Modern inorganic chemistry

30 STRUCTURE AND BONDING

Table 2.1

Atomic

number

3

11

19

37

55

Element

Li

Na

K

Rb

Cs

Atomic

radius (s)*

0.152

0.186

0.227

0.248

0.263

Radius* of

M+/ \ ion

(nm)

0.060

0.095

0.133

0.148

0.169

lonisation energies (kJ mol ' )

1st 2nd 3rd

520

496

419

403

376

7297

4561

3069

2650

2420

11800

6913

4400

3900

3300

• Atoms (and ions), unlike ordinary solid spheres, do not have fixed radii; their electron distributions are
  affected by the other atoms (or ions) to which they are bonded, and by the nature of this bonding. However,
  approximate values of atomic size are clearly of value. For a metal, the radius quoted is the 'metallic radius', this
  being half the average mtcrnuclcar distance in the metal For gaseous diatomic molecules joined by a single
  covalent bond (for example Ct Cl), half the Internuclear distance is taken as the 'covalent radius of the atom.
  In the solid noble gases, chemical bonds do not exist, and the solids are held together by weak 'van der Waal's'
  forces (p. 471). Half the internuclear distance is then called the 'van der Waal's' radius. For solid non metals, the
  'atomic radius* may refer to the bulk solid (as for a metal), or to a molecular species such as I 2 , P 4 , or to the free
  atoms. Measurements of the internuclear distance in a solid ionic compound MX gives the sum of the ionic
  radii of M and X. For most purposes, it is sufficient to assume that ionk radii are constant; with this assumption,
  individual ionic radii can be calculated if the radius of one ion can be determined. This can be done by several
  methods which lie outside the scope of this book. Ionic radii quoted in this book are based on Pauling's value for
  the O^2 " ion.

Table 2.2

Atomic

number

4

12

20

38

56

«..

Be

Mg

Ca

Sr

Ba

Atomic

radius

(s)*

0.112

0.160

0.197

0.215

0.221

Radius* of

M2+ion

(nm)

0.031

0.065

0.099

0.113

0.135

lonisat ion energies (kJ mol ')

1st

899

738

590

549

502

2nd

1758

1450

1 146

1064

965

3rd

14850

7731

4942

4200

—

4th

21000

10540

6500

5500

—

' See footnote to Table 2.1.

Table 2.3

Atomic

number

5

13

31

49

81

B

Al

Ga

In

Tl

Atomic

(nm)

0.079

0.143

0.153

0.167

0.171

Radius* of

M^3 + inn

(nm)

(0.020)

0.045

0.062

0.081

0.095

lonisation energies (kJ mol *)

1st

801

578

579

558

589

2nd

2428

1817

1979

1820

1970

3rd

3660

2745

2962

2705

2880

4th

25020

11580

6190

5250

4890

Sec footnote to Table 2,1.