6 Electronegativity And Electron Affinity

1. 6 Electronegativity and
Electron Affinity

2. Electronegativity (p 344)
Electronegativity is the ability of an atom in a covalent bond
to attract electrons to itself.
Fluorine is the most electronegative element - caesium the
least

3. Trends

5. The bigger the difference between the electronegativities of
the elements in an ionic compound the ionic the
compound
The compound with the highest ionic character would be
ceasium fluoride
Electronegativity F 4.0
Cs 0.7
difference 3.3
Compare this to methane (a covalent compound)
C 2.5
H 2.1
difference 0.4

6. So we can find out the % ionic character of a compound by
finding the electronegativity difference between the elements
of the compound
Non-polar0
Covalent<0.4
Polar covalent0.4-1.7
Ionic>1.7
Ionic characterEN

8. Electron affinity (p 310)
Electron affinity is the energy change accompanying the
addition of 1 mol of electrons to 1 mol of gaseous ions or
atoms.
Atom
(g)
+ e
-
 ion
-
(g)

In most cases energy is released when an electron is added
because it is attracted to the positive charge in the
nucleus.

10. Influence of electronegativity on
properties
As the electronegativity decreases from F to C the polarity
of the hydrides will decrease
HF H
2
O NH
3
CH
4
 decreasing polarity
 decreasing hydrogen bond strength

11. We can also compare boiling points of hydrides down
groups. For example for group 6 (O to Te). Only oxygen at
the top of the group is electronegative enough to form a
polar molecule  only water has hydrogen bonds and the
boiling point is much higher than expected compared to
other hydrides down the group.
The other hydrides do not form hydrogen bonds so their bp
is influenced only by their size.  the trend for the rest of
the group is increasing boiling points due to increasing
van der waals forces

13. For group 4 (C to Pb ) CH
4
does not form hydrogen bonds 
the trend depends solely on Van der Waals forces and is
shows an increase in bp’s down the group.
You may have noticed that, although HF forms stronger
hydrogen bonds than water the bp of water is higher than
the bp of HF. This is due to the fact that water forms more
hydrogen bonds than HF due to the larger no of atoms in
the molecule.