BELLRINGER
Explain in complete sentences the
difference between various
Structures of chemical bonds
Form an ionic bond between
the following
• Na and Cl
• Cl and F
• Na and Mg
• Mg and Br
+
• NH4 OH
• NH4+ SO42-
Covalent Bonding
The Octet Rule
• The octet rule says that atoms tend to
gain, lose or share electrons so they have
eight electrons in their outer shell.
Covalent Bonding
• Instead of gaining or losing electrons atoms
can get stable by sharing electrons
• This is always between two non-metals.
• Two fluorine atoms, for example, can form
a stable F2 molecule in which each atom
has 8 valence electrons by sharing a pair of
electrons.
• In covalent bonds they can share
more than two electrons
Single, Double and Triple Bonds
• With Covalent bonds the elements can share
two or more electrons
• A Single Bond is when 2 electrons are shared
they are represented by a single line in bond
diagrams
• A Double bond is when 4 electrons are shared
they are represented by two lines in bond
diagrams
• A Triple bond is when 6 electrons are shared
they are represented by three lines in bond
diagrams
• Oxygen has six valence electrons and is
seeking an octet. Therefore it combines
with two hydrogen atoms, each with one
valence electron.
Representing covalent molecules
• Chemists use two types of diagrams to
represent molecules:
1. Electron dot formula (Lewis structure) –
each outer shell (valence) electron is
represented by a dot.
2. Valence structure – lines are used to
represent electrons: one line represents
one pair of electrons.
Lewis Dot Structure
• In order to draw covalent bonds you must be
able to write Lewis Dot structure
• This shows the number of valence electrons
around the symbol
• Put one electron on each side before
doubling up.
• We can see that helium does not follow this
rule exactly.
• First, we must determine whether CO2 is a
covalent or ionic bond.
• Because nonmetals form covalent bonds, CO2
will be covalent.
• Ionic bonds form when combining both metals
and a nonmetals.
• Begin by drawing the Lewis dot structure of
each individual atom.
•
•
•
•
How many electrons does the carbon want?
4
How many electrons does each oxygen want?
2
•
•
•
•
Determine which is the central atom
All three of these atoms want an octet.
Carbon needs to “share” four electrons.
Carbon will share two of its electrons with
the oxygen on the right and two with the
oxygen on the right.
• The carbon dioxide Lewis Dot Structure:
:C:
• Take note of the boxed in electrons below.
:C:
• Now each atom has eight electrons.
What type of bonds does CO2 have?
Double bonds
Draw and complete the following table
Name/
No.
E dot
Formulae Valence diagram
electrons
HCl
H=1
Cl = 7
Methane C = 4
CH4
H=1
Oxygen
(02)
Bond
diagram
Type of
bond
O=
Include the following compounds CS2, NH3 , CF4.
Naming Covalent formulas
1. First name generally stays the same.
2. Second non metal name changes to –ide.
3. If there is one or more of the second
element it will have prefix in front
e.g. Mono (means 1) or di (2) or tri (3) tetra (4)
penta (5) e.g. CO2 Carbon Dioxide
3. If there are two or more of the first element
it must have a prefix
e.g. di (2) or tri (3) tetra (4) penta (5) Dihydrogen
sulfide (H2S)
Naming Covalent compounds
•
•
•
•
•
•
What is dihydrogen monoxide?
H2O
Name CO
Carbon Monoxide
Name P2S3
Diphosphourous trisulfide
Valence electrons
• In covalent bonds when electrons are shared the
electrons spend most of their times between the two
nuclei of the atoms. The electrons are called
localised electrons.
• The shared electrons are called the bonding
electrons
• The valence electrons not involved in bonding are
Lone pair
called the non-bonding electrons.
• Pairs of non-bonding electrons
are called lone pairs.
Pair of bonding
electrons
Your turn
• How many bonding electrons does Cl2
have?
• How many non-bonding electrons does Cl2
have
• How many lone pairs does Cl2 Have?
Covalency
• Chlorine needs to share one electron to
give it a stable outer shell. Therefore its
has a covalency of one.
• The covalency of an atom is generally the
number of electrons it shares when
bonding with another non metal atom.
Draw and Complete the following table
Name/
Formulae
Fluorine
(F2)
Methane
CH4
No.
No. of
No. of
bonding non
lone
electrons bonding pairs
electrons
Covalency
C=
H=
Oxygen
(02)
Include the following compounds HCl, CS2, NH3 ,
CF .
Charge Cloud Model
• This Model of the atom pictures the
electrons moving around the nucleus in a
region called an electron cloud.
• The electron cloud is a cloud of varying
density surrounding the nucleus. The
varying density shows where an electron
is more or less likely to be.
