Bonds as a Continuum
Bonds form because the positively charged nucleus of each atom attracts the negatively charged electrons of other atoms.
Atoms of some elements attract the shared electrons to a much greater extent than do atoms of other elements.
Electronegativity (EN): a measure of an atom’s ability to attract the shared electron pair (bonding electron pair). The higher the value the greater the attraction. Chart pg 174 (not including the noble gases).
1)Electronegativity generally increases across a period (left to right) and up a group.
2)Metals tend to have lower electro negativity values and non metals have higher values.
3)Fluorine has the highest value (3.98)
4)Francium has the lowest value (0.7)
The type of bond that will occur between two elements can be predicted by finding the electronegativity difference between the two elements. (Δ EN)
How to calculate ΔEN
Electronegativity difference (Δ EN) / Example / Bond Type0 / Br Br
Br – Br
ΔEN = ENBr - ENBr
= 2.96 – 2.96
= 0 / Nonpolar Covalent (electrons shared equally)
0 < ΔEN < 1.70 / O H
O – H
ΔEN = ENO – ENH
= 3.44 – 2.20
= 1.24 / Polar Covalent (electrons shared but not equally)
ΔEN > 1.70 / Na Cl
Na – Cl
ΔEN = ENCl – ENNa
= 3.16 – 0.93
= 2.23 / Ionic
( electrons transferred not shared)
Page 178- #7
Polarity of Bonds
H – Br Δ EN = ENBr – ENH
= 2.96 – 2.20
= 0.76 (polar covalent)
δ is the lower case Greek letter delta
δ+ δ- δ- slightly negative
H - Br δ+ slightly positive
The charge in a polar covalent bond is slightly positive at one end and slightly negative at the other. This is a bond dipole.
The δ+ goes above the element with the smaller EN.
The δ- goes above the element with the larger EN.
An arrow can also be used to indicate the direction of the bond dipole (+ to -)
H - Br
Example: hydrogen oxygen bond
O HΔEN = ENO – ENH
= 3.44 – 2.20
O H = 1.24
polar covalent
page 178 - #8,9
Polarity of Molecules
Molecules that seem to be similar may have different properties. e.g. H2O and CO2
The polarity of the molecules can be used to predict properties of covalent compounds. (for example solubility)
Molecules are polar if they have a positive and negative end, these are called dipoles.
To determine if a molecule is polar;
1)Draw the Lewis diagram for the molecule.
2)Draw a structural (line) diagram for the molecule.
3)Using the electronegativity values, determine if the bonds are polar.
4)Use arrows in the molecule to indicate the bond dipoles.
5)Use δ+ and δ- to show the dipoles in a polar molecule.
Examples: H2O, CCl4 and C2H3OH