Biology 12

Appendix 6 • MHR 563

6

APPENDIX

Electronegativity and the

Periodic Table of the Elements

Chemist Linus Pauling, winner of the Nobel prize,
first defined the concept of electronegativity in
the 1930s. The electronegativity of an element
is a measure of its ability to attract electrons in
a bond. This chemical property is relevant only
to bonding between atoms, not to individual
atoms. Pauling created a quantitative scale of
the electronegativity values of the elements
listed in the periodic table. In general,
electronegativity increases from left to right
along each row (period) of the periodic table.
Electronegativity also tends to increase from
the bottom to the top of each column (group).
Fluorine has the highest electronegativity
value (4.0), so fluorine is said to be the most
electronegative element. Francium and cesium
each have the lowest electronegativity value (0.7),
so they are the least electronegative elements.
The following table lists the electronegativity
values of some biologically important elements.

Electronegativity and

Chemical Bonds

Two atoms of the same element have the same

electronegativity, or the same ability to attract

electrons in a bond. Therefore, if two atoms of

the same element form a bond, as in a molecule

of hydrogen (H 2 ), oxygen (O 2 ), or nitrogen (N 2 ),

the bonding electrons are equally shared

between the atoms. A bond in which electrons

are shared between atoms is called a covalent

bond. If the electrons are equally shared, the

bond is said to be non-polar covalent.

If a covalent bond is formed between atoms

of two different elements, such as hydrogen and

chlorine in hydrogen chloride (HCl), the

electronegativities of the elements determine

how the bonding electrons are distributed.

Because the electronegativities are different, the

bonding electrons are shared unequally. A

covalent bond in which electrons are shared

unequally is called a polar covalent bond. As

shown in Table A6.1, chlorine is more

electronegative than hydrogen, so chlorine has

the stronger attraction for the bonding electrons

in a hydrogen chloride molecule. For this

reason, the chlorine atom becomes slightly

negative, and the hydrogen atom becomes

slightly positive. The slightly charged ends of

the bond are known as poles. Because two

poles are present, the bond is said to have a

dipole. The slightly positive and negative ends

of the polar covalent bond can be represented

using the symbols δ+and δ−, as shown in

Figure A6.1.

Figure A6.1The bond between hydrogen and chlorine

in a hydrogen chloride molecule is polar covalent.

If two elements form a covalent bond, the

difference between their electronegativities

determines how polar the bond is. Table A6.1

shows that the electronegativity difference

between carbon and oxygen is 1.0, whereas the

ClH

δ −δ +

Element Symbol Electronegativity

potassium

sodium

calcium

iron

hydrogen

phosphorus

carbon

sulfur

chlorine

nitrogen

oxygen

K

Na

Ca

Fe

H

P

C

S

CI

N

O

0.8

0.9

1.0

1.8

2.1

2.2

2.5

2.6

3.0

3.0

3.5

Table A6.1

Electronegativity values of some biologically

important elements

Electronegativity:

A Quick Reference for the Biology Student