• an 'electron cloud' may be a single, double
or triple bond, or a lone pair of electrons
What is the Electron Cloud Model?
Diagram 1:
What is the Electron Cloud Model?
Diagram 2:
Shapes of molecules
• The shape of a molecule describes the way
in which the atoms are arranged
• Molecules can be linear, angular v-shaped
or bent, triangular pyramid and tetrahedral.
• There are more shapes but you only need
to the know the above
http://www.youtube.com/watch?v=FhVkCH9COZo
http://www.youtube.com/watch?v=q6g7BUruUK0&feature=r
elated
http://www.youtube.com/watch?v=ra0A8Ni1DAA&feature=re
lated
Shape of Name of
Molecule Shape
No. of
Lone
pairs
Bond
angle
Examples
Linear
0
180
CO2, HCN
Bent/
Angular
Triangular
pyramid
2
<109.5
H2O, SCl2
1
<109.5
NH3, PCl3
109.5
CH4, SiF4
Tetrahedral 0
Trigonal Planar 0 120° sulfur trioxide
The molecule is all in the same plane
and is two dimensional.
Activity
• Use Balloons and string to make the shape
of a molecule you have been assigned
• Use the molecular models to make all the
other shape molecules
• x
Valence
Shell
Electron
Pair
Repulsion
Trigonal Planar
Theory
Tetrahedral
Trigonal
bipyramidal
Octahedral
The valence shell electron pair
repulsion (VSPER) model
• VSPER is used for predicting the shapes of
individual molecules, based upon the extent of
electron-pair repulsion (to push away).
• a lone pair of electrons is a non-bonding pair of
electrons
• 'electron clouds' are negatively charged since the
electrons are negatively charged, so electron
clouds repel (push away) from one another and try
to get as far away from each other as possible.
The valence shell electron
pair repulsion (VSPER) model
RULES
1. Lone electron pairs in the outer shell of an atom in a
molecule will repel each other the most due to their
negative charge
2. lone pairs of electrons exert a greater repelling effect
than bonding pairs do
3. lone pair-bonding pair repulsion is greater than
bonding pair-bonding pair repulsion
lone pair-lone pair repulsion > lone pair-bonding pair
repulsion > bonding pair-bonding pair repulsion
How to determine shapes of molecules
1. Decide which atom is the central atom
2. Determine the number of valence electrons for each
atom
3. Arrange the electrons to include bonding
4. Draw the bonding electrons and lone pairs so they are
as far apart as possible
5.Count the total number of electron pairs around the
central atom, including both the bonding pairs and
lone pairs. Double and triple bonds are counted as on
bonding pair.
6. Use the VSEPR geometry table to predict the
geometry of the molecule (it can only be one of 5
shapes!!).
How to determine shapes of
molecules
• Sometimes the molecules are represented by
AXY, A is the central atom, X is the 2nd atom and
Y is the # of peripheral atoms
• AX2 = linear or bent
• AX3 = trigonal planar or trigonal pyrimadial
• AX4 = tetrahedral (tetra = 4 faces)
• AX5 = trigonal bipyramidal (2 pyramids)
• AX6 = octahedral (octa = 8 faces)
Which shapes do you recognise?
• x
Your turn
• Use Lewis structures, VSPER and the
molecular models to help you determine
the shape of the following molecules.
• Carbon tetrachloride
• Silicon disulfide
• Dicarbon dihydride
• Boron trihydride
• Phosphorous trifluoride
What is Electronegativty?
• Electronegativity: is a measure of an
elements ability to attract electrons
• The electronegativity of elements tends to
increase from left to right across a period.
• Electronegativities decrease from top to
bottom within a group.
• Fluorine is the most electronegative atom
Electronegativity Values
• The electronegativity values can be found
in the periodic table
• The higher the value the higher the
electrotnegativity
• The Pauling scale is used to measure
electronegativity. It is a relative scale
running from 0.7 to 4.0 (hydrogen = 2.2).
• The units for electronegativity are Pauling
units.
• x
There are two types of covalent
bonding
1. Non-polar bonding with an equal sharing
of electrons.
2. Polar bonding with an unequal sharing of
electrons. The number of shared electrons
depends on the number of electrons
needed to complete the octet.
1. Non Polar Bonds
• What does polarised/polarity mean?
• Opposites
• Non polar bonding results when two identical
non-metals equally share electrons between
them.
• One well known exception to the identical
atom rule is the combination of carbon and
hydrogen in all organic compounds.
• Non polar bonding exists between carbon and
hydrogen
• Iodine (I2) and oxygen (O2) forms a
diatomic (di – two, atomic – atoms) nonpolar covalent molecule.
