An Introduction to Environmental Chemistry

Again, it is easy to see that glucose might form a six-carbon cyclic unit. However
this molecule cannot be considered aromatic because it lacks the double bonds
found in the benzene ring. Compounds like glucose that contain elements other
than carbon in the ring structure are called heterocyclic.

The drawing shows a three-dimensional representation of the molecule with,
for example, some OHs sticking up from the ring and some hanging downward.
Common sugars, for example sucrose—the main component of domestic sugar
—are made of short chains of these six-carbon (C 6 ) units. In the case of sucrose just
two C 6 units are present (disaccharide), whereas polymers such as cellulose (see
Box 4.10) are made of much longer chains, giving rise to the term polysaccharide.
Use of the simple carbohydrate formula to represent organic matter has
the advantage of simplifying reactions, but it also means the reactions are very
approximate representations of the complexities found in nature (see e.g. Box
4.10).

2.8 Radioactivity of elements

Where the number of both protons and neutrons in an atom is known we are
able to identify a specific isotope of a specific element and this is termed a nuclide.
Some naturally occurring elements are radioactive and specific isotopes of these
elements are called radionuclides. This term implies that their nuclei are unsta-
ble and spontaneously decay, transforming the nucleus into that of a different
element. Radioactive decay is written in equations that look a little like those for
chemical reactions, but they need to express the atomic mass of the elements
involved and the type of rotation emitted. A number of modes of radioactive
decay are possible, and here we outline some of the common ones. The decay of
potassium (^40 K) can be written:

eqn. 2.12

In this transformation an electron of the potassium is captured by the nucleus,
a proton within it is converted to a neutron and excess energy is lost as a g

(^40) KÆ (^40) Ar+g

O

CH 2 OH

OH

OH

Glucose

OH

OH

HCCCCCCO, which gives the forumla C 6 H 12 O 6

HHHOHHH

OHHOHOHOH

Environmental Chemist’s Toolbox 27