5.
CHEMICAL BONDING
All particles are held together by chemical bonds of some kind.
Water molecules are made up to two hydrogen atoms and one oxygen atom always. The
three atoms are held together by chemical bonds, which are strong and therefore not
easily broken.
How then do ice, liquid water and steam differ? They all contain water molecules.
Gas particles move far away from each other. Liquid particles are closer together. Solid
particles are very close together, held in a fixed position.
The main reason for the difference is that bonds can occur between molecules, called
intermolecular forces, and they increase in strength as the molecules get closer together.
These bonds are much weaker in strength than chemical bonds.
Chemical bonds form between atoms to form stable partnerships, since most atoms are
not stable alone. Chemical bonds exist between the hydrogen and oxygen atoms in the
stable water molecule. Chemical bonds involve the outer electron shells of atoms.
What makes an Atom Chemically Stable?
The elements in the group at the right-hand end of the periodic table - He, Ne, Ar, Kr, Xe
are called the rare, noble or inert gases. They are inert because they are extremely
unreactive. This means that they don’t form compounds except under rare circumstances.
They are extremely stable as monatomic elements - as single, separate atoms.
Electron
configuration :
(2)
(2,8)
(2,8,8)
In each of these atoms all electron shells present are full. The outer shell of electrons is
called the valence shell.
Chemical stability is obtained when an atom has a complete (full) outer shell of
electrons.
1
5.2
Loss and Gain of Electrons – The Ionic Bond
Sodium and chlorine are extremely reactive as pure elements, but the compound they
form, sodium chloride, is extremely stable. The sodium atom has one electron in its
valence shell, while the chlorine atom has seven electrons in its valence shell (which is
one short of a full shell). A transfer of this one electron from the sodium atom to the
chlorine atom results in the formation of more stable particles, now charged ions, which
have a full outer electron shells. The sodium atom loses an electron to become a positive
sodium ion, while the chlorine atom gains the electron to become a negative chloride ion.
Both the sodium ion and the chloride ion have full outer shells of electrons. Since only
the outer electron shells are involved in chemical bonding between atoms, only the outer
shell of electrons is often shown as an Electron Dot Formula. This concept is also
referred to as a Lewis Structure
Sodium atom 
Sodium ion + e -
Chlorine atom + e -  Chlorine ion
2
Sharing Electrons – The Covalent Bond
The other way for an atom to fill its outer shell of electrons is to share electrons with other
atoms. When sharing electrons each atom involved contributes an equal number of
electrons - one, two or three - to form a single, double or triple covalent bond. A pair of
shared electrons is represented as a dash ( – ) in a structural formula, ie a single bond.
Some elements have their atoms covalently bonded in simple molecules (such as oxygen)
or even giant networks (such as carbon) to form stable units.
The following common elements have stable diatomic molecules: H2, N2, O2, F2, Cl2 Br2, I2.
Written Exercise:
Lewis Electron Dot Formulae
Use Lewis Structures to illustrate covalent bonds for the following molecular elements:
H2
O2
N2
F2
Structural formula:
H–H
Structural formula:
O=O
Simple molecular compounds contain covalent bonds within their molecules.
Complete the following examples:
H2O
NH3
CH4
Structural formula:
CO2
HCl
3
HCN
Which Chemical Bond Will Form?
When atoms of one element need to gain electrons to become stable, and atoms of
another element need to lose electrons to become stable then an ionic bond will form.
Ionic bonds usually form between metals and non-metals. Metals will not bond chemically
with other metals to form compounds but they can form mixtures called alloys.
When the atoms of both elements need to gain electrons to become stable then a
covalent bond is formed. This situation arises when compounds form between nonmetals.
+
Exception: The ammonium ion NH4 is a positive ion containing non-metals, however it
bonds ionically with any negative ion.
Written Exercise: Type of chemical bond?
Identify the nature of chemical bond that will form between the following elements.
Mg and Cl
...............................
K and S
...............................
C and Br
...............................
O and O
...............................
H and Cl
...............................
Na and Ca
...............................
take care !
The Metallic Bond
A Model of a Metallic Lattice
Metal atoms generally have few electron shells (1  3) occupying outer valence shells. In
theory these electrons need to be lost in order for the metal to form a stable ion. However
in the pure element there are no atoms to gain them since all are identical in electron
configuration (arrangement).
4
The outer electrons are freed from the metal atoms and form a negative cloud which is the
glue that holds the resultant positive ions (ie nucleus + inner electron orbitals) together in
a regular lattice structure.
