The Group 1 Elements

11.1 The Elements

The Group 1 elements, the alkali metals, are lithium, sodium, potassium, rubidium, caesium (cesium), and francium. Sodium and potassium have high natural abundances, occurring widely as salts such as chlorides. Lithium is relatively rare, occurring mainly in the mineral spodumene, LiAlSi₂O₆. Rubidium and caesium are rarer still but occur in reasonable concentrations in some minerals such as the zeolite pollucite, Cs₂Al₂Si₄O₁₂·nH₂O.

All the Group 1 elements are metals with valence electron configuration ns¹. They conduct electricity and heat, are soft, and have low melting points that decrease down the group. Their softness and low melting points stem from the fact that their metallic bonding is weak because each atom contributes only one electron to the valence band.

Properties of Group 1 Elements

Atomic Radius Trend

Flame Tests

Flame tests are commonly used for the identification of alkali metals and their compounds. Electronic transitions occur within the metal atoms and ions formed in the flames with energies that fall in the visible part of the spectrum:

Reactivity with Water

The thermodynamic tendency to form M⁺ in aqueous conditions is confirmed by the standard potentials of the couples M⁺/M, which are all large and negative, indicating that the metals are readily oxidized. All the elements must be stored under a hydrocarbon oil to prevent reaction with atmospheric oxygen.

11.2 Simple Compounds

Hydrides

Group 1 elements form ionic (saline) hydrides with the rock-salt structure; the anion present is the hydride ion, H⁻.

Halides

All the Group 1 elements form halides, MX. They can be obtained by direct combination of the elements or more normally from solutions. Most of the halides have the 6:6-coordinate rock-salt structure, but CsCl, CsBr, and CsI have the 8:8-coordinate CsCl structure as the larger caesium ion is able to fit a larger number of halide anions around it.

Oxides

The Group 1 elements react vigorously with oxygen:

• Li forms the normal oxide: Li₂O
• Na forms the peroxide: Na₂O₂ (contains O₂²⁻)
• K, Rb, Cs form superoxides: MO₂ (contain paramagnetic O₂⁻)

Other Compounds

The metals react with sulfur to form compounds with the formula M₂S_x, where x lies in the range 1 to 6. Lithium readily forms a nitride, Li₃N, when heated in nitrogen, but the other alkali metals do not react with nitrogen gas. Only Li reacts directly with carbon to form a carbide Li₂C₂ containing the dicarbide (acetylide) anion, C₂²⁻.

Solutions in Liquid Ammonia

Sodium dissolves in liquid ammonia without evolving hydrogen, producing deep-blue solutions that contain solvated electrons. These solutions survive for long periods at and below the normal boiling point of ammonia (−33°C). Concentrated metal–ammonia solutions have a metallic bronze colour and have electrical conductance close to that of a solid metal.

11.3 The Atypical Properties of Lithium

The lightest member of a group often displays properties that are markedly different from those of its congeners. This difference can often be expressed as a diagonal relationship with the element to its lower right in the periodic table (Li with Mg).

Lithium's Unique Properties

11.4 Occurrence and Extraction

Extraction Methods

11.5 Uses of the Elements and Their Compounds

Lithium Applications

• Alloys: Al containing ~2% Li has 6% lower density, used in aircraft parts and aerospace
• Batteries: Primary and rechargeable lithium batteries (LiCoO₂, LiFePO₄, LiC₆)
• Medical: Lithium carbonate for treating bipolar conditions
• Lubricants: Lithium stearate in automotive industry
• Optics: LiMO₃ (M=Nb, Ta) for nonlinear optical devices

Sodium and Potassium Applications

• NaCl: Food flavoring, road de-icing, production of NaOH
• NaOH: One of top 10 industrial chemicals; soap, paper, organic chemistry
• KOH: Soap manufacture ("soft" liquid soaps)
• KCl, K₂SO₄: Fertilizers
• KNO₃: Gunpowder, fireworks, matches

Rubidium and Caesium Applications

• Glass for fiber optics in telecommunications
• Night-vision equipment and photoelectric cells
• Caesium clock (atomic clock): International standard measure of time
• High-density drilling fluids (due to high atomic mass of Cs)

11.6 Hydrides

The Group 1 elements react with hydrogen to form ionic (saline) hydrides with the rock-salt structure; the anion present is the hydride ion, H⁻.

