COVALENT BONDING
COVALENT BONDING
•
This occurs when two non-metallic atoms _________ electrons in order to obtain
the stable number of eight electrons in their outer shells.
•
The atoms share pairs of electrons in overlapping partially filled atomic orbitals.
Each shared pair of electrons forms a ______ bond and the molecule formed is a
covalent molecule.

E.g.
chlorine gas – Cl2. p112
LEWIS DIAGRAMS

Lewis diagrams (or Electron Dot Diagrams) – only include the ________ or outer
shell electrons to simplify the above drawings.

The nucleus and inner-shell electrons are represented by the element ________
and the valence electrons are shown as dots.
Eg. phosphorus
Unpaired – bonding electrons
Paired – non-bonding electrons or ‘lone pairs’
..
. P .
.
LEWIS SYMBOLS

The outer shell electron arrangements of the elements in periods 2
and 3 can be simplified into LEWIS SYMBOLS.
Cl
Ar
S
C
N
O
F
Ne
NOTE: A shared pair of electrons is represented by a ________ _______ in a
structural formula : Cl – Cl
•When more than one pair of electrons are shared a multiple covalent bond is
formed.
•Two pairs = double bond
Eg. oxygen O2
•three pairs  triple bond
E.g. nitrogen N2
•Covalent bonding also occurs between different elements to form covalently
bonded compounds such as water, carbon dioxide and methane.
The formation of water:
NB.
Bonding electrons and non-bonding electrons or (lone pairs.)
•Now you draw the following molecules – hydrogen H2, hydrogen chloride HCl,
ammonia NH3, methane CH4, difluoromethane CH2F2, carbon dioxide CO2,
ethane C2H6. Electron dot diagrams and structural formulae.
COVALENT BONDING TO FORM MOLECULAR
ELEMENTS/DIATOMIC MOLECULES
Single bond
Single Bond
Triple bond
COVALENT MOLECULES
METHANE
AMMONIA
WATER
HYDROGEN
FLUORIDE
SHAPES OF COVALENT MOLECULES:
•Covalent molecules are _______ dimensional in shape. These shapes are
extremely important in predicting and understanding the properties of many
covalent substances.
• Diamond (an allotrope of carbon) is very hard and brittle while graphite
(another form of carbon) is a good lubricant.
•These predictions are made using the VSEPR.
VALENCE SHELL ELECTRON PAIR REPULSION THEORY (VSEPR):
•This theory states that the electron pairs in the molecule _______ each other and
take up positions as far apart as possible (multiple bonds only count as one pair).
Eg. Methane and water
NOTE: Table 6.2 p112
SHAPES OF SOME COMMON MOLECULES
NAMING COVALENT COMPOUNDS
MOLECULAR ELEMENTS
•
Not all non-metallic elements are molecular. E.g. Helium and carbon.
•
All molecular elements are non-metallic elements.
•
These non-metallic molecules are known by their elemental name.
•
E.g. N2 is nitrogen
MOLECULAR COMPOUNDS
•
Molecular compounds contain discrete __________ units.
•
Binary molecular compounds are those which contain _____ elements.
•
There are four rules for naming binary covalent compounds:
→
The first element in the formula is named in ______.
→
The second element is named as if it was an anion and given the suffix – __.
→
The number of each type of atom in the molecule is indicated by a ______.
These prefixes are listed on page119 Note that the prefix mono is never
used for the first element.
→
If the name of the second element begins with a vowel and the prefix for
that name ends with a or o, these letters are dropped to make
pronunciation easier.
ELECTRONEGATIVITY
•
•
•
Different atoms have different electron _________ powers.
The relative attraction that an atom has for shared electrons in a
covalent bond is known as ____________.
The Pauling scale of electronegativities are shown below:
NON-POLAR AND POLAR MOLECULES
Differences between the electron attracting powers of the atoms in
covalent bonding is very important as it leads to ________ SHARING of
the pair of electrons forming the bond. When this happens the bond
becomes POLARIZED with a small ________ charge at one end and a
small negative charge at the other.
 Eg. the electron sharing in a H – O bond in water:
+






