TOPIC 4
How Atoms Combine
Covalent Bonding
 When Atoms join they do so by a Chemical Bond
 When non - metal atoms join they form a Covalent
Bond
 When Atoms join it is their outer electrons that are
directly affected
 Remember that atoms join so as to try and achieve
their nearest most stable electron arrangement.
 To try to do this non-metal atoms overlap and
share Electron Clouds
Covalent Bonding
 Let us consider two Chlorine atoms joining
Cl
**
*
*
Cl
**
Each Chlorine atom has
seven electrons in its outer
energy level
*
**
**
Chlorine atoms
overlap and
electrons are
shared equally
*
*
*
*
*
Cl -Cl
It is the attraction of the
outer unpaired electrons
for the nearby atoms
positive nucleus that pulls
the atoms together
Ionic Bonding
 Ionic Bonding occurs between Metal and a Non-Metal
atoms.
 They do so this to get to their nearest most stable Electron
This leaves the Na
Arrangement ie. 2,2,8 or 2,8,8 etc.
with one less
Consider
Electron arrangement
of each atom
electron and there
fore positively
charged and the and
Cl
the Cl atom with one
2,8,7(17p,17e) more and therefore
negatively charged.
Sodium and Chlorine joining together
Na
2,8,1( 11p, 11e)
e
In order to do this an electron
Na wants to get to an
is transferred from the
EA of 2,8
metal to the non-metal
+
Opposite charges attract to give
Na -Cl
Cl wants to get to
an EA of 2,8,8
Valency
 The Valency is the Combining Power of an atom.
 It is calculated form the number of electrons that an atom
wishes to gain or lose so as to get to its most stable Electron
Arrangement
 Group 1, 2 and 3 metals have a valency of 1+,2+ and 3+
respectively. Groups 7, 6 and 5 have valencies of 1-,2- and 3since they are more likely to receive electrons in order for
them to get to their most stable Electron Arrangement.
 Group 0 elements have valency of zero since they are already
in their most stable EA.
 Valencies are used to write down Chemical Formulae
Chemical Formulae
 Rules for Writing
Chemical Formulae
 1. Write it down in WORDS
 2. Put in the chemical symbols.
Write down the chemical formula
for Aluminium Chloride
Aluminium Chloride
Al
 3. Write in the valencies
 4. Balance the valencies by cross
multiplying.
 5. Write down the formula
Cl
-
3+
Al
Cl
Al
Cl
1
Al 3+
3
( Cl - )
3
Valency of Transition Metals
 Transition Metals can have more than one
valency. Valencies of 1,2,3,4,5,6 & 7+ are
not uncommon. When writing a chemical
formula you will be told the valency to use. It
will be in the form of a Roman Numeral
 eg Zinc(II)Chloride – here the valency of
Zinc is two ie Zn2+(Cl-)2
Iron(III)Oxide - here the valency of Iron is
3+
2
three ie Fe 2O 3
Complex ions
 A complex ion is formed when atoms collect together
to form one ion.
2-
 eg Sulphate – SO4
Carbonate –
When writing
formulae with
complex ions the
complex ion should
be treated just the
same as a single ion
eg
Nitrate - NO3
CO 3
( Na
Cu
+
2+
2-
)
2
-
Phosphate- PO4
SO42-
( NO3 )
2
3-