is one HED.
3. Determine the arrangements of LONE PAIRS and ATOMS around the central atom. • VSEPR is a guide to the arrangement.
4. Determine the
molecular
shape around the central atom.
• Ignore the lone pairs of electrons.
5. Adjust molecular shape for the effect of any lone pairs (repulsions).
Counting regions of HIGH ELECTRON DENSITY
VSEPR Theory: Geometries/Shapes
•
Main guiding principle: Regions of high electron density around the central atom are arranged as far apart as possible to minimize repulsions
.
• There are
six
basic electronic arrangements based on the number of regions of high electron density around the central atom.
VSEPR Theory
Two
regions of high electron density around the central atom.
LINEAR
VSEPR Theory
Three
atom.
regions of high electron density around the central
TRIGONAL PLANAR
VSEPR Theory
Four
regions of high electron density around the central atom.
TETRAHEDRAL
VSEPR Theory
Five
regions of high electron density around the central atom (Note: This has to be an EXCEPTION because it means 10 valence electrons instead of 8.)
Trigonal bypramidal
VSEPR Theory
Six
regions of high electron density around the central atom. (Note: This has to be an EXCEPTION because it means 12 valence electrons instead of 8.)
OCTAHEDRAL
VSEPR Theory
Molecular shape
determined by the arrangement of atoms around the central atom(s).
Electron pairs are determination, only the positions of the atoms are used.
not
used in the molecular shape
NOW, Take out your Green “VSEPR” Sheet.
Example
Shape if you were considering the atoms AND the unshared pairs of electrons H · · ·· O ·· H · · Shape of molecule with only ATOMS and WITHOUT the unshared pairs HED: Bonded atoms: 4 4 4 2 Electronic arrangement : tetrahedral tetrahedral (this is the column marked “Geometry of Electron Pairs” on your green “VSEPR” sheet.) Molecular shape : tetrahedral bent (this is the column marked “Geometry of Atoms” on your green “VSEPR” sheet.)
Both have “tetrahedral” arrangements of both atoms and electrons because HED = 4
H · · ·· O ·· H · ·
Tetrahedral
H H H C H Bent
Molecular shapes
VSEPR Theory and Shape
• Lone pairs of electrons (unshared pairs) require more volume than shared pairs.
Repulsion of other atoms
CH
4
vs. NH
3 Note: Both CH 4 and NH 3 have four regions of HED and have tetrahedral electronic arrangements. Note how the angles change (but just a little)!
Molecular Shapes and Bonding
• In the next sections we will use the following terminology:
A = central atom B = bonding pairs around central atom E = lone pairs around central atom
• For example: AB 3 E designates that there are 3 bonding pairs and 1 lone pair around the central atom.
Linear Molecules: AB 2 Species (No Lone Pairs of Electrons on A) • Some examples of molecules with this arrangement are:
BeCl 2
, BeBr 2 , BeI 2 , HgCl 2 , CdCl 2 • All of these examples are linear, molecules.
Linear Molecules: AB
2
Species
Molecular shape H H H C H
Linear Molecules: AB
3
Species (no lone pairs)
• Some examples of molecules with this arrangement are:
BF 3
, BI 3 BCl 3 BBr 3 • All of these examples are planar molecular
Molecular shape
BCl
3
H H H C H
So Which on is the shape of BF
3
according to VSEPR Theory?
A or B
Answer: A
• • Because in the shape of “B”, the electrons are not spaced evenly apart from each other.
A B
Review of Strategy:
Lewis dot structure # Regions high electron density Show where the Atoms and Lone Pairs are on around the Central Atom Lone pairs on central atom?
Molecular shape
Tetrahedral Molecules AB 4 : (Four regions of high electron density) • Some examples of molecules with this arrangement are: CH 4 , CF 4 , CCl 4 , SiH 4 , SiF 4 • All of these examples are tetrahedra
Dot Formula
CH 4
# HED = 4
What is the shape of CH
4
?
