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A Level Chemistry
Chemical Bonding

Learning Outcome I
Ionic Bonding
• describe ionic bonding, as in sodium
chloride, magnesium oxide and calcium
fluoride, including the use of ‘dot-andcross’ diagrams

Ionic Bond
• The positive ion and the negative ion are strongly
attracted to each other
...
The electrons
are transferred from one atom to the other
...


Ionic Bond
Sodium Chloride

Na
——> Na+ + e¯
1s2 2s2 2p6 3s1 1s2 2s2 2p6

and

Cl + e¯ ——> Cl¯
1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p6 3s2 3p6

An electron is transferred from the 3s orbital of sodium to the 3p orbital of
chlorine; both species end up with the electronic configuration of the nearest
noble gas the resulting ions are held together in a crystal lattice by
electrostatic attraction
...

The arrangement of ions in a
crystal lattice depends on the
relative sizes of the ions
...


DOT & CROSS DIAGRAM
• Decide how many valence (outer shell) electrons are possessed by
each atom in the molecule
...

• Arrange the electrons so that each atom contributes one electron to a
single bond between each atom
...

• If the octets are incomplete, and more electrons remain to be shared,
move one electron per bond per atom to make another bond
...

• Redraw the dots so that electrons on any given atom are in pairs
wherever possible
...

• Charge of ion, written at the top right-hand corner of
the square brackets
...

a) Potassium Chloride
b) Sodium Oxide
c) Calcium Fluoride
d) Magnesium Chloride, MgCl2
e) Magnesium Oxide

[AS Nov 2006 Paper I Q7]

Magnesium oxide may be used for the lining of an electric
furnace for making crockery
...
Which type of bond needs to be broken
for magnesium oxide to melt?
A
...
Covalent
C
...
Metallic

[AS June 2008 Paper I Q4]

When barium metal burns in oxygen, the ionic compound
barium peroxide, BaO2, is formed
...

• The chemical bond that forms between non-metal atoms
when they share electrons is called a covalent bond
...

• So, each atom has a stable outer energy level some of the
time
...

• In diatomic molecules (like H2 or Cl2), the electronegativity
of both atoms are exactly the same so electrons are shared
equally
...

• Only nonmetals and hydrogen
...


Multiple Bonds
• Sometimes atoms share more than one pair of valence
electrons
...

• A triple bond is when atoms share three pairs of electrons
(6 total)
...

Cl2
2
...

CH4
4
...

CO2
6
...

HCl
8
...

At ground state, most elements follow the octet rule
...

• Sulfur and Phosphorus are good examples of elements that
commonly exceed the octet rule
...
Phosphorus (III) chloride
2
...
Sulfur hexafluoride
4
...

Donor species will have lone pairs in their outer shells
Acceptor species will be short of their “octet” or maximum
...

• The N now has a +ive charge as
• - it is now sharing rather than owning two electrons
...

It exists as a dimer (two molecules joined together)
...


Coordinate covalent bond
Class Exercise
Draw dot and cross diagrams to show the formation of coordinate bond between the followings:
a) Ammonium chloride, NH4Cl
b) Amino boron trifluoride, F3BNH3

Aluminium chloride sublimes at 178C
...

• Explain and predict the shapes of, and bond
angles in, molecules by using the VSEPR theory
using as simple examples: BF3 (trigonal), CO2
(linear), CH4 (tetrahedral), NH3 (pyramidal), H2O
(non-linear), SF6 (octahedral), PF5 (trigonal
bipyramidal)

Activity 1 (5 minutes)
In the mahjong paper provided,
1
...

2
...


VSEPR
Valence Shell Electron Pair Repulsion
• Predicts the three dimensional shape of molecules
...


– Valence shell = outside electrons
...


VSEPR
• Maximum angular separation between valence electron
pairs around the central atom
...

• Repelling power : lone pair–lone pair > lone pair–bond pair > bond
pair–bond pair

VSEPR
Things to remember:
• Based on the number of pairs of valence electrons, both
bonded and not bonded
...

• VSEPR can only be used with the central atom
...


To predict the shape of the molecules
...
Draw a dot and cross diagram to show total valence
electrons ( includes the charge)
2
...

3
...


VSEPR
• Names of Shapes:
• tetrahedral
• trigonal pyramidal
• Bent
• Linear
• trigonal planar
• trigonal bipyramidal
• octahedral

To predict the shape of the molecules
...
Draw a dot and cross diagram to show total valence
electrons ( includes the charge)
2
...

