Covalent bond

Chapter 8
Chemical bonding: covalent bonding
8.1 Molecules and atomicity
8.2 Covalent bonding and covalent compounds
8.3 Dative covalent bond
8.4 Formulae and names of covalent compounds
P. 1 / 75
8.5 Relative molecular mass and formula mass
8.6 Chemical bonding and constituent particles in
different substances — a summary
Key terms
Progress check
Summary
Concept map
P. 2 / 75
8.1 Molecules and atomicity
Molecules in compounds and elements
Molecules in compounds
Compounds made up of only non-metals
usually consist of neutral particles called
molecules.
A molecule of a compound consists of atoms
of different kinds.
P. 3 / 75
Example:
Water consists of water molecules.
Each molecule consists of two hydrogen atoms
and one oxygen atom chemically joined together.
Formula of water: H2O
8.1 Molecules and atomicity
P. 4 / 75
a water molecule
oxygen atom
hydrogen atom
Figure 8.1 Water consists of water molecules, H2O. Each H2O molecule
consists of two H atoms and one O atom.
8.1 Molecules and atomicity
P. 5 / 75
H
O
C
O
H
N
H
H
carbon dioxide
ammonia
H
C
H
methane
H
Cl
H
hydrogen chloride
Figure 8.2 Molecules of some compounds.
8.1 Molecules and atomicity
P. 6 / 75
Molecules in elements
Some compounds consist of ions, while some
consist of molecules.
Elements consist of either atoms or molecules.
All metals consist of atoms.
Most non-metals consist of discrete (or
separate) molecules.
8.1 Molecules and atomicity
P. 7 / 75
chlorine atoms
a chlorine atom cannot exist
on its own under room
conditions
one chlorine molecule
Figure 8.3 Chlorine consists of chlorine molecules. Each chlorine
molecule Cl2 consists of two Cl atoms chemically joined together.
8.1 Molecules and atomicity
P. 8 / 75
A molecule of an element consists of atoms of
the same kind.
For example, a sulphur molecule consists of
sulphur atoms only.
Key point
molecule is the smallest part of an element
A ___________
or a compound which can exist on its own under
room conditions.
8.1 Molecules and atomicity
P. 9 / 75
Atomicity
The number of atoms in a molecule of an element
or a compound is called atomicity.
Note that:
argon (Ar) is monoatomic
oxygen (O2) is diatomic
ozone (O3) is triatomic
8.1 Molecules and atomicity
P. 10 / 75
Element
Simple model
Formula Atomicity
of molecule
Argon
Ar
1
Hydrogen
H2
2
Chlorine
Cl2
2
Oxygen
O2
2
Table 8.1 The atomicity and the simple model of molecules of some elements.
8.1 Molecules and atomicity
P. 11 / 75
Molecule
Simple model
Formula Atomicity
of molecule
Nitrogen
N2
2
Ozone
O3
3
(White
phosphorus)
P4
4
Sulphur
S8
8
Phosphorus
Table 8.1 The atomicity and the simple model of molecules of some elements.
Class practice 8.1
8.1 Molecules and atomicity
P. 12 / 75
8.2 Covalent bonding and covalent
compounds
Covalent bonding in some non-metal elements
Formation of covalent bond in a chlorine molecule (Cl2)
Example: Chlorine
Under room conditions, chlorine atoms cannot
exist by themselves.
Animation (Formation of covalent bonds)
P. 13 / 75
Chlorine gas consists of discrete molecules.
Each molecule consists of two chlorine atoms
chemically joined together.
Each chlorine atom shares its outermost shell
electron with another chlorine atom (one
electron from each chlorine atom), forming
single covalent bond.
Therefore, a chlorine molecule (Cl2) consists of
two chlorine atoms.
8.2 Covalent bonding and covalent compounds
P. 14 / 75
Each chlorine atom has a stable octet.
a shared pair of electrons
forms a single covalent bond
electron
sharing
+
2,8,7
2,8,7
chlorine atom (Cl)
chlorine atom (Cl)
2,8,8
a lone pair
of electrons
2,8,8
chlorine molecule (Cl2)
Figure 8.4 Electron diagrams showing the sharing of a pair of electrons
in the formation of a chlorine molecule (only the outermost shell
electrons are shown).