Molecular shape CH 4 H H H C H
Tetrahedral Molecular Shape: CH
4
The shape looks like a tetrahedron!
Other Examples of Tetrahedral Shapes
Moving ahead….
• Trigonal bipyramidal and Octahedral • Molecules that have
unshared
electrons on the central atom (lone pair)
“Bent” Molecules: AB 2 E Species
Recall BCl 3
H H H C H
“Bent” Molecules: AB 2 E Species • Consider SO 2
.. ..
.. .. ..
(Remember that A is the central atom, X =bonded atoms and E = unshared pair of electrons)
# HED = 3
Both have the same arrangement (trigonal planar) when you consider lone pairs and atoms but DIFFERENT molecular shapes:
Trigonal planar Angular
H H H C H H H H C H BF 3 SO 2
Trigonal Pyramidal Molecules: AB 3 E Species (One Lone Pair of Electrons on A) NH 3 , NF 3 , PH 3 , PCl 3 , AsH 3 • All have one lone pair of electrons on the central atom.
• NH 3 and NF 3 are trigonal pyramidal
Bent Molecules: AB 2 E 2 Species (Two Lone Pairs of Electrons on A) • Some examples of molecules with this arrangement are: H 2 O , OF 2 , H 2 S • These molecules are our first examples of central atoms with
two
lone pairs of electrons.
H 2 O
H 2 O Molecular shape H C H H H
(You can’t see the lone pairs, so the molecule looks like an inverted vee.)
Trigonal Bipyramidal Molecular Shape Some examples of molecules with this arrangement are: PF 5 ,
AsF 5
, PCl 5 , etc.
• These molecules are examples of central atoms with
five
regions of high electron density:
expanded octet
! This means they have 10 electrons instead of 8.
Molecular shape
(This is the best way to arrange five atoms around a center atom)
Trigonal Bipyramid
PF 5 Trigonal Bipyramidal Electronic arrangement
Trigonal Bipyramidal with Lone Pairs.
AB 5 , AB 4 E, AB 3 E 2 , and AB 2 E 3 • If lone pairs are incorporated into the trigonal bipyramidal structure, there are three possible new molecular shapes. 1. One lone pair - Seesaw shape 2. Two lone pairs - T-shape 3. Three lone pairs – linear 4.
THESE ARE JUST INTERESTING, BUT I WON’T ASK YOU TO DO ANY OF THESE.
Example: SF 4
# HED = 5 Arrangement with the Lone Pairs : trigonal bipyramidal Molecular shape: seesaw
Ex. ClF 3
# HED = 5 Arrangement with the Lone Pairs: trigonal bipyramidal Molecular shape: t-shaped
Ex. XeF 2 H H H C H
# HED = 5 Arrangement with the Lone Pairs: trigonal bipyramidal Molecular shape: linear
Octahedral Molecular Shapes with and without lone pairs: AB 6 , AB 5 E, and AB 4 E 2 • Some examples of molecules with this arrangement are: SF 6 , SeF 6 , SCl 6 , etc.
• These molecules have central atoms with
six
regions of high electron density. This means they have 12 electrons instead of 8!
• You should be able to do only AB 6
• • Octahedral Electronic arrangement: AB 6 , AB 5 E, and AB 4 E 2 If lone pairs are incorporated into the octahedral structure, there are two possible new molecular shapes. 1. One lone pair - square pyramidal 2. Two lone pairs - square planar All octahedral shapes are exceptions because they need 6 bonds, which means 12 total valence electrons instead of 8.
SeF 6 or SF 6
e # HED = 6 Molecular shape: octahedral
IF 5
# HED = 6. This is an exception because I has 10 total electrons!!!
Molecular shape: square pyramidal
XeF 4
# HED = 6, This molecule is an EXCEPTION Xe doesn’t normally make bonds because it is a Noble gas. But here it has 12 total electrons!! It would take a lot of Energy to do this.
Molecular shape: square planar