An Introduction to Environmental Chemistry

Thus bond formation can be envisaged as a result of attaining noble-gas-type

structures that have particularly stable configurations of electrons. Symbolically

this covalent bond is written F–F. We can think of the bonding electrons, which

tend to sit between the two nuclei, as shielding the repulsive forces of the protons

in the nucleus.

Oxygen and nitrogen are a little different:

eqn. 2.2

For oxygen the argon-like structure requires two electrons from each atom and

the double bond formed is symbolized O=O. For nitrogen we have:

eqn. 2.3

Gases in the atmosphere, water and organic compounds (Section 2.7) are typi-

cally formed with these kinds of covalent bonds.

2.3.2 Ionic bonding, ions and ionic solids

Unlike oxygen (O 2 ) and nitrogen (N 2 ), where individual atoms bond by sharing

electrons, many crystalline inorganic materials bond by donating and accepting

electrons. In fact, it can be argued that these structures have no bond at all,

because the atoms entirely lose or gain electrons. This behaviour is usually

referred to as ionic bonding. The classic example of an ionic solid is sodium chlo-

ride (NaCl):

eqn. 2.4

As in equations 2.1–2.3 the dots represent electrons.

The theory behind this behaviour is that elements with electronic structures

close to those of inert (noble) gases lose or gain electrons to achieve a stable (inert)

structure. In equation 2.4, sodium (Na; Z=11) loses one electron to attain the

electronic structure of neon (Ne; Z=10), while chlorine (Cl; Z=17) gains one

electron to attain the electronic structure of argon (Ar; Z=18). The compound

NaCl is formed by the transfer of one electron from sodium to chlorine and the

solid is bonded by the electrostatic attraction of the donated/received electron.

The compound is electrically neutral.

Crystalline solids, for example NaCl, are easily dissolved in polar solvents

such as water (see Box 4.1), which break down the ionic crystal into a solution of

separate charged ions:

eqn. 2.5

(Note: Most equations in this book will not show the electrons (·) on individual

ions and atoms, only their charge (+/-).)

Positively charged atoms like Na+are known as cations, while negatively

charged ions like Cl-are called anions. Thus, metals whose atoms have one, two

or three electrons more than an inert gas structure form monovalent (e.g. potas-

sium, K+), divalent (e.g. calcium, Ca^2 +) or trivalent (e.g. aluminium, Al^3 +) cations.

Na.+:Cl..

..-

.ª

H 2 O

Na+()aq +:Cl..

..

:()-aq

Na.+:Cl..

..

.ÆNa+:Cl..

..-

:

:N. ̇.+:N ̇..ª: N:::N:

symbolizedN∫N a triplebond()

:O.. ̇.+:O ̇...ª:O..:: O..:

20 Chapter Two