8.2 Covalent bonding and covalent compounds
P. 15 / 75
Covalent bond is the strong directional
electrostatic attraction between:
the shared electrons (negatively charged)
the two nuclei (positively charged) of the
bonded atoms
Key point
covalent bond forms by the sharing of
A ______________
outermost shell electrons between two atoms.
8.2 Covalent bonding and covalent compounds
P. 16 / 75
Bond pair of electrons (or a bond pair)
a pair of electrons shared between two
atoms
Lone pair of electrons (or a lone pair)
a pair of outermost shell electrons that is not
shared between two atoms
In a chlorine molecule, there are:
one bond pair and
three lone pairs in each chlorine atom
8.2 Covalent bonding and covalent compounds
P. 17 / 75
Formation of covalent bond in an oxygen molecule (O2)
Each oxygen atom shares two of its outermost
shell electrons with another oxygen atom, forming
a double covalent bond.
An oxygen molecule (O2) consists of two oxygen
atoms.
Each oxygen atom has a stable octet.
8.2 Covalent bonding and covalent compounds
P. 18 / 75
2 shared pairs of electrons
form a double covalent bond
electron
sharing
+
2,6
2,6
oxygen atom (O)
oxygen atom (O)
2,8
a lone pair of
electrons
2,8
oxygen molecule (O2)
Figure 8.5 Electron diagrams showing the sharing of two pairs of electrons in
the formation of an oxygen molecule (only the outermost shell electrons are
shown).
8.2 Covalent bonding and covalent compounds
P. 19 / 75
Formation of covalent bond in a nitrogen molecule
(N2)
Each nitrogen atom shares three of its
outermost shell electrons with another nitrogen
atom, forming a triple covalent bond.
Therefore, a nitrogen molecule (N2) consists of
two nitrogen atoms.
Each nitrogen atom has a stable octet.
8.2 Covalent bonding and covalent compounds
P. 20 / 75
3 shared pairs of electrons
form a triple covalent bond
electron
sharing
+
2,5
2,5
nitrogen atom (N)
nitrogen atom (N)
a lone pair of
electrons
2,8
2,8
nitrogen molecule (N2)
Figure 8.6 Electron diagrams showing the sharing of three pairs of
electrons in the formation of a nitrogen molecule (only the outermost shell
electrons are shown).
8.2 Covalent bonding and covalent compounds
P. 21 / 75
Molecular formula and structural formula
A bond pair of electrons is represented by a stroke
(–) between the atomic symbols.
Examples:
chlorine molecule Cl2, Cl–Cl
oxygen molecule O2, O=O
nitrogen molecule N2, N≡N
8.2 Covalent bonding and covalent compounds
P. 22 / 75
Cl2, O2 and N2 are the molecular formulae of
the elements.
Cl–Cl, O=O and N≡N are the structural
formulae of the elements.
Learning tip
When we say the ‘formula’ of a molecular substance,
we usually refer to its ‘molecular formula’.
8.2 Covalent bonding and covalent compounds
P. 23 / 75
Key point
The ________________
molecular formula of a molecular substance
is the formula which shows the actual number of
each kind of atoms in one molecule of the
substance.
Key point
structural formula of a molecular substance
The ________________
is the formula which shows how the constituent
atoms are joined up in one molecule of the
substance.
8.2 Covalent bonding and covalent compounds
P. 24 / 75
Covalent bonding in some covalent compounds
What is a covalent compound?
For compounds containing two or more non-metal
elements, the atoms are usually held together by
covalent bonds.
These compounds are known as covalent
compounds.
Concept check
8.2 Covalent bonding and covalent compounds
P. 25 / 75
Example: hydrogen chloride (HCl)
One electron from a hydrogen atom and one
electron from a chlorine atom are shared to form
a single covalent bond.
