Interrelationships between the Lightest Metals

Nicholas C. Boyde and Timothy P. Hanusa

Vanderbilt University, Nashville, TN, USA

1. 1 Introduction
2. 2 Nucleosynthesis and Abundance of the Lightest Metals
3. 3 Discovery, Initial Isolation, and Commercial Production
4. 4 Isodiagonal Relationships
5. 5 Solid-State Structures of the Metals
6. 6 Comparative Physical Properties
7. 7 Ceramics, Glasses, and Alloys
8. 8 Conclusions
9. 9 Acknowledgment
10. 10 Glossary
11. 11 Related Articles
12. 12 Abbreviations and Acronyms
13. 13 References

1 Introduction

Use of the expression “the lightest metals” requires some explanation, especially because the terms “light metals” and “heavy metals”, although often encountered in chemical and engineering contexts, are not uniquely defined. Magnesium, aluminum, and titanium (and sometimes beryllium) are traditionally considered among the “light metals” in metallurgical settings, and they are indeed “light” if measured by density (grams per cubic centimeter) as compared to iron or copper (e.g., Mg is only 22% as dense as Fe). However, the use of density alone as a classifier leads to some awkward groupings. For example, yttrium has nearly the same density as titanium (both about 4.5 g cm−3), and barium (3.5 g cm−3) is less dense than either, but neither Y nor especially Ba is ever counted among the “light” metals [with some irony, the name barium, despite the element's low density, comes from the Greek barys, meaning “heavy”; the heaviness refers to the oxide, BaO (5.7 g cm−3), not the metal itself]. Consequently, the “lightest” metals are defined here as those pretransition metals of lowest atomic number; that is, lithium, beryllium, sodium, magnesium, aluminum, potassium, and calcium (atomic no. 3–20). Although they indeed possess relatively low density, they are not a set of the least dense metals in the periodic table [elemental cesium (1.9 g cm−3), e.g., is only 70% as dense as aluminum (2.7 g cm−3)].

Taken as a group, the “lightest metals” are among the most important elements on the Earth and are critical both to life and civilization. Much of their importance stems from their ubiquity: Al is the most abundant metal in the Earth's crust (8.3% by weight) and is a constituent of the widely distributed aluminosilicates such as feldspars, garnets, and kaolin clays.1, 2 The metals Ca, Mg, Na, and K constitute the fifth through eighth most abundant elements in the crust. It is perhaps not surprising that their ions (Ca2+, Mg2+, Na+, and K+) are the most common metal species in biological systems, where they play myriad roles, from the formation of bones to conversion of solar energy through photosynthesis.3, 4 The use of compounds of these metals has been foundational to the development of human society: limestone, marble, and concrete building materials (containing CaCO3, CaO, and MgO) have been associated with cultures from ancient Mesopotamia and China (e.g., in the Great Wall) to the present. The semiprecious lapis lazuli, a mineral whose deep blue color arises from the S3− ions in lazurite, a complex tectosilicate that contains three of the lightest metals (with the formula (Na,Ca)8[(S,Cl,SO4,OH)]2(Al6Si6O24)), has been mined as a gem for at least 6000 years and served as the source of the natural pigment ultramarine since the early European Middle Ages (Figure 1). In the form of alum (KAl(SO4)2·12H2O), aluminum has been used as a mordant in dying cloth for over two millennia.5 The widespread availability of aluminum metal itself at the end of the nineteenth century transformed the modern transportation and building industries, and the aircraft and 100+-story buildings that are known today would not be possible without aluminum and its alloys. Lithium, although far less abundant than sodium or potassium, has become prominently associated with consumer electronics in the form of the lithium-ion battery, powering everything from music players, cell phones, and digital cameras to electric automobiles (see Lithium-Ion Batteries: Fundamentals and Safety). Beryllium, the least abundant of the “lightest” group (although about as common as arsenic), has restricted uses owing to the toxicity of its salts (see Beryllium Metal Toxicology: A Current Perspective), but is a component of widely used copper alloys, and, in the form of emeralds, has been treasured as a precious gemstone for millennia.

Figure 1 Lapis lazuli, the mineral source of the pigment ultramarine, has been used in the arts for millennia; a polished obelisk of the material is behind the natural stone

(By owner's permission)

In the briefest summary, the lightest metals are all lustrous, silvery elements that, with the exception of beryllium, are relatively soft (Mohs hardness 1.5–2.75; that for Be is 5.5, about the same as molybdenum) (see Table 1). The metals are almost never found in the elemental state in nature, as they react in air, and all rapidly form an oxide coating on their surface. The oxide layer of beryllium and aluminum passivates the metals and inhibits further reaction with oxygen or water. The other metals react with water with various degrees of vigor; magnesium reacts with steam, and calcium reacts slowly with cold water. Lithium, sodium, and potassium all float on water and release hydrogen gas as they react with it; in the case of sodium or potassium, the hydrogen will often be ignited from the heat of reaction. With a few important exceptions (see Low Oxidation State Chemistry), the metals all display their “expected” oxidation state in compounds (e.g., +1 for Li and Na, +2 for Be and Mg, and +3 for Al).

Table 1 Atomic and physical properties of the lightest metals

Li

Be

Na

Mg

Al

K

Ca

Atomic number

3

4

11

12

13

19

20

No. of naturally occurring isotopes

2

1

1

3

1

3

6

Atomic mass (g mol−1)

6.94

9.01

23.00

24.31

26.98

39.10

40.08

Electron configuration

[He]2s1

[He]2s2

[Ne]3s1

[Ne]3s2

[Ne]3s23p1

[Ar]4s1

[Ar]4s2

Ionization energy (kJ mol−1)

520.2 (1st)

899.4 (1st); 1757 (2nd)

495.8 (1st)

737.7 (1st); 1451 (2nd)

577.5 (1st); 1816.7 (2nd); 2744.8 (3rd)

418.8 (1st)

589.8 (1st); 1145.4 (2nd)

Metal radius (Å)

1.52

1.12

1.86

1.60

1.43

2.27

1.97

Ionic radius (six-coordination) (Å)

0.76

0.45

1.02

0.72

0.535

1.38

1.00

E° for Mn+(aq) + ne− → M(s) (V)

−3.05

−1.97

−2.71

−2.36

−1.78 (n = 3)

−2.93

−2.84

Melting point (°C)

181

1287

97.7

650

660

63.7

842

Boiling point (°C)

1342

2469

883

1090

2519

759

1484

Density (20 °C)

0.53

1.85

0.97

1.74

2.70

0.86

1.55

ΔHfus (kJ mol−1)

2.93

15

2.64

8.9

10.7

2.39

8.6

ΔHvap (kJ mol−1)

148

309

99

127

294

79

155

Electrical resistivity (20 °C)/μΩ cm

9.5

3.7

4.89

4.5

2.7

7.4

3.4

Data from Ref. 6.

In this article, various chemical and physical properties of both the elemental and ionic forms of the lightest metals are compared to each other and to the related metals and their compounds. The subject is vast, and specifics can be found in various textbooks.7–9 We highlight here some of the distinctive features of this special group of elements.

2 Nucleosynthesis and Abundance of the Lightest Metals

Nucleosynthesis is the process by which elements in the universe heavier than hydrogen have been generated. The lightest elements are generally thought to have been produced during the “big bang”, at the start of the formation of the universe, in the process of big bang nucleosynthesis (BBN).10, 11 The BBN model seeks to explain the genesis...