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1.2.2. Covalent Bonding.
Another way to combine elements is to share
electrons between the elements. The compound
formed is a covalent compound. The interaction
of the elements is termed a covalent bond.
e.g. in hydrogen H2
2 H 
H:H
Electrons are equally shared between the H atoms.
In a covalent bond between different elements, the
electrons will not be equally shared.
2
They will spend more time closer to one atom than
the other because the elements will not attract the
electrons equally. This is because the elements will
have:
(i)
different numbers of protons in the nucleus
(which attract electrons)
(ii) different numbers of electrons in the orbitals
(which repel electrons).
Each element (type of atom) will have a different
combination of attractive and repulsive forces,
e.g. for the compound hydrogen chloride, the
electrons are more attracted to the chlorine atom:
H : Cl
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A measure of the ability of an atom in a compound
or molecule to attract bonding electrons to itself is
called the Electronegativity of the element, symbol
 chi.
Definition – the ability of an atom in a molecule to
attract electrons to itself (Chap. 8, p. 312)
Quantitative scales of Electronegativity have been
devised from both experimental data and theoretical
calculations.
The most commonly used scale is that devised by
Pauling.
See Fig. 8.6, p. 312
Trends in electronegativity
Decreases going down the Group e.g. from F
(4.00) to Br (2.8)
Increases going from left to right across the
Periodic Table e.g. from Li (1.00) to F (4.00)
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1.2.3. Polar Covalent Bonding.
Chap. 8, Section 8.4, p. 313-314
Now consider the compounds NaCl, H2 and HCl
The bonding in the first is ionic Na+/Cl(Cl - Na = 3.0 - 0.9 = 2.1)
Large difference in electronegativity
H2 is covalent
(H - H = 2.1 - 2.1 = 0)
No difference in electronegativity (same element)
and HCl?
(Cl - H = 3.0 - 2.1 = 0.9)
Small difference in electronegativity.
Bonding in HCl is described as polar covalent.
Clearly, there must be gradations from covalent to
ionic; the form of bonding will depend on the
elements in the compound, i.e.
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A:A
covalent
A+ :Xpolar covalent
A+
Xionic
In general the type of bonding can be predicted
from the following "rules".
(a) Electronegativity difference between two atoms
> 1.7, then bonding is ionic; if < 1.7, covalent or
polar covalent - the most common type.
Or
(b) Compound formed from a metal and a nonmetal - ionic, if two non-metals combine then
covalent/polar covalent.
(Note: Pure metals and compounds between two
or more metals have another type of bonding called
metallic - we will not consider this type of bonding
further in this course).
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Question
Predict the nature of the bonding present in the
following:
1. CaO
2. BCl3
3. NCl3
4. [NO3]-
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1.3 Calculation of Oxidation States (Oxidation
Numbers) in Main Group Covalent Compounds.
Chap. 4,Section 4.4 p. 139-140
Definition of Oxidation State (O.S.) in a Covalent
Compound:- O.S. is a positive or negative number
assigned to an element in a molecule or ion on the
basis of a set of formal rules
Rules by which oxidation states are assigned:
(1) The O.S. of any atom in its neutral elemental
form is zero
e.g. in H2, the O.S. of H is zero.
(2) The O.S. of fluorine is always -1
(the most electronegative element, i.e. electrons
are always attracted to it).
(3) The sum of the O.S.s of all the atoms in a
neutral compound is zero. The sum of the O.S.s
of all the atoms in an ion is equal to the charge
on the ion.
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(4) The more electronegative element has the
negative O.S. e.g. N is -3 in NH3 but +3 in NF3.
(5) The O.S. of monoatomic ions is the charge on
the ion :
all Group 1 ions = +1, e.g. Na+, O.S. = +1
 all Group 2 ions = +2, e.g. Ca2+, O.S. = +2
 all Group 17 ions = -1 when combined with
elements which are less electronegative, e.g.
Cl- in HCl, O.S. = -1
(6) The O.S. of Oxygen is usually -2 except
(i) in compounds containing O-F bonds
(ii) in compounds containing single O-O
bonds
Notes:
A) These numbers are a formality. May not
correspond to the actual charges on the
atoms in compounds.
B) Usually find a greater range of O.S.s for
elements in covalent compounds than in
ionic compounds. See below.
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Common Oxidation States of the Main group
Elements
1
H
+1
-1
Li
+1
2
12
13
14
15
16
17
18
He
Be
+2
B
+3
F
-1
Ne
Mg
+2
Al
+3
N
5
to
-3
P
+5
+3
-3
O
-1
-2
Na
+1
C
+4
to
-4
Si
+4
S
+6
+4
-2
Ar
K
+1
Ca
+2
Ga
+3
Ge
+4
As
+5
+3
-3
Se
+6
+4
-2
Cl
+7
+5
+3
+1
-1
Br
+7
+5
+3
+1
-1
Zn
+2
Kr
+4
+2