Structural formula: H–Cl
8.2 Covalent bonding and covalent compounds
P. 26 / 75
a shared pair of electrons
forms a single covalent bond
electron
sharing
+
1
2,8,7
hydrogen atom
(H)
chlorine atom (Cl)
2
a lone pair of
electrons
2,8,8
hydrogen chloride (HCl)
Figure 8.7 Electron diagrams showing the sharing of electrons in the
formation of a hydrogen chloride molecule (only the outermost shell
electrons are shown).
8.2 Covalent bonding and covalent compounds
P. 27 / 75
Electron diagrams showing
covalent bond formation
Molecular
formula
Structural
formula
H2O
NH3
CH4
Table 8.2 Electron diagrams showing the formation of covalent bonds in
molecules of some simple molecules (only the outermost shell electrons are
shown).
8.2 Covalent bonding and covalent compounds
P. 28 / 75
Electron diagrams showing
covalent bond formation
Molecular
formula
Structural
formula
CCl4
CO2
Table 8.2 Electron diagrams showing the formation of covalent bonds in
molecules of some simple molecules (only the outermost shell electrons are
shown).
8.2 Covalent bonding and covalent compounds
P. 29 / 75
Number of electrons contributed for sharing
Atom
Number of
outermost
shell
electrons
Additional number of
Number of
electrons needed to get the
electrons
electronic arrangement of contributed by an
the nearest noble gas
atom for sharing
Hydrogen
1
1
1
Carbon
4
4
4
Nitrogen
5
3
3
Oxygen
6
2
2
Fluorine
7
1
1
Table 8.3 Number of electrons contributed by an atom for sharing when
forming covalent bonds.
8.2 Covalent bonding and covalent compounds
P. 30 / 75
Simple models of molecules
There are two common kinds of models:
ball-and-stick models
space-filling models
8.2 Covalent bonding and covalent compounds
P. 31 / 75
Molecule
H2
Cl2
O2
N2
Ball-andstick
model
Spacefilling
model
Table 8.4 Ball-and-stick models and space-filling models for some simple
molecules.
8.2 Covalent bonding and covalent compounds
P. 32 / 75
Molecule
H2O
NH2
CH4
Ball-andstick model
Space-filling
model
Table 8.4 Ball-and-stick models and space-filling models for some simple
molecules.
Experiment 8.1
Class practice 8.2
8.2 Covalent bonding and covalent compounds
P. 33 / 75
8.3 Dative covalent bond
In some compounds, a covalent bond forms by
sharing a pair of electrons coming from the
same atom.
Key point
A dative covalent bond (or coordinate bond) is
a covalent bond formed between two atoms
where both electrons of the shared pair are
contributed by the same atom.
P. 34 / 75
Dative covalent bond in ammonium ion (NH4+)
An ammonia molecule combines with a hydrogen
ion to form an ammonium ion.
Dative covalent bond forms between the lone pair
of electrons on:
N atom in NH3
H+ ion
Animation (Formation of dative covalent bond)
8.3 Dative covalent bond
P. 35 / 75
The symbol ‘
covalent bond.
’ is used to represent the dative
ammonium ion
dative covalent bond
Figure 8.8 Electron diagrams and structural formulae showing the
formation of an ammonium ion (NH4+).
8.3 Dative covalent bond
P. 36 / 75
Ammonium chloride (NH4Cl) is a common ionic
compound that contains ammonium ion (NH4+).
It contains both
ionic bonds (between NH4+ and Cl– ions)
covalent bonds (four N–H bonds)
8.3 Dative covalent bond
P. 37 / 75
Three of the N–H bonds are normal covalent
bonds and one is dative covalent bond.
Dative and normal covalent bonds differ only in the
way they form.
All the four N–H bonds in ammonium ion are
identical.
Learning tip
The ammonium ion (NH4+) has an overall charge of
+1 distributed all over the structure.
8.3 Dative covalent bond
P. 38 / 75
Dative covalent bond in hydronium ion (H3O+)
A dative covalent bond forms between:
a H+ ion
a lone pair of electrons on the O atom in H2O
A more stable ion, hydroxonium ion or
hydronium ion (H3O+), is obtained as a result.
