Transcript Slide 1

© www.chemsheets.co.uk
AS 018
09-Jul-12
Electronegativity = the power of an atom
to attract the electrons in a covalent bond
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.
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.
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
Non-polar bond
Polar bond
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.
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
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