The freedom of these electrons to move (even in the solid state) is the reason why metals
are good conductors of electricity. It also helps to explain properties of malleability and
ductility as the free electrons can flow to bind a distortion in the lattice.
Summary of Bonding
Atoms are held together in compounds by Chemical Bonds. Chemical bonds result from
the sharing or transfer of valence electrons between pairs of atoms. Bonded atoms attain
the stable electron configuration of a noble gas. The noble gases themselves exist as
isolated atoms because that is their most stable condition. The transfer of one or more
electrons between atoms produce positively and negatively charged ions: cations and
anions.
The attraction between a cation and an anion is an ionic bond. A substance with ionic
bonds is an ionic compound. Nearly all ionic compounds are crystalline solids at room
temperature, having high melting points. These solids consist of positive and negative
ions packed in an orderly arrangement. The total positive charge is balanced by the total
negative charge, and therefore the ionic compound (also called a salt) is electrically
neutral.
When atoms share electrons to gain stable electron configuration of a noble gas, they
form covalent bonds. A shared pair of electrons constitutes a single covalent bond.
Sometimes two or three pairs of electrons are shared to give double and triple covalent
bonds.
Metals consist of positively charged ions packed together and surrounded by a sea of
their valence electrons. This arrangement constitutes the metallic bond. The valence
accounts for the excellent electrical conductivity in metals and helps explain why metals
are malleable and ductile.
5
Written Exercises:
1.
Write electron dot structures for each of the following elements.
(a)
2.
Cl
5.
6.
8.
(c)
Al
(d)
Li
(e)
C
(b) Al
(c) Na
(d) Li
(e) Ba
(f) Mg
(b) S
(c) N
(d) Cl
(e) P
(f) O
Write the formula for the stable ion formed from each of the following elements.
(a) aluminium
(b) lithium
(c) barium
(d) potassium
(e) calcium
(f) strontium
(g) bromine
(h) oxygen
(i) arsenic
(j) nitrogen
(k) selenium
(l) sulfur
Which of the following pairs of elements are likely to form ionic compounds?
(a) magnesium and bromine
(b) chlorine an iodine
(c) potassium and helium
(d) nitrogen and sulfur
(e) lithium and fluorine
(f) carbon and oxygen
Draw Lewis Structures of the compounds formed from these pairs of ions.
(a) K+ ; S2-
7.
S
How many electrons must be gained by each of the following atoms to attain a
noble gas configuration (full electron shell)?
(a) I
4.
(b)
How many electrons must be lost by each of the following atoms to form an ion?
(a) Ca
3.
Formulae and Chemical Bonding
(b) Ca2+ ; O2-
(c) Al3+ ; Cl-
Write formulae for the ions in the following compounds.
(a) KCl
(b) NaNO3
(c) BaSO4
(d) MgBr2
(e) KOH
(f) Li2CO3
Draw Lewis electron structures of the following. Classify each as ionic or covalent.
(a) H2O
(b) Na2O
(c) CS2
(d) CaS
(e) NH3
(f) SO2
(g) Al2O3
6
Compound Chemical Formulae
A chemical formula represents one molecule or formula unit of an element or compound.
A chemical formula can feature:
FEATURE
DEFINITION
EXAMPLES
Symbols
identify element(s) present.
C
Subscripts
i) indicate number ratios of
elements or polyatomic formula
units
CO2 or C6H12O6
ii) states of matter
Mg
NaCl or CaO etc
O2 (g) H2O (l) AgCl (s) or HCl (aq)
Superscripts
indicate charge of cation or anion
Coefficient
Numbers
represent mole quantity ratios in
balanced chemical formulae and
equations
Na
+
or
CO3
2-
CuSO4.3H2O
N2 (g) + 3H2 (g)  2NH3 (g)
Brackets
express multiple polyatomic ion
units in a chemical formula.
Roman
Numerals
clarify valency of transition metal Iron III Oxide,
elements in word formulae
Prefix Names
indicate number ratios of non
metal elements in covalent
molecular compounds
Al (NO3)3 (aq)
Fe2O3
Dinitrogen Pentoxide
N2O5
A molecule is a discreet group of atoms which are bonded together in a fixed ratio
defined by chemical formula.