8.3 Dative covalent bond
P. 39 / 75
hydronium ion
dative covalent bond
Figure 8.9 Electron diagrams and structural formulae showing the
formation of hydronium ion (H3O+).
Class practice 8.3
8.3 Dative covalent bond
P. 40 / 75
8.4 Formulae and names of covalent
compounds
Formulae of covalent compounds
The formula of a covalent compound indicates the
ratio of one atom to the other(s) in the molecule.
Problem-solving strategy 8.1
Class practice 8.4
P. 41 / 75
Names of covalent compounds
The system for naming covalent compounds is
different from that used for naming ionic
compounds.
If a compound forms from only two non-metal
elements, the compound should be named
according to the following rules:
8.4 Formulae and names of covalent compounds
P. 42 / 75
1. The element that occurs first in the following
series is named first:
B, Si, C, P, N, H, S, I, Br, Cl, O, F
2. The name of the second element should end up
with –ide.
3. A prefix (mono, di, tri, tetra and so on) is usually
used to indicate the number of atoms of that
element in a molecule of the compound.
8.4 Formulae and names of covalent compounds
P. 43 / 75
Example:
Chlorine trifluoride (ClF3)
ClF3
Name:
chlorine
tri fluoride
indicates that there are 3 fluorine atoms
in a molecule
8.4 Formulae and names of covalent compounds
P. 44 / 75
Example:
Dinitrogen tetraoxide (N2O4)
N2O4
Name:
di nitrogen
indicates that there are
2 nitrogen atoms in a
molecule
tetra oxide = tetroxide
indicates that there are 4 oxygen atoms
in a molecule
Learning tip
The prefix ‘mono’ is usually not added to the name of
the first element in the molecule of the compound.
8.4 Formulae and names of covalent compounds
P. 45 / 75
Formula of covalent
compound
CH4
NH3
H2O
H2O2
Name of covalent
compound
Methane
Ammonia
Water
Hydrogen peroxide
Table 8.5 Names of some covalent compounds. The name of these
compounds cannot be predicted using the above rules.
Class practice 8.5
8.4 Formulae and names of covalent compounds
P. 46 / 75
8.5 Relative molecular mass and formula
mass
What is relative molecular mass?
Relative atomic mass
Weighted average of the relative isotopic
masses of all the naturally occurring isotopes
of that element on the
12C = 12.00 scale.
P. 47 / 75
Relative molecular mass
Mass of one molecule of it on the
12C = 12.00 scale
It carries no unit.
It can also be called molecular mass.
Key point
Sum of relative atomic
Relative molecular
masses of all atoms
mass of an element = present in a molecule of
or a compound
the element or compound
8.5 Relative molecular mass and formula mass
P. 48 / 75
Example: water (H2O)
Relative molecular mass of H2O
= 1.0 × 2 + 16.0 = 18.0
relative atomic
mass of hydrogen
relative atomic
mass of oxygen
8.5 Relative molecular mass and formula mass
P. 49 / 75
hydrogen atom
oxygen atom
Figure 8.10 The mass of a water molecule (H2O) is equal to the sum of
the mass of one oxygen atom and the mass of two hydrogen atoms.
8.5 Relative molecular mass and formula mass
P. 50 / 75
What is formula mass?
We use formula mass to describe the relative
masses.
Formula mass carries no unit.
The formula mass of a substance (or species) is
the mass of one formula unit of it on the 12C =
12.00 scale.
8.5 Relative molecular mass and formula mass
P. 51 / 75
For a pure substance with a formula, the
simplest unit is its formula unit.
For ionic compounds, like magnesium chloride,
Formula unit: MgCl2
It consists of one Mg2+ ion and two Cl– ions.
Key point
Sum of relative atomic
Formula mass
masses of all atoms
of a substance = present in a formula unit of
(or species)
the substance (or species).