3
...


Activity 2 (6 minutes)
1
...

2
...

3
...

4
...

5
...


https://goo
...
The four H
atoms are arranged about the C
atom in a tetrahedral shape
...

The 109
...


BeCl2
The central Be atom has two bond pairs
Repulsion between these two pairs
causes the molecule to be linear with
180o
...


BF3

The central B atom has three bond
pairs
Minimizing the repulsion causes this
molecule to have a trigonal planar
shape with 120o

NH3

...

Due to the greater repelling character of
lone pairs, the H atoms in NH3 are bent
closer together than the normal tetrahedral
angle of 109
...


H 2O

Oxygen has 2 bond pairs and 2 lone pairs
which makes the shape bent with angle
104
...

The angle is even smaller than NH3
because H2O has two lone pair at the
central atom
...

This shape
bypyramidal
...


SF6

Sulfur has 6 bond pairs
...
of
bonding pair

No
...


1
...
AlCl3
3
...
NH4+
5
...
H3O+
7
...
SO2
9
...
OH-

Q1: Which of the following molecules and ions have a regular
trigonal planar shape?
1
...
CH3+
3
...

Which set of data would the hydrogen selenide molecule be
expected to have?

• [AS Nov 2003 Paper I Q7]

Q3: Which molecule contains only six bonding electrons?
A
...
C2F6
C
...
NH3
• [AS Nov 2003 Paper I Q6]

Q4: In which sequences are the molecules quoted in order of
increasing bond angle within the molecule?
1
...
H2O
3
...
What is the pattern of electron pairs in this ion?

• [AS June 2007 Paper I Q5]

Which is the most likely shape of a molecule of hydrazine, N2H4?

• [AS Nov 2002 Paper I Q6]

Learning Outcome IV
• describe covalent bonding in terms of
orbital overlap, giving σ and π bonds,
including the concept of hybridisation to
form sp, sp2 and sp3 orbitals

Orbitals Overlapping
A Bonding Orbital is an orbital that can be occupied by two
electrons of a covalent bond (it’s the space in between the
two atoms)
There are 2 types of bonding orbitals: sigma and pi

σ (sigma) bond
• A covalent bond formed by the overlap of
orbitals along the bond axis end to end
...


orbital containing 1
electron

orbital containing 1
electron

overlap of orbitals provides a region
in space which can contain a pair of
electrons

The greater the overlap the stronger the bond
...


1s

1s
σ

H
H

H
H

•

Example: Bonding in H2S
...


Hybrization of Orbitals
• The merging of several atomic orbitals to form the
same total number of hybrid orbitals
...

_
_
_
_
sp3 sp3 sp3 sp3

sp3 hybrid orbital

Copyright©2000 by Houghton
Mifflin Company
...

• Each hybrid sp3 orbital
contains one electron and
is available to bond with
another atom
...

• Each of these bonds is
called a σ (sigma) bond
...
5o

sp2 hybridization

sp2 hybridization
When an s and two p orbitals are mixed to form a set of
2
three sp orbitals, one p orbital remains unchanged and is
perpendicular to the plane of the hybrid orbitals
...

• The pi bond is made up of the parallel
overlap of the p orbitals
...

• A double bond is made up of a sigma
bond and a pi bond
...


sp hybridization

sp hybridization

sp hybridization
The orbitals of an sp hybridized carbon atom
...


Q: 1
(b) Covalent bonding occurs when two atoms share a pair of
electrons
...

(i) How are the two atoms in a covalent bond held together? In
your answer, state which particles are attracted to one another
and the nature of the force of attraction
...


(d) The bonding in ethene may be described as a mixture of σ and
π bonding
...


On the diagram, sketch the π bond that is also present in ethene
...


Atoms arrange in regular close packed
3-dimensional crystal lattices
...
The electron cloud binds the newlyformed positive ions together
...


Metallic Bonding
Metals are excellent conductors of electricity
For a substance to conduct electricity it must have mobile ions or electrons
...
Ionic bonds
2
...
Lattice of ions
• [AS Nov 2002 Paper I Q32]

Learning Outcome VI
• describe hydrogen bonding, using ammonia and water as
simple examples of molecules containing N–H and O–H
groups
• understand, the concept of electronegativity and explain the
properties of molecules such as bond polarity the dipole
moments of molecules and the behaviour of oxides with
water
...