Examples:
Elements
Compounds
H2 - Hydrogen
H2O - Water
P4 - Phosphorus
C6H12O6 - Glucose (a sugar)
S8 - Sulfur
NH3 - Ammonia
7
A formula unit represents the simplest ratio of atoms in a substance which may be a
giant lattice, ie network structure.
Examples: Each of these substances is present as a giant lattice structure.
Substance
Type
Formula
Diamond
element
C
Sodium Chloride
compound (ionic)
NaCl
Silica
compound
(network covalent)
SiO2
Ions (charged particles) use superscripts to show the electrical charge.
Examples:
• Mg2+ means a magnesium ion having a charge of +2.
• CO3 2- means one carbon atom and three oxygen atoms forming an ion (called a
carbonate ion) with a charge of -2.
Further Examples:
1.
2 atoms of
hydrogen
H2SO4
1 atom of
sulfur
2.
4 atoms of
oxygen
Ca(OH)2
2 groups of OH
1 atom of
calcium
2 atoms of
oxygen
8
2 atoms of
hydrogen
Written Exercise – Interpreting chemical formulae:
1.
(NH4)2SO4
( ......... groups
of .................... )
............ atoms of
............. atoms of
.............. atoms of ............ atoms of
..........................
............................
............................. ...........................
2.
CH4 represents ......................................................................................................
3.
Sn3(PO4)4 represents ..............................................................................................
4.
CuSO4.5H2O represents..........................................................................................
5.
Cl- represents ........................................................................................................
6.
OH represents ...........................................................................................................................
Valency
Valency is a numerical measure of combining power in determining correct compound
formula. Cations have positive valency, Anions negative. Atoms combine together
according to their values of valency to form stable compounds, commonly of neutral
charge, but not always. . .
Where an element can have more than one valency, it is given as a Roman Numeral in
the name of the compound. This commonly occurs with transition elements.
9
VALENCY TABLE
Polyatomic Ions – contain 2 or more elements making up their charged arrangement.
CATIONS
1+
2+
3+
4+/-
Name
Formula
Name
Formula
Name
Formula
Name
Formula
Hydrogen
H+
Calcium
Ca2+
Iron (III)
Fe3+
Carbon
C 4+/-
Lithium
Li+
Magnesium
Mg2+
Aluminium
Al3+
Silicon
Si 4+/-
Sodium
Na+
Copper II
Cu2+
Potassium
K
Cobalt II
Co
* * Carbon and Silicon
Ammonium
NH4+
Zinc II
Zn2+
do not form ions,
Silver I
Ag+
Iron II
Fe2+
rather covalent bonds
Copper I
Cu
Lead II
Pb
+
+
2+
2+
ANIONS
1Name
2Formula
–
Name
3Formula
2–
Name
Formula
Nitride
N3–
Fluoride
F
Oxide
O
Chloride
Cl–
Sulfide
S2–
Phosphide
P3–
Bromide
Br–
Carbonate
CO32–
Phosphate
PO43–
Iodide
I–
Sulfate
SO42–
Hydroxide
OH–
Sulfite
SO32–
Ethanoate
CH3COO–
Oxalate
C2O42–
Permanganate
MnO4–
Chromate
CrO42-
Hydrogen
carbonate
HCO3
Dichromate
Cr2O7
Nitrate
NO3–
Nitrite
NO2–
Cyanide
CN–
–
10
2-
Chemical formulae and naming
The valency of an atom or polyatomic ion can be viewed as a bonding position which must
be filled to form a stable compound. In order to find the formula of a compound you must
recognise the parts or constituents from the name. Since the name is in two parts this
recognition is easy as long as you realise that the ending of the name of the second atom
is often changed to -ide.
Oxygen becomes oxide; sulfur becomes sulfide; chlorine becomes chloride etc.
Na
will join the ratio 1:1
Na
Cl
Cl
 Sodium’s valency is 1+
 Chloride’s valency is 1-
2.
Formula is Na1Cl1 or
NaCl (1 is automatically assumed)
Aluminium nitrate is a compound made from aluminium and nitrate polyatomic ion
NO3
Al
NO3
will join in the ratio 1:3
Al
NO3
NO3
Aluminium has a valency of 3+
Nitrate has a valency of 1-
Formula will be Al(NO3)3
3.
Lead IV oxide is a compound made from lead (valency 4) and oxygen.
Pb
O
will join in the ratio 1:2
O
Pb
O
Lead has a valency of 4+
Oxygen has a valency of 2-
The formula will be PbO2
11
Cross Multiply Method
Step 1.