8.5 Relative molecular mass and formula mass
P. 52 / 75
Example: magnesium chloride (MgCl2)
Formula mass of MgCl2 = 24.3 + 35.5 × 2 = 95.3
relative atomic
mass of
magnesium
relative atomic
mass of
chlorine
Learning tip
Never speak of the relative molecular mass of
magnesium chloride, as there are no molecules in
this compound.
Example 8.1
8.5 Relative molecular mass and formula mass
P. 53 / 75
Formula mass is a general term applicable to all
substances (or species) with a formula.
Relative molecular mass only applies to
molecular substances.
Class practice 8.6
8.5 Relative molecular mass and formula mass
P. 54 / 75
8.6 Chemical bonding and constituent
particles in different substances — a
summary
Metallic bond, ionic bond and covalent bond — a
summary
Metallic bond
electrostatic
attraction between a
‘sea’ of negatively
Nature
charged electrons
and positively
charged metal ions
Ionic bond
electrostatic
attraction
holding the
oppositely
charged ions
together
Covalent bond
electrostatic
attraction between
the shared
electrons and the
two nuclei of the
bonded atoms
Table 8.6 A summary of information about metallic bond, ionic bond and
covalent bond.
P. 55 / 75
Metallic bond
Formation
escape of
outermost shell
electrons from
metal atoms
Directional/
nonnon-directional
directional
Ionic bond
Covalent bond
transfer of one
or more
electrons from
one atom (or
group of atoms)
to another
sharing of
outermost shell
electrons
between two
atoms
non-directional
directional
Table 8.6 A summary of information about metallic bond, ionic bond and
covalent bond.
Concept check
8.6 Chemical bonding and constituent particles in different
substances — a summary
P. 56 / 75
Particles that make up matter — a summary
elements
Pure
substances
metals
non-metals
compounds made
up of only
non-metals
compounds
compounds made
up of metal(s) and
non-metals
Figure 8.11 Constituent particles of various substances.
8.6 Chemical bonding and constituent particles in different
substances — a summary
P. 57 / 75
Elements
Constituent
particles
Examples
metals
atoms
copper (Cu)
non-metals
molecules
(exception:
carbon)
argon (Ar)
chlorine (Cl2)
sulphur (S8)
Figure 8.11 Constituent particles of various substances.
8.6 Chemical bonding and constituent particles in different
substances — a summary
P. 58 / 75
Compounds
Constituent
particles
compounds
made up of only
non-metals
usually
molecules
Examples
water (H2O)
ammonia (NH3)
compounds made ions potassium oxide (K2O)
sodium chloride (NaCl)
up of metal(s)
and non-metals
Figure 8.11 Constituent particles of various substances.
Think about
Class practice 8.7
8.6 Chemical bonding and constituent particles in different
substances — a summary
P. 59 / 75
Key terms
1.
2.
3.
4.
5.
6.
7.
8.
atomicity 原子數
ball-and-stick model 球棒模型
bond pair of electrons (or bond pair) 鍵合電子
對
covalent bond 共價鍵
covalent compound 共價化合物
dative covalent bond/coordinate bond 配位共價
鍵/配位鍵
diatomic 雙原子的
double covalent bond 共價雙鍵
P. 60 / 75
9.
10.
11.
12.
13.
14.
15.
16.
17.
formula mass 式量
formula unit 式單位
lone pair of electrons (or lone pair) 孤電子對
molecular formula 分子式
molecule 分子
monoatomic 單原子的
relative molecular mass/molecular mass 相對
分子質量/分子質量
single covalent bond 共價單鍵
space-filling model 填空模型
Key terms
P. 61 / 75
18.
19.
20.
structural formula 結構式
triatomic 三原子的
triple covalent bond 共價三鍵
Key terms
P. 62 / 75
Progress check
1. What is a molecule?
2. How does a covalent bond form?
3. How can we describe the formation of single
covalent bond using electron diagrams?
4. How can we describe the formation of double
covalent bond using electron diagrams?
5. How can we describe the formation of triple
covalent bond using electron diagrams?
6. How do we write the molecular formulae and
structural formulae of molecular substances?
