Transcript Lesson 1 - Bonding in compounds overview

(B) Bonding and Structure
Bonding in Compounds overview
(C) Bonding and Structure
Bonding in Compounds
After completing this topic you should be able to :
Overview
•
•
Learn how the elements can form bonds in compounds and examine properties
of bonding types.
An introduction to the variety of intermolecular forces which exist between
molecules.
The Chemical Bond
Chemical Bond
Intramolecular
(Within)
Intermolecular
(between)
Van der Waals
Metallic
Hydrogen bonding
Covalent
Permanent Dipole- Permanent
Dipole interactions
Polar covalent
Ionic
London dispersion forces
Ionic Compounds
Ionic Compounds
Ions
- metals lose electrons and form positive ions
- non-metals gain electrons to form negative ions
- electrons are transferred from metals to non-metals
transfer
-
+
Na
+
Cl
Na atom + Cl atom
(2.8.1)
(2.8.7)
Na
Cl
Na+ ion + Cl- ion
(2.8)
(2.8.8)
Ionic Compounds
The positive and negative ions are attracted (electrostatic
bond ) to each other.
Ionic bond (electrostatic attraction)
Na+
Cl-
A giant lattice structure is formed.
Each Na+ ion is surrounded by 6 Cl- ions.
While each Cl- is surrounded by 6 Na+ ions.
Ionic bonding is the electrostatic force of
attractionbetween positively and negatively
charged ions.
This ionic network compound has many ionic
bonds so ionic compounds have high m.p.s
Ionic Compounds
A giant lattice structure is formed when each Na+ ion is
surrounded by 6 Cl- ions and each Cl- ion is surrounded by
6 Na+ ions.
Sodium Chloride
The formula of sodium chloride
is NaCl, showing that the ratio
of Na+ to Cl- ions is 1 to 1.
The m.p. of NaCl is 801 0C
The size of the ions will effect the strength of the ionic bond
and how the ions pack together. e.g. NaF m.p. 1000oC, NaI 660oC
Molecular Ions, e.g. SO4
Oxygen
A single covalent bond.
Sulphur
O
2 additional electrons
e.g. Copper can donate
the extra 2 electrons
needed.
Cu
Cu
2+
+ 2e
O
S
O
O
Copper sulphate contains the Cu2+ and the SO42- ions. There is,
therefore, covalent bonding and ionic bonding in copper sulphate
A solution of copper sulphate can conduct electricity.
Molten ionic compounds can also conduct electricity.
Bond Strengths
Bond Type
Strength (kJ mol –1)
Metallic
80 to 600
Ionic
100 to 500
Covalent
100 to 500
Hydrogen
40
Dipole-Dipole
30
London’s forces
1 to 20
Covalent Molecular
Compounds
Covalent Bonding
Sharing electrons
•
takes place between non-metal and non-metal
•
shared electrons count as part of the outer shell
of both Atoms
•
shared electrons attract the nuclei of both atoms
•
this attraction is called the covalent bond
Hydrogen chloride
H
H
Cl
(linear)
Cl
HCl
Ammonia
H
H
H
N
H
H
(pyrimidal) NH3
H
N
Water
O
H
H
(bent) H2O
H
O
H
Draw electron dot cross diagrams for the
following molecules and structural formula
1. SCl2
2. CO2
3. CH4
X X
S
X X
H
Cl-S-Cl
O=C=O
H C H
H
Bond Strengths
Bond Type
Strength (kJ mol –1)
Metallic
80 to 600
Ionic
100 to 500
Covalent
100 to 500
Hydrogen
40
Dipole-Dipole
30
London’s Forces
1 to 20
Covalent Molecular Compounds
Discrete molecules are formed when two or more atoms share
electrons.
The atoms are non-metal elements. An example is methane.
Methane: CH4
H
H
H
C
H
H
H
C
H
H
Methane has strong intra-molecular and weak inter-molecular.
It’s b.p. is -183oC
Covalent Molecular Compounds
Non- metals elements can form double and triple covalent bonds.
ethane C2H6
H
H
H
C
C
H
H
H
ethene C2H4
H
H
H
C
C
H
H
H
H
H
C
C
H
H
H
H
C
C
H
H
H C
Double covalent bond
Covalent molecular compounds have low m.p.’s because the weak
forces holding the molecules together require only small
amounts of thermal energy to break them.
Covalent Molecular Compounds
Properties
Low m.p.’s and b.p.’s., this increases with size of the molecule
and the increasing number of atoms in the molecule.
m.p.’s of the carbon halides
171
Temp
/ oC
90
-23
-183
CF4
CCl4
CBr4
CI4
m.p.’s increase because the strength of the London dispersion
forces increase with the increasing size of the molecule. So more
Energy is needed to separate molecules.
Covalent Network
Compounds
Silicon Carbide SiC
Silicon, like carbon, can form giant covalent networks.
Silicon carbide exist in a similar structure to diamond.
Tetrahedral
shape
C
Covalent
Bond
C
Si
C
C
The 4 carbon atoms are
available to bond with
another 4 silicon atoms.
This results in a COVALENT NETWORK COMPOUND
Silicon Carbide SiC
Silicon carbide (carborundum) has a
chemical formula is SiC. As this compound is
linked by strong covalent bonding, it has a
high m.p. (2700oC).
It is a hard substance as it is very difficult
to break the covalent lattice.
SiC is used as an abrasive for smoothing
very hard materials.
Each Si is bonded to 4 C’s and
each C is bonded to 4 Si’s.
Hence the chemical formula, SiC
Silicon Dioxide SiO2
Silicon and oxygen make up nearly 75% of the Earth’s crust.
They are therefore the most common elements in the
Earth’s crust.
They combine together to make a covalent network compound
called silicon dioxide.
This is usually found in the form of sand or quartz.
Each Si atom is bonded to 4 O atoms, and each O atom is
bonded to 2 Si atoms. Hence the chemical formula, SiO2 .
Silicon dioxide (silica) also has a
high m.p. (1610 oC) and like SiC,
it is very hard and used as an
abrasive.
It is relatively un-reactive.
New Higher Chemistry E Allan J Harris