Bond Length
•

•
•
•

The distance between the centre of two atoms joined by a
covalent bond
...

Double bonds have greater quantity of negative charges
between two atomic nuclei
...


Bond Energy
The quantity of energy required to break one mole of covalent
bonds in a gaseous species, usually expressed in kJ/mol
...
93 kJ/mol

A large bond dissociation energy corresponds to a strong
bond which makes it unreactive
...


Learning Outcome VI
• Recall the shapes of molecules
...

• Explain the terms bond polarity and compare the reactivity
of covalent bonds

ACTIVITY 1
• Recall the shapes of the molecule
...
In a group of four, discuss the dot & cross
diagram and predict the shapes and bond
angle of the molecules in the worksheet
provided
...

• When two different atoms bond covalently, there is an
unequal sharing
– the more electronegative atom will have a stronger
attraction, and will acquire a slightly negative charge
– called a polar covalent bond, or simply polar bond
...
1
Cl = electronegativity of 3
...

• The effect of polar bonds on the polarity of the entire
molecule depends on the molecule shape
– carbon dioxide has two polar bonds, and is linear =
nonpolar molecule

Polar molecules
– water has two polar bonds and a bent shape; the highly
electronegative oxygen pulls the e- away from H = very
polar!

ACTIVITY 2
1
...


EXIT CARD – Bond Polarity
1
...

– Carbon disulfide
– chloromethane
– Boron trihydride
– Oxygen dichloride
...

Electrostatic interactions between polar molecules

Dipole-dipole interactions
•

In a sample containing many polar molecules

A balance of attraction and
repulsion
holding
the
molecules together

Instantaneous dipole
An instantaneous dipole is a temporary dipole that exists as a
rise and fall irregularly
result of fluctuation in the electron cloud
...

Permanent dipole

Strength of van der Waals’ forces
Depends on three factors
1
...
Surface area of molecule
3
...
Size of Molecule
Molecule

Rel
...
7
58
...
6
-42
...
Polarity of molecules
For molecules with comparable molecular sizes and shapes,
dispersion forces are approximately equal
...
g
...


2
...
0

2
...
5

2
...
0

Electrostatic attractions exist between this partial
positive charge and the lone pair electrons on a
highly electronegative atom (i
...
fluorine, oxygen or
nitrogen) of another molecule
...


Structure and bonding of ice
The lone pairs of oxygen atom
of each water molecule forms
hydrogen bonds with two
hydrogen atoms of nearby
water molecules

a water
molecule
hydrogen
bond

hydrogen
bond

hydrogen atom
oxygen atom

Each H2O molecule is bonded tetrahedrally to four
H2O molecules
1
4
2

3

The oxygen atoms in the
structure of ice are arranged in
a hexagonal shape
...


Effect of hydrogen bonding on properties of
water
1
...

A lot of energy is required to overcome the hydrogen
bonds between water molecules and separate them
...
Surface tension

High surface tension of water allows water striders to
‘walk’ on it
...


2
...

•Stronger intermolecular forces leads to higher
surface tension
Water molecules at the surface are strongly
attracted by neighboring molecules on the
same surface
...

each water molecule is attracted to neighboring
water molecules in all directions and there is a
balance of force
...
Surface tension
There is an imbalance of force for
the molecules at the surface
...

As a result, water forms droplets
rather than spreading out on leaf
...


3
...


The higher the viscosity of a liquid, the more slowly it
flows
...


Q2: Which physical properties are due to hydrogen bonding
between water molecules?
1
...

2
...

3
...

• [AS Nov 2009 Paper 12 Q31]

Q3: In which process are hydrogen bonds broken?
A
...
NH3 (l)  NH3 (g)
C
...
CH4 (g)  C (g) + 4H (g)
• [AS June 2006 Paper I Q5]

Q5: The diagram shows the variation of the boiling point of the
halogen halides
...
The bond energy of HF molecules is greater than in other
hydrogen halides
B
...
The electronegativity of fluorine is much higher than for other
elements in the group
D
...
Hydrogen sulphide is covalent, melting
at -85C and boiling point at -60C
...

(b) Predict the shape of the H2S molecule
...

Suggest why the melting and boiling points of water, H2O are
much higher than those of H2S

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