P. 63 / 75
7. What is a covalent compound?
8. How can we describe the formation of dative
covalent bond in NH4+ by means of electron
diagrams?
9. How can we describe the formation of dative
covalent bond in H3O+ by means of electron
diagrams?
10. How do we write the names and formulae of
covalent compounds based on their constituent
atoms?
Progress check
P. 64 / 75
11. What is the meaning of relative molecular mass?
12. What is the meaning of formula mass?
13. How can we perform calculations related to
relative molecular mass and formula mass?
14. What are the differences between metallic bond,
ionic bond and covalent bond?
Progress check
P. 65 / 75
Summary
8.1
Molecules and atomicity
1.
A molecule is the smallest part of an element or
a compound which can exist on its own under
room conditions.
Compounds made up of only non-metals
usually consist of molecules. Elements are
made up of either atoms or molecules. All
metals consist of atoms. Most non-metals
consist of discrete molecules.
The number of atoms in a molecule of an
element or a compound is called atomicity.
2.
3.
P. 66 / 75
8.2
4.
Covalent bonding and covalent compounds
A covalent bond forms when one or more pairs
of outermost shell electrons are shared between
two atoms. For example,
1 hydrogen atom + 1 chlorine atom
5.
1 hydrogen chloride molecule
Covalent bond is the strong directional
electrostatic attraction between the shared
electrons and the two nuclei of the bonded
atoms.
Summary
P. 67 / 75
6.
A shared pair of electrons (bond pair) makes a
single covalent bond, e.g. H–Cl.
Two shared pairs of electrons make a double
covalent bond, e.g. O=C=O.
a double covalent bond
Three shared pairs of electrons make a triple
covalent bond, e.g. N≡N.
a triple covalent bond
Summary
P. 68 / 75
7.
Some atoms have unshared pairs of
outermost shell electrons. These are known
as lone pairs, e.g.
a lone pair of electrons
8.
The molecular formula of a molecular
substance shows the actual number of each
kind of atoms in one molecule of the
substance, e.g. CH4.
Summary
P. 69 / 75
9.
The structural formula of a molecular
substance shows how the constituent atoms
are joined up in one molecule of the
substance, e.g.
8.3
10.
Dative covalent bond
A dative covalent bond (or coordinate bond) is
a covalent bond formed between two atoms
where both electrons of the shared pair are
contributed by the same atom.
Summary
P. 70 / 75
8.4
11.
12.
Formulae and names of covalent compounds
The formulae of covalent compounds can often
be constructed using a quick method. Refer to
‘Problem-solving strategy 8.1’ on p.14.
The system for naming covalent compounds is
different from that used for naming ionic
compounds. Refer to the rules stated on p.15.
Summary
P. 71 / 75
8.5
13.
14.
Relative molecular mass and formula mass
Relative molecular mass of an element or a
compound
= Sum of relative atomic masses of all atoms
present in a molecule of it on the 12C = 12.00
scale
Formula mass of a substance (or species)
= Sum of relative atomic masses of all atoms
present in a formula unit of it on the 12C =
12.00 scale
Summary
P. 72 / 75
8.6
15.
Chemical bonding and constituent particles in
different substances — a summary
All matter is made up of particles: atoms,
molecules or ions.
Summary
P. 73 / 75
Concept map
Outermost
_____________
Shared
_____________
electron shell
comes pair(s) of electrons
of atoms
nuclei
The _______of
the
two bonded atoms
from
Single covalent bond
Double covalent bond
______
electrostatic
is the______________
attraction between
can
be
COVALENT BOND
Triple covalent bond
______
Non-metallic
elements, e.g. H2,
Cl2
______
can be found in
Covalent compounds,
H2O
e.g. CH4,_______
P. 74 / 75
Non-metallic
elements, e.g. H2,
Cl2
___________
Covalent compounds,
H2O
e.g. CH4,_______
both exist as
Relative
molecular
mass
molecules
is the sum
of relative
atomic
masses of
all atoms
present in
can be
represented
by
Molecular formulae
________
Structural formulae
Concept map
P. 75 / 75

Download Report