An Introduction to Environmental Chemistry

74 Chapter Four

(MgSiO 3 ). As in monomer silicates, bonding within chains is stronger than

bonding between chains, which is between metal ions and non-bridging oxygens.

Double-chain silicates

In this structure the single chains are cross-linked, such that alternate tetrahedra

share an oxygen with the neighbouring chain (Fig. 4.4d). Consequently, this

structure has 1.5 non-bridging oxygens, since, for every four tetrahedra, two

share two oxygens and the other two share three oxygens. The overall Si : O

Box 4.2 Electronegativity

Electronegativity is a measure of the

tendency of an atom to attract an additional

electron. It is used as an index of the

covalent (see Section 2.3.1) or ionic (see

Section 2.3.2) nature of bonding between

two atoms. Atoms with identical

electronegativity, or molecules such as

nitrogen (N 2 ) consisting of two identical

atoms, share their bonding electrons equally

and so form pure covalent bonds. When

component atoms in a compound are

dissimilar, the bonds may become

progressively polar. For example, in hydrogen

chloride (HCl) the chlorine (Cl) atoms have

a strong affinity for electrons, which are to

a small extent attracted away from the

hydrogen (H) towards chlorine. The bonding

electrons are still shared, but not equally as

in N 2.

Hence the chlorine atom carries a slight

negative charge and the hydrogen atom a

slight positive charge. Extreme polarization

means that the bond becomes ionic in

character. A bond is considered ionic if it has

more than 50% ionic character.

Elements that donate electrons (e.g.

magnesium, calcium, sodium and potassium)

rather than attract them are called

electropositive.

H Cl N N bonding electrons

polar bond alent bond

:::

cov

(= )

( )( )

Table 1Partial list of electronegativities and percentage ionic character of bonds with oxygen.

Electro- % Ionic Electro- % Ionic Electro- % Ionic

Ion negativity character Ion negativity character Ion negativity character

Cs+ 0.7 89 Zn^2 + 1.7 63 P^5 + 2.1 35

K+ 0.8 87 Sn^2 + 1.8 73 Au^2 + 2.4 62

Na+ 0.9 83 Pb^2 + 1.8 72 Se^2 - 2.4 —

Ba+ 0.9 84 Fe^2 + 1.8 69 C^4 + 2.5 23

Li+ 1.0 82 Si^4 + 1.8 48 S^2 - 2.5 —

Ca^2 + 1.0 79 Fe^3 + 1.9 54 I- 2.5 —

Mg^2 + 1.2 71 Ag+ 1.9 71 N^5 + 3.0 9

Be^2 + 1.5 63 Cu+ 1.9 71 Cl- 3.0 —

Al^3 + 1.5 60 B^3 + 2.0 43 O^2 - 3.5 —

Mn^2 + 1.5 72 Cu^2 + 2.0 57 F- 4.0 —

Measurements of % ionic character are not applicable for anions since their bonds with oxygen are

predominantly covalent.