Introduction

o A chemical bond is the force that holds a
group of atoms together.15/29

Evaluation of bond polarity

I The electronegativity scale, developed by L.Pauling in the 1930s,
was based on measurements of the strengths of covalent bonds
between di

erent elements

I Fluorine was arbitrarily set an electronegativity of 4.0 (now refined
to 3.98), thereby creating a scale in which all elements have values
between 0 and 4.0
o Bond polarity is evaluated by determining the electronegativity
di
erence between two adjacent atoms that compose the bond

o Regarding the electronegativity di

erence (

EN) between them, a

bond can be classified as:
I ionic if

EN O 2.0
I polar (covalent) if 0.4<

EN2.0

I covalent if

EN AE 0.4

16/29

Example 5

Given the relative electronegativity values (EN) below:
Atoms H C K F Cl
EN 2.1 2.5 0.8 4.0 3.0

classify the following bonds as ionic, polar or covalent : (a) HCl , (b) KF,
and (c) C!=C and H!=C bonds in C2H6

17/29

Dipole moment

I The asymmetrical charge distribution in a polar substance such as
HCl produces a dipole moment (u).For example:
I C(C)C bond has a length of 120.3 pm in C2H2 molecule
I C=C bond has a length of 133.9 pm in C2H4 molecule
I C-C bond has a length of 153.5 pm in C2H6 molecule

19/29

Lewis structure

o A Lewis structure represents the covalent bonding in a molecule in
which :
I only valence electrons are shown
I shared electrons between two atoms are shown either as lines or as
pairs of dots
I lone pairs are shown as pairs of dots

20/29

Drawing Lewis structure

o Lewis structures of compounds such as HF or Cl2 are simple to
draw; however, many complexes molecules and ions (CH2O, CO2!=
3

... ) are not
o Therefore, basic steps should be followed to draw these complexes
compounds:
1.o The ions are oppositely charged; therefore they attract each other to
form ionic networks, lattices
o Generally, ionic compound are formed from the reaction of metals
with non-metals:
I electrons are transferred from the metal to the non metal
I the metal becomes + ion and the non-metal becomes != ion
o For example, Li (Z=3) reacts with F (Z=9) as follow:

o the VE 2s1 of Li is transferred to the F atom to form the LiF ionic
compound.o For example the elements lithium (Z = 3),
beryllium (Z=4) and boron (Z=5) are
written using the Lewis dot symbols
o The Lewis dot symbols explains why :
I lithium, with one unpaired valence
electron, tends to form one bond
I beryllium, with two unpaired valence
electrons, tends to form two bonds
I boron, with three unpaired valence
electrons, tends to form three bonds

5/29

Example 1

1- Draw the Lewis dot symbols for these elements C, N, O, F, Ne.
2- Predict the number of bonds that can be formed in a compound from
these elements ? I and metallic bonds: Cu, Fe ... (solid
pure metals)

o In the 20th century, a system has been
devised to predict the number of bonds
formed by most elements in their
compounds, the Lewis dot symbols

4/29

The Lewis dot symbols

o The Lewis symbols consists of the
chemical symbol for the element with a
dot for each valence electron.o The covalent bond is:
I non-polar in H2 molecule because the electrons are equally shared
I polar in HF molecule because the electrons are not equally shared
o Bond polarity is evaluated by determining the electronegativity
di
erence between two adjacent atoms that compose the bond.o With regard to the electronegativity
di
erence (

EN) between two atoms in a
compound, the formed bond can be: I ionic (Na-Cl), I polar (H-Cl) I or non-polar (Cl-Cl, H-H, C-C,...)

14/29

Electronegativity variation

o In a periodic table, electronegativity increases from lower left to
upper right.o For example, the formation reaction of the molecule Cl2 is as follow :

I each Cl atom has 7 VEs; but only the unpaired electron is involved in
the covalent bond
I the non-bonding electrons are called lone pairs, which are not
involved in covalent bond
I Each Cl in Cl2 molecule has three lone pairs of electrons

11/29

Example 4

1.I atoms such as F, Cl and Br (p block) have higher electronegativities
I atoms such as Li, Na and K (s block) have lower electronegativities
I electronegativity of the d block elements is intermediate between the
alkali and the halogens elements.o A covalent bond between many-electron atoms involves only the
VEs.Explain
2.