Chemistry, Third edition

4 · BONDING BETWEEN ATOMS

Here,means ‘slightly’: the hydrogen end of the bond is slightly positively charged

and the other end of the bond is slightly negatively charged. The molecule is neutral

overallbecause the charges at either end balance out. A covalent bond which has a

positive end and a negative end is called a polar covalent bondand a molecule which

has a positive end and a negative end is said to possess a dipole(two poles).

58

BOX 4.6

Pauling’s electronegativity values

The ability of an atom to attract a pair of shared electrons in a covalent bond is called the

electronegativityof an atom.

Linus Pauling calculated relative electronegativity values for a number of elements. He

assigned a value of 4 to fluorine, the most electronegative element. Electronegativity values

are shown below:

H

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

KCa Se Br

0.8 1.0 2.5 2.8

If two elements combine, the difference in their electronegativity values can help decide

whether the bonds holding them together are ionic or covalent:

1.The difference between the electronegativity values for sodium and chlorine is

3.00.9 2.1.

This is a large difference and, in general, differences greater than 1.7 indicate that compounds

are ionic.

2.The difference in electronegativity values between the two atoms in a hydrogen molecule is

2.12.1 0.0

A value of 0 shows that the compound is completely covalent.

These examples are two extremes. By calculating these differences for covalent bonds

between atoms, we can compare the polaritiesof covalent bonds. For example, the H–Cl bond

(difference in electronegativities of H and Cl 0.9) is much more polar than the H–Br bond

(difference in electronegativities of the elements 0.7).

Electronegativities can also show, for a polar covalent bond, which ends can be marked +and

. The atom that has the highest electronegativity, or has the greatest attraction for the

shared electrons, is marked . In the HBr molecule, Br has the largest electronegativity value

(2.8) and so the polarity in the molecule is shown as

H——Br

Comparing the

polarity of bonds

Work out the differences in

electronegativities

between the elements in

the following covalent

bonds, rank them in order

of increasing polarity, and

show the direction of each

dipole:

(i) Be——H

(ii)O——H

(iii)C——H

(iv)C——Cl

(v)C——O.

Exercise 4H

Electronegativity values

Box 4.6 gives you only a selection of elements together with their calculated

electronegativity values. One way of determining electronegativities is to measure the

polarities of bonds between various atoms. You would not be able to use this method,

however, to measure the electronegativities of the noble gases He, Ne or Ar. Why do you

think this is?

Exercise 4I