Write the symbols which represent each part of the name.
Step 2.
Write the valencies as a superscript to each part.
Step 3.
Cancel by dividing the valencies by any common factor
Step 4.
Cross over the numbers to form subscripts, using brackets where necessary.
Examples:
1+
1.
Na
2.
Sodium Chloride
Aluminium Nitrate
1Cl
3+
1-
Al
NO3
4+
3.
Lead (IV) Oxide
Written Exercises:
Pb
O
2 1 4
Formula is Na1Cl1 or NaCl
Formula is Al1(NO3)3 or Al(NO3)3
2Formula is Pb1O2 or PbO2
2
Write chemical formulae for the following compounds.
(a) Hydrogen bromide ………………………
(g) Ammonium nitride ………………………
(b) Calcium chloride ………………………..
(h) Iron (III) hydroxide ………………………
(c) Zinc (II) sulfate ………………………..
(i)
Potassium sulfite ………………………..
(d) Magnesium oxide ………………………
(j)
Nickel (II) carbonate …………………….
(e) Lead (IV) sulfide ………………………
(k) Silver (I) phosphate ……………………..
(f) Barium phosphide ………………………
(l)
12
Tin (II) nitrate ………………………….
Prefix Numbering
When two non-metals combine to form a compound, they can often form several different
compounds depending on the prevailing conditions. For example sulfur and oxygen can
form SO2 or SO3. These compounds are named so that the number of each atom present
is indicated by a prefix.
SO2 is sulfur dioxide
SO3 is sulfur trioxide
The prefixes used are :
mono
1
hexa
6
di
2
hepta
7
tri
3
octa
8
tetra
4
nona
9
penta
5
deca
10
The prefix mono is not used for the first atom, but is used for the second.
For example, CO is carbon monoxide - notice also that the final “o” from the prefix name
is dropped so that it is easier to pronounce.
Written Exercises:
1.
Formulae and Naming (also termed “Nomenclature”)
Write formulae for the following compounds:
(a) potassium nitrate ……………….
(b) sodium carbonate …………………………
(c) cobalt (II) sulfate ………………
(d) ammonium carbonate …………………….
(e) calcium phosphate ………………
(f) disulfur dichloride .....……………………….
(g) nitrogen triodide .........................
(h) iodine heptafluoride .................................
(i) silicon dioxide ............................
(j) selenium dibromide ...................................
2.
Name the following compounds:
(a) SiF4 .............................................
(b) CaSO4 …………………………………….…
(c) FeO …………………………………
(d) CuCl ………………………………………….
13
(e) AgCl ………………………………..
(f) NaHCO3 ……………………………………..
(g) NO .................................................. (h) P2O5 ............................................................
(i) PCl3 ................................................. (j) CS2 ...............................................................
3.
Complete the following table:
Formula
Name
Name
MgO
sodium oxide
CaS
calcium carbonate
CO
ammonia
CO2
aluminium hydroxide
Csl
dinitrogen tetroxide
CaBr2
ammonium sulfate
H2O
aluminium oxide
NO2
copper (I) bromide
N2O4
chromium (III) fluoride
AsCl3
manganese (IV) oxide
AgNO3
mercury (II) sulfide
KBr
oxygen difluoride
Ca(OH)2
iron (II) chloride
CoCl2
iron (III) iodide
HI
copper (II) sulfate
SnO2
carbon tetrachloride
CCl4
strontium chloride
SnCl2
potassium nitrate
BaF2
calcium phosphate
14
Formula
Activity – MODELLING MOLECULES AND COMPOUNDS
Aim: To classify and construct models to represent some common substances.
Task:
1. Correspond a chemical formula to each chemical name.
2. Classify each substance according to chemical bond type.
3. Draw a structural representation of each molecular model.
CHEMICAL NAME /
FORMULA
CLASSIFICATION
(Ionic / Covalent)
Chlorine gas
………….…………….
Oxygen gas
……………………………
Nitrogen gas
……………………………
Ammonia
……………………………
Sodium Chloride
……………………………
Lithium Oxide
……………………………
15
STRUCTURE
Sodium Hydroxide
……………………………
Carbon Dioxide
……………………………
Potassium Sulfate
……………………………
Methane
……………………………
Ethanol
……… C2H5OH ……
Ethanoic Acid
……… CH3COOH ……
Question – Consider and write down some of the advantages and disadvantages of using
molecular model kits to understand the nature of chemical substances.
16