An Introduction to Environmental Chemistry

electrons. These elements form doubly charged or divalent ions (Be^2 +, Mg^2 +and

Ca^2 +) and are known as alkali earth metals (column IIa in the Periodic Table).

Continued progression up each energy ramp in Fig. 2.1b results in predictable

patterns. For example, Mg is followed by aluminium (Al) which has three valence

electrons, and then silicon with four valence electrons. Progressively more energy

16 Chapter Two

HHe

Li

Be

B C N O F

Ne

Na

Mg

Al

Si P SCl Ar

KCa

10 15 20

Atomic number Z

5

(a)

‘Expected’ first ionization energy

H

He

Li

Be

B

C

N

O

F

Ne

Na

Mg

Al

Si

P

S

Cl

Ar

K

Ca

10 15 20

Atomic number Z

5

(b)

First ionization energy/10

6

J mol

–1

0.5

1.0

1.5

2.0

2.5

1

Fig. 2.1(a) Expected first ionization energy plotted against atomic number (Z), up to Z=20.

This plot disregards the effects of increasing nuclear charge with Z. (b) Variation of measured

first ionization energy with atomic number Z, up to Z=20. The ramped profile (arrows) of

increasing ionization energy following the abrupt drops reflects the increasing nuclear charge.

After Gill (1996), with kind permission of Kluwer Academic Publishers.