Box 42 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 (N2) 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 N2.

H : Cl N : N (: = bonding electrons) (polar bond) (covalent bond)

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.

Table 1 Partial 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

Zn2+

1.7

63

P5+

2.1

35

K+

0.8

87

Sn2+

1.8

73

Au2+

2.4

62

Na+

0.9

83

Pb2+

1.8

72

Se2-

2.4

Ba+

0.9

84

Fe2+

1.8

69

C4+

2.5

23

Li+

1.0

82

Si4+

1.8

48

S2-

2.5

Ca2+

1.0

79

Fe3+

1.9

54

I-

2.5

Mg2+

1.2

71

Ag+

1.9

71

N5+

3.0

9

Be2+

1.5

63

Cu+

1.9

71

Cl-

3.0

Al3+

1.5

60

B3+

2.0

43

O2-

3.5

Mn2+

1.5

72

Cu2+

2.0

57

F-

4.0

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

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

(MgSiO3). 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

(a) SiO4 tetrahedron

(a) SiO4 tetrahedron

(b) Simplified structure for olivine

Fig. 4.4 (a) The schematic SiO4 tetrahedron in Fig. 4.2a can be represented as a tetrahedron, each tip representing the position of oxygen anions. The sketches represent monomer structure in (b) and progressive polymerization of adjacent tetrahedra to form (c) chains, (d) cross-linked double chains and (e) sheets. The cross-linked structures form hexagonal rings which can accommodate anions such as OH-. After Gill (1996), with kind permission of Kluwer Academic Publishers.

Fig. 4.4 (a) The schematic SiO4 tetrahedron in Fig. 4.2a can be represented as a tetrahedron, each tip representing the position of oxygen anions. The sketches represent monomer structure in (b) and progressive polymerization of adjacent tetrahedra to form (c) chains, (d) cross-linked double chains and (e) sheets. The cross-linked structures form hexagonal rings which can accommodate anions such as OH-. After Gill (1996), with kind permission of Kluwer Academic Publishers.

ratio is therefore 4:11, giving a general formula Si4O11. The amphibole group of minerals has double-chain structure —for example, tremolite (Ca2Mg5Si8O22(OH)2).

Sheet silicates

The next step in polymerization is to cross-link chains into a continuous, semi-covalently bonded sheet, such that every tetrahedron shares three oxygens with neighbouring tetrahedra (Fig. 4.4e). This structure has one non-bridging oxygen and the overall Si:O ratio is 4:10, giving a general formula Si4O10. The hexagonal rings formed by the cross-linkage of chains are able to accommodate additional anions, usually hydroxide (OH-). This structure is the basic framework for the mica group—for example, muscovite (Mg3(Si4O10)(OH)4)—and all of the clay minerals. These minerals are thus stacks of sheets, giving rise to their 'platy' appearance.

Framework silicates

In this class of silicates every tetrahedral oxygen is shared between two tetrahe-dra, forming a three-dimensional semicovalent network. There are no non-bridging oxygens, the overall Si: O ratio being 1:2, as in the simplest mineral formula of the class, quartz (SiO2). Substitution of aluminium into some of the tetrahe-dral sites (the ionic radius of aluminium is just small enough to fit) gives rise to a huge variety of aluminosilicate minerals, including the feldspar group, the most abundant mineral group in the crust. Substituting tetravalent silicon for trivalent aluminium causes a charge imbalance in the structure, which is neutralized by the incorporation of other divalent or monovalent cations. For example, in the feldspar orthoclase (KAlSi3O8), one in four tetrahedral sites is occupied by aluminium in place of silicon. The charge is balanced by the incorporation of one K+ for each tetrahedral aluminium.

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