Electronegativity
http://liakatas.org/chemblog/?page_id=17#
Videos
AQA AS Specification
Lessons
1-2
3
3-5
Topics
Bonding
•understand ionic bonding, covalent bonding, co-ordinate bonding and metallic
bonding in terms of electrons and forces.
Electronegativity
•understand electronegativity and that the electron distribution in a covalent
bond may not be symmetrical
• know that covalent bonds between different elements will be polar to different
extents
• understand qualitatively how molecules may interact by dipole forces and
hydrogen bonding
• understand the importance of hydrogen bonding in determining the boiling
points of compounds and the structures of some solids (e.g. ice) and to
understand changes of state
5-7
Bonding and structure
•recognise the four types of crystal and know the structures of the following
crystals: sodium chloride, magnesium, diamond, graphite, iodine and ice
•be able, in terms of electron pair repulsion, to predict the shapes of, and bond
angles in, simple molecules and ions.
3.3 Electronegativity
How desperate are you for this lesson to end? (tick one)
Very
Slightly
What group do you belong to?
Group I & VII
Not at all
Group II & VI
N/A
Group III & V
Group 4
If you answered “very”, then you
are like elements in Groups I and
VII. They are very desperate to
lose or gain an electron!
This demonstrates the concept of electronegativity:
"The power of an atom in a molecule to attract electrons to itself."
Electronegativity = the power of an atom
to attract the electrons in a covalent bond
The Pauling scale
Increases
Decreases
Why are the halogens
missing?
What does electronegativity
depend upon?
Describe the trends of
electronegativity across the
PT
1. Nuclear charge
2. Distance between the nucleus
and the outer electrons
3. The shielding by inner electrons
H
He
2.1
Li
Be
B
C
N
O
F
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Na
Mg
Al
Si
P
S
Cl
0.9
1.2
1.5
1.8
2.1
2.5
3.0
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
0.8
1.0
1.3
1.5
1.6
1.6
1.5
1.8
1.8
1.8
1.9
1.6
1.6
1.8
2.0
2.4
2.8
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
0.8
1.0
1.2
1.4
1.6
1.8
1.9
2.2
2.2
2.2
1.9
1.7
1.7
1.8
1.9
2.1
2.5
Cs
Ba
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.2
2.2
2.2
2.4
1.9
1.8
1.8
1.9
2.0
2.2
Ne
Ar
Kr
Xe
Rn
Factors affecting electronegativity
9+
1+
H–F
Factors affecting electronegativity
1) Nuclear charge – the more protons, the stronger the
attraction from the nucleus to the bonding pair of
electrons.
2) Atomic radius – the closer the bonding electrons to the
nucleus, the stronger the attraction from the nucleus to the
bonding pair of electrons.
3) Shielding – the less shells of electrons shielding (repelling)
the bonding electrons, the stronger the attraction from the
nucleus to the bonding pair of electrons.
Atomic radii – what’s the link?
Factors affecting electronegativity
1) Nuclear charge – more protons, stronger attraction
between nucleus and bonding pair of electrons.
2) Atomic radius – closer to the nucleus, stronger attraction
between nucleus and bonding pair of electrons.
3) Shielding – less shells of electrons between the nucleus and
the electrons, less shielding (less repulsion), stronger
attraction between nucleus and bonding pair of electrons.
Rules
1. The smaller the atomic radius, the closer the nucleus is to the shared
electrons, the larger the electronegativity
2. The larger the nuclear charge (for a given shielding effect), the greater the
electronegativity
Trend down a group
Electronegativity decreases
• Atomic radius increases
• More shielding
•  Less attraction between nucleus and bonding pair of
electrons
Trend across a period
Electronegativity increases
• Atomic radius decreases
• More nuclear charge
•  Stronger attraction between nucleus and bonding pair of
electrons
Formation of a covalent bond
Inequality
What happens if the partner of oxygen doesn’t
want to give away it’s electron??
The molecule becomes
polar
Polar molecules
What molecule is this?
HCl
Is it polar, or non polar?
polar
How many lone pairs
are there?
three
Electron probability
(density) map
What other bonding
possibilities are there
HCl polar bond showing the unequal
sharing of a cloud of electron
density.
HCl polar bond showing the partial
(+) change on hydrogen and the
partial ( -) change on chlorine.
HCl polar bond showing the
direction of the dipole with an
arrow pointing toward the more
negative atom. The + on the
opposite end also reminds us which
atom is more positive.
Non-polar bond
Polar bond
Electronegativity Difference
0
X
Y
Pure covalent
X
4
+
Y
Polar covalent
Electrons not
equally shared
Polarisation of covalent bonds
X-
Y+
Polar ionic
Distorted ions
X-
Pure ionic
Polarisation of ions
Favoured by small,
highly charged +ve
ions, e.g. Li+, Be2+
Y+
100
90
% ionic characater
80
70
Covalent
bonding
60
50
Ionic
bonding
40
30
20
10
0
0
0.5
1
1.5
2
2.5
3
difference in electronegativity
3.5
4
Question:
Which of the following molecules will be polar?
All of them, except CCl4
HCl, H2O, CCl4, CH2 O
How do we know if a molecule is going to be polar?
Step 1: Draw a reasonable Lewis structure for the substance
Step 1: If the difference in electronegativity for the atoms in a bond is
greater than 0.4, we consider the bond polar.
Step 3: If there are no lone pairs on the central atom, and if all the
bonds to the central atom are the same, the molecule is nonpolar.
If the central atom has at least one polar bond and if the groups
bonded to the central atom are not all identical, the molecule is
probably polar.
Step 4: Describe the polar bonds with arrows pointing toward the
more electronegative element. Use the length of the arrow to show
the relative polarities of the different bonds. Check for symmetry.
Mpt
Structure
NaCl
MgCl2
AlCl3
SiCl4
801ºC
714ºC
190ºC
-70ºC
Polar ionic
Polar
covalent
Covalent
Ionic
Difference in electronegativity decreases
+ve ion gets smaller and more highly
charged, so –ve polarised more
Mpt
BeCl2
MgCl2
CaCl2
SrCl2
BaCl2
401ºC
714ºC
782ºC
870ºC
963ºC
Ionic
Ionic
Ionic
Ionic
Polar
Structure
covalent
Difference in electronegativity decreases
+ve ion gets smaller and more highly
charged, so –ve polarised more
H2O
-
H
+
O
H
+
Bonds: polar
Molecule: polar
NH3
-
H
+
N
H
H
+
+
Bonds: polar
Molecule: polar
CO2
-
O
+
C
-
O
Bonds: polar
Molecule: non-polar
CCl4
-
Cl
Cl
-
C
Cl
+
Cl
-
-
Bonds: polar
Molecule: non-polar
Summary of bonding