Reactions of Hydrides

The hydrides react violently with water:

Finely divided sodium hydride can even ignite if exposed to humid air. Such fires are difficult to extinguish because even carbon dioxide is reduced when it comes into contact with hot metal hydrides.

Hydrides are useful as non-nucleophilic bases and reductants:

11.7 Halides

Radius Ratio and Structure

Italic = rock-salt structure expected; Bold = CsCl structure (γ > 0.732)

Enthalpies of Formation

The halides are all soluble in water with the exception of LiF, which is only sparingly soluble. This low solubility of LiF can be traced to the fact that the high lattice enthalpy due to the small ionic radii is not offset by the enthalpy of hydration.

11.8 Oxides and Related Compounds

Reactions with Oxygen

Properties of Oxygen Species

Reactions with Water

Air Purification

Potassium superoxide, KO₂, absorbs carbon dioxide, liberating oxygen:

This reaction is exploited to purify air in submarines and breathing apparatus. For aerospace applications, lithium peroxide is often used to reduce weight.

Suboxides

Partial oxidation of Rb and Cs yields suboxides such as Rb₆O, Rb₉O₂, Cs₄O, and Cs₇O. These compounds are dark, highly reactive metallic conductors—some of the earliest metal cluster compounds characterized.

11.9 Sulfides, Selenides, and Tellurides

All the alkali metals form a simple sulfide of stoichiometry M₂S; those of the smaller ions (Li⁺ to K⁺) adopt the antifluorite structure with simple S²⁻ ions. The polysulfides, M₂S_n, with n ranging from 2 to 6, are also known for the heavier alkali metals where the softer acids, M⁺, stabilize the soft bases S_n²⁻.

11.10 Hydroxides

All the hydroxides of Group 1 elements are white, translucent, deliquescent solids. They absorb water and carbon dioxide from the atmosphere in an exothermic reaction. Lithium hydroxide forms the stable hydrate LiOH·8H₂O.

CO₂ Absorption

Industrial Uses of NaOH

• Reagent in organic chemical industry
• Preparation of inorganic chemicals
• Papermaking industry
• Food industry (protein breakdown)
• Oven and drain cleaners

11.11 Compounds of Oxoacids

(a) Carbonates

The Group 1 elements form the only soluble carbonates (with the exception of NH₄⁺), although lithium carbonate is only sparingly soluble.

Solvay Process for Na₂CO₃

The overall reaction uses NaCl and CaCO₃ as feedstocks through a complex stepwise route involving ammonia.

Thermal Decomposition

The carbonates of the heavier elements only decompose significantly when heated above 800°C. This stabilizing influence of a large cation on a large anion can be explained in terms of trends in lattice energies.

(b) Hydrogencarbonates

Applications of sodium hydrogencarbonate:

• Fire extinguisher (smothers flames and releases CO₂)
• Baking (released gases cause dough to rise)
• Antacid to combat indigestion

(c) Other Oxosalts

• Na₂SO₄: Wood pulp processing, glass manufacture, detergents
• NaNO₃: Fertilizers, explosives (deliquescent)
• KNO₃ (saltpetre): Gunpowder (since 12th century), fireworks, matches

11.12 Nitrides and Carbides

Lithium Nitride

The structure consists of sheets of composition Li₂N, containing six-coordinate N³⁻ ions. Li⁺ ions are highly mobile, making it a fast ion conductor—studied for solid electrolytes and rechargeable batteries.

Hydrogen Storage

Li₃N stores up to 11.5% by mass of hydrogen:

Lithium Carbide

Lithium reacts directly with carbon at high temperatures to form Li₂C₂, containing the dicarbide (acetylide) anion, C₂²⁻.

Fullerides

The alkali metals Na to Cs react with fullerene, C₆₀, to form fullerides such as Na₂C₆₀, Cs₃C₆₀, and K₆C₆₀. The structure of K₃C₆₀ contains K⁺ ions in all the octahedral and tetrahedral holes of a close-packed array of C₆₀³⁻ anions; this material becomes superconducting below 30 K.

11.13 Solubility and Hydration

All the common salts of the Group 1 elements are soluble in water. The solubilities cover a wide range of values, some of the most soluble being those for which there is the greatest difference between the radii of the cation and anion.

