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5/28/2018 Chapter 4 (Part 1)
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Chemical Bonds
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Learning Outcomes describe ionic (electrovalent) bonding, as in sodium chloride and magnesium oxide,
including the use of dot-and-cross diagrams
describe, including the use of dot-and-cross diagrams,
(i) covalent bonding, as in hydrogen, oxygen, chlorine, hydrogen chloride, carbon
dioxide, methane, ethene
(ii) co-ordinate (dative covalent) bonding, as in the formation of the ammonium
ion and in the Al2Cl 6molecule
explain the shapes of, and bond angles in, molecules by using the qualitative model
of electron-pair repulsion (including lone pairs), using as simple examples: BF3
(trigonal), CO2 (linear), CH4 (tetrahedral), NH3 (pyramidal), H2O (non-linear), SF6
(octahedral)
describe covalent bonding in terms of orbital overlap, giving and bonds
explain the shape of, and bond angles in, the ethane, ethene and
benzene molecules in terms of and bonds
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Learning Outcomes predict the shapes of, and bond angles in, molecules analogous to those specified
above.
describe hydrogen bonding, using ammonia and water as simple examples of
molecules containing N-H and O-H groups
explain the terms bond energy, bond length and bond polarity and use them to
compare the reactivities of covalent bonds
describe intermolecular forces (van der Waals forces), based on permanent and
induced dipoles, as in CHCl 3(l); Br2(l) and the liquid noble gases
describe metallic bonding in terms of a lattice of positive ions surrounded by
mobile electrons describe, interpret and/or predict the effect of different types of bonding (ionic
bonding, covalent bonding, hydrogen bonding, other intermolecular interactions,
metallic bonding) on the physical properties of substances
deduce the type of bonding present from given information
show understanding of chemical reactions in terms of energy transfers associatedwith the breaking and making of chemical bonds
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Chemical Bonds
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Chemical Bonds Chemical bond is the intermolecular forces that hold atoms together in
molecules.
Bonding involves only the valence electrons. There are 2 main types of chemical bond:
a) Ionic bond
- electrons are transferred from one atom to another
b) Covalent bond
- bonding occurs when atoms share electrons
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Ionic Bonding Ionic bonds are formed by one atom transferring electrons to another
atom to form ions. Ions are atoms, or groups of atoms, which have lost or
gained electrons.
Ions of opposite charge will attract one another, thus creating an ionic
bond.
It is the electrostatic force that hold the ions of opposite charge together.
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Ionic BondsExample: NaCl
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Ionic compound
NaCl, have an extended crystal lattice with non-metal anions
electrostatically attracted to adjacent metal cations and metal cations
electrostatically attracted to adjacent non-metal anions
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ExerciseDescribe the ionic bond formation in the following compounds:
a) Magnesium oxide
b) Calcium sulfide
c) Potassium nitride
d) Magnesium chloride
e) Calcium phosphide
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Covalent bond
Covalent bonds are formed as a result of the sharing of one or
more pairs of bonding electrons.
Example : H2
Two hydrogen atoms share their single electrons and form a
covalent bond.
H + H H H H-H
electrons to share a shared pair a covalent bond
of electrons
The Covalent Bond Model
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Covalent BondsExample : H2and H2O molecule
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Bonding electrons
Pairs of valence electrons that are shared
between atoms in a covalent bond
Nonbonding electrons
Pairs of valence electrons on an atom that are
not involved in electron sharing
Bonding and Nonbonding Electrons
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Number of covalent bonds depends on the
number of unpaired valence electrons of the
central atom.
To obey octet rule by sharing electrons from
other atoms in the molecule.
Covalent Compounds
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Single covalent bond
A covalent bond in which 2 atoms share 1 pair of
electron
Double covalent bond
The sharing of two pairs of electrons
Triple covalent bond
Three pairs of electrons are shared
Multiple Covalent Bonds
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Multiple Covalent Bond
Example of covalent compound with a double bond is oxygen gas.
O = O
Example of covalent compound with a triple bond is nitrogen gas.
N N
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Explain the bond formation in the following compounds.
a) Hydrogen fluoride
b) Methane
c) Carbon dioxide
d) Ethene
Exercise
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Covalent Compound
Silicon dioxide, SiO2, is a molecular compound. It is also a mineral called quartz (left).
Quartz is found in nearly every type of rock. Most sand grains (center) are bits of quartz.
Glass is made from sand.
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Both electrons of a shared pair come from one of the
two atoms involved in the bond
One atom has a pair of nonbonding electrons Another atom has two or more empty space in its
valence shell.
The result of overlap of a filled and a vacant orbital.
Co-ordinate Covalent Bond
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Co-ordinate covalent bond is represented
by an arrow.
Once a co-ordinate bond is formed, it
cannot be distinguished from other
covalent bonds. All electrons are identical and all the bonds
are of the same strength.
Co-ordinate Covalent Bond
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Both electrons from the nitrogen are shared with the upperhydrogen
Other example: Carbon monoxide
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Co-ordinate Covalent Bond
At high temperatures aluminium chloride exists as molecules withthe formula AlCl3(electron deficient).
At lower temperatures two molecules of AlCl3 combine to form a
molecule with the formula Al2Cl6 .
The molecules are able to combine because lone pairs of electronson two of the chlorne atoms form co-ordinate bonds with the
aluminium atoms.
AlCl
Cl Cl
Cl Al Cl
Cl
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Geometry of the atoms around a central atom can be
predicted by the electron-pair repulsion theory.
This theory is used to determine the shape whichlooks at the geometry of the electron groups around a
central atom.
Molecular Geometry
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Bond angles
The shape and bond angles of a covalently bonded molecule
depend on:
a) the number of pairs of electrons around each atom
b) whether these pairs are lone pairs or bonding pairs
Lone pairs occupy more space than bonding electron pairs.
Double bonds occupy more space than single bonds.
LP-LP > LP-BP > BP-BP
Lone pairs are more repulsive than bonding pairs
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Shape of Molecules
Sets
(group of bonding
pairs/number of
bonded atoms)
Lone
Pairs
Shape
2 0 Linear 180o
2 2 Bent 104.5o
3 0 Trigonal planar 120o
3 1 Trigonal pyramidal 107.3 o
4 0 Tetrahedral 109.5o
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Shape of Molecules
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Shapes of Molecules
Silicon tetrachloride has a tetrahedralstructure.
4 bonding pairs of electrons and no lone
pairs
Equal repulsive forces of each bonding
pair of electrons
All ClSiCl bond angles being 109.5o
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Shapes of Molecules
Lone pairbond pair repulsion is greater than bond pairbond
pair repulsion
Bonding pairs of electrons are pushed closer together.
The HNH bond angle is about 107o
3 bonding pairs of electrons
1 lone pair
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Examples
Arrangement of
electron pairs on
central atom
Number of
bonding
electron pairs
Example
Linear 2 BeCl2
Trigonal planar 3 BCl3
Tetrahedral 4 CH4
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Learning Check
1. Predict the shapes for the following molecules:
a) hydrogen sulfide, H2S
b) phosphine, PH3
c) boron chloride, BCl3 (SPECIAL!! BCl3 does not fulfill octet rule)
d) carbon dioxide, CO2
e) nitrogen trichloride, NCl3
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