O
:
H
3.5
2.1
Oxygen gains an unequal share of the electron pair and therefore
gains a small negative charge ( -). Bonds of this type are called
_______ BONDS.
Molecules which contain polar bonds and which are not symmetrical
will be polarized and contain a permanent dipole.
Eg. the ammonia molecule NH3:
N
PERMANENT
3.0
DIPOLE
QUESTIONS 13-15
P.122
H
H
H
 - 2.1
INTERMOLECULAR ATTRACTIONS
•
How do molecules ‘stick’ together?
•
The attractions between molecules are called __________ forces of
attractions and these forces of attractions hold molecules to each
other.
•
They are ________ than covalent or ionic bonds but they are
responsible for whether the compound exists in the solid, liquid or
gaseous states.
•
The stronger the intermolecular forces the higher the melting or
boiling point.
•
The three types of weaker intermolecular forces are known as van
der Waal forces and these include _________ forces, dipole-dipole
interactions and ________ bonding.
DISPERSION FORCES
•
These are _______ bonds which exist between all molecules due to
the formation of ‘instantaneous dipoles’ within molecules from
random unequal distribution of their electrons.
•
These instantaneous dipoles are very ______ lived but may occur
thousands of times a second.
•
The resulting bonds are weak but ___________ with the number of
electrons in the molecules (more possible instantaneous dipoles)
and the similarity of the molecular shapes (fit more closely
together).
DISPERSION FORCES
DIPOLE-DIPOLE INTERACTIONS
•
When two dipolar molecules interact, the ______ end of one molecule attracts
the negative end of the other molecule (and visa versa) and the molecules
become bonded together by a dipole – dipole bond.
HYDROGEN BONDING
•
This is a special case of dipole – dipole interactions where _______ is bonded
to a highly electronegative element such as F, O and N, which also has nonbonding electron pairs available.
•
Quite a ________ bond forms between the  + charge on the H atom and the
non –bonding electron pair on the F, O or N atoms.
 These bonds are quite strong
and result in H-bonded
substances having ______ melting
points and boiling points than we
might expect. Eg. HF and H2O
PROPERTIES OF MOLECULAR COVALENT SUBSTANCES:
•
Strong bonding within molecules – weak bonding between
molecules.
•
The physical properties of a molecular substance is due largely to the
strength of the ____________ attractions.
•
A few generalisations can be made: (summarise dot points on P 128)
GIANT COVALENT LATTICES

TWO TYPES:
(i)
THREE-DIMENSIONAL
(ii) TWO-DIMENSIONAL

THREE DIMENSIONAL GIANT COVALENT NETWORK LATTICES
−
These consist of millions of atoms bonded to each other in a three
dimensional (3-D) array in all directions such that no individual molecules
exist and the array forms one giant ‘molecule’. Eg. a diamond p129, silicon
dioxide (quartz or sand), tungsten carbide (cutting tools).
−
COPY P 129 PROPERTIES OF COVALENT NETWORK LATTICES
TWO DIMENSIONAL COVALENT LAYER LATTICES

These consist of atoms held strongly together in ____ dimensional layers by covalent bonds,
while between the layers there are only weak Van der Waals bonds (dispersion forces). This
allows the layers to slide ‘freely’ over each other.

GRAPHITE (an allotrope of carbon) is a classical layer lattice: graphite is made of _____
sheets of C atoms bonded into hexagonal rings to three other C atoms.

This leaves one unused valence electron which is _________ among the other atoms in the
layer and gives the graphite many of it’s properties.

The unused electrons are ____________ but travel only along the layer occupied by their
parent by their ‘parent’ atom.
• The arrangement
around each
carbon atom in
graphite.
• Models of part of the
sheet of hexagonal
rings found in
graphite.
THE LAYERS OF GRAPHITE
•Copy the properties of graphite from P 131.
•Do the multiple questions and chapter review questions.