• x
2. Polar Bonds
• Polarised bonds occur in covalent molecules.
One atom has greater electronegativity and
therefore has a partial (slight) negative charge
because it attracts the bonding electrons more
strongly (remember electrons have a –ive
charge).
• The other atom has a slight
positive charge.
Partial
positive is
represent
ed by this
symbol
Partial negative
is represented
By this symbol
• x
Which is more electronegative H or
Cl?
• Cl has greater electronegativity
• The bonding electrons therefore spend more
time near the Cl atom
• So the Cl side of the molecule becomes slightly
negative (partial negative
charge) and the H side becomes
slightly positive making a
polarised bond
• What about HF?
Polar molecules
• Polar molecules are those containing polar
bonds that do not cancel each other out
• Molecular polarity is dependent on the
difference in electronegativity between atoms
in a compound and the asymmetry of the
compound's structure.
• For example, a molecule of water is polar
because of the unequal sharing of its
electrons in a "bent" structure, whereas
methane is considered non-polar because
the carbon shares the hydrogen atoms
uniformly.
Polar
• The covalent bond between HCl is said to
be polarised and the molecule is called a
polar molecule
• It is also said to be a dipole
• because is has two charged ends
Non polar molecules
• Non polar molecules are those that
contain only non-polar bonds
OR
• Molecules in which polar bonds cancel
each other out
Quiz
• What happens to the electrons in a polar
bonds?
• How do you know if a bond is polar?
• What is a non polar molecule?
• What is a dipole?
• What is the symbol for slightly negative?
• Can non polar molecules have polar
bonds?
How to determine polar molecules
There are two important factors
1. The polarity of the individual bonds in the
molecule;
2. The shape or geometry of the molecule.
Steps to take
a) Determine if a given individual bond is polar,
Look at the difference between
electronegativity of the atoms in the perioidc
table. If the difference is:
0.3 < non polar
0.3 -1.7 = polar
b) Determine the shape of molecule. Then
there are 3 option
i) if all bonds are non-polar, then the whole
molecule is non-polar regardless of its shape.
ii) If there is symmetry in the molecule so that the
polarity of the bonds cancels out, then the
molecule is non-polar. (symmetry arround the
central atom)
iii) If there are polar bonds but there is no
symmetry the overall molecule is polar.
Which molecules are polar?
Determine if BF3 is polar or non
polar
• B-F bonds are polar but they are
symmetrically arranged around the central
fluorine atom. No side of the molecule has
more negative or positive charge than
another side, and so the molecule is
nonpolar.
Predict whether the following molecules
are polar or nonpolar:
(a) BrCl; (b) SO2;
• SOLUTION (a) Chlorine is more electronegative
than bromine. Consequently, BrCl will be polar with
chlorine carrying the partial negative charge:
• (b) Because oxygen is more electronegative than
sulfur, the molecule has polar bonds.
• For each of these, the VSEPR model predicts a
bent geometry. Because the molecule is bent, the
bond dipoles do not cancel and the molecule is
polar
Which of the following molecules
contains polar bonds but is nonpolar?
a) H2O
b) NH3
c) F2
d) CCl4
e) CH2Cl2
• In CCl4, there are 4 bonds arranged
symmetrically around the central carbon
atom.
• Each bond is polar, you can tell due to the
electronegativity differences between
carbon and chlorine.
• Because the 4 atoms of chlorine are
arranged symmetrically, the polarity of the
bonds cancel each other out, making a
non-polar molecule.
Determine if OF2 is polar or non polar
• The electronegativities of oxygen and
fluorine, 3.44 and 3.98, respectively,
produce a 0.54 difference that leads us to
predict that the O-F bonds are polar. The
molecular geometry of OF2 is bent. Such
an asymmetrical distribution of polar
bonds would produce a polar molecule.
• When there are no polar bonds in a
molecule, there is no permanent charge
difference between one part of the
molecule and another, and the molecule is
nonpolar.
What is the difference between a
polar bond and a polar molecule?
• A polar bond is a charged bond. There is a
positive and negative end that do not
cancel each other out
• A polar molecule contains polar bonds that
do not cancel each other out.
ACTIVITY
Discuss in a group and write a report about a molecule or
compound given to you. Present your report to class.
Distribution of work: person 1 – geometric structure of
molecule or compound, 2 – properties of molecule or
compound, 3 – properties if element 1, 4 - properties of
elements 2 and 3.
Distribution for groups:
Group I – H2O, NaCl; II – CO, O3; III – KBr, NH3;
IV – LiF, SO2; V – HCl, O2; VI – SO, F2; VII – SO2,Cl2
VIII – CO2, Br2