• Li halides: solubility increases F → Br
• Cs halides: solubility decreases F → I

Most Group 1 salts are anhydrous because their cations have low charge density. Exceptions include:

• LiOH·8H₂O
• Na₂SO₄·10H₂O (Glauber's salt)
• LiX·3H₂O (X = Cl, Br, I)

11.14 Solutions in Liquid Ammonia

The deep blue color originates from the tail of a strong absorption band in the near infrared. Concentrated solutions have a metallic bronze colour with electrical conductance close to that of a solid metal (~10⁷ S m⁻¹). These have been described as "expanded metals."

Reducing Properties

Blue metal–ammonia solutions are excellent reducing agents:

Alkalide Ions

When ethylenediamine (en) is used as a solvent:

The alkalide ions (M⁻) have the spin-paired ns² valence-electron configuration and are diamagnetic.

11.15 Zintl Phases Containing Alkali Metals

Zintl phases are formed when a Group 1 element is combined with a p-block metal from Groups 13 to 16. They are ionic compounds in which electrons are transferred from the alkali metal atom to a cluster of p-block atoms to form a polyanion. These compounds are normally diamagnetic, semiconducting or poor conductors, and brittle.

Examples

• M₄E₄: Contains tetrahedral E₄⁴⁻ anion (e.g., K₄Ge₄)
• M₄E₉: Contains monocapped square antiprismatic anion (e.g., Cs₄Ge₉)
• Rb₂In₃: Contains In₆ octahedra
• Cs₅Bi₄: Contains Bi₄⁵⁻ chains
• Na₉₆In₉₁M₂: Fullerene-type structures (M = Ni, Pd, Pt)

11.16 Coordination Compounds

The Group 1 ions, particularly Li⁺ to K⁺, are hard Lewis acids. Most complexes form from Coulombic interactions with small, hard donors such as O or N atoms. Monodentate ligands are only weakly bound.

Crown Ethers and Cryptands

Macrocycles and crown ethers form strong complexes with Group 1 elements provided their ions have the correct radius to fit into the ligand coordination environment.

Selectivity

The dominant factor is the fit between the cation and the cavity in the ligand:

• 2.2.1-crypt: Favors Na⁺ (smaller cavity)
• 2.2.2-crypt: Favors K⁺ (larger cavity)

Biological Importance

Na⁺ and K⁺ ions cross the hydrophobic cell membrane through embedded protein molecules containing donor-lined cavities. The naturally occurring molecule valinomycin selectively coordinates K⁺: the resulting hydrophobic complex transports K⁺ through bacterial cell membranes, acting as an antibiotic.

Sodides and Electrides

The complexation of sodium with a cryptand can prepare solid sodides such as [Na(2.2.2)]⁺Na⁻. It is also possible to crystallize solids containing solvated electrons—electrides.

11.17 Organometallic Compounds

Group 1 elements form organometallic compounds that are unstable in the presence of water and are pyrophoric (spontaneously ignite) in air. They are prepared in organic solvents such as tetrahydrofuran (THF).

Formation

The cyclopentadienide anion is an important intermediate in synthesis of d-block organometallic compounds.

Organolithium Compounds

Organolithiums are by far the most important Group 1 organometallic compounds. They are:

• Liquids or low-melting solids
• Most thermally stable of the entire group
• Soluble in organic and nonpolar solvents (THF)

Synthesis

Structures

A feature of many main-group organometallic compounds is the presence of bridging alkyl groups:

• Li₄(CH₃)₄: Tetrahedron of Li atoms with bridging CH₃ groups (in ethers)
• Li₆(CH₃)₆: Octahedral arrangement of Li atoms (in hydrocarbon solvents)

These alkyllithiums are electron-deficient compounds containing 3c,2e bonds.

Industrial Applications

• Nucleophiles attacking carbonyl groups in organic synthesis
• Converting p-block halides to organoelement compounds
• Stereospecific polymerization of alkenes to form synthetic rubber
• Synthesis of pharmaceuticals (vitamins A and D, analgesics, antihistamines, antidepressants, anticoagulants)
• Introduction of alkyl groups into d-metal organometallic compounds

Exercises

Chapter Summary

Key Trends Down Group 1