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.
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