Alkali Metals – Configuration, Properties, Applications

Alkali metals are s-block elements that are found on the periodic table’s left side. Because alkali metals readily lose electrons, they are among the most reactive elements on the planet. The term alkali refers to the basic or alkaline nature of their metal hydroxides in general. The compounds are known as alkali metals because when they react with water, they produce alkalis, which are strong bases that can easily neutralise acids.

Alkali metals have a corresponding [Noble gas] ns¹ electronic configuration. 

They are located in the first column of the periodic table. Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Ru), Cesium (Cs), and Francium (Fr) are alkali elements that occupy successive periods from one to seven. Francium is a radioactive element that has an extremely short half-life. The main reason hydrogen is not considered an alkali metal is that it is most commonly found as a gas at normal temperatures and pressures. When exposed to extremely high pressure, hydrogen can exhibit properties or transform into an alkali metal.

Electronic Configuration of Alkali Metals

The valence shell of alkali metals contains one electron. ns¹ specifies the electronic configuration.

They have a proclivity to lose the outer shell electron, resulting in cations with charge +1. (monovalent ions). As a result, they are the most electropositive elements, and as a result, they are not found in their pure state.

Trends in Physical Properties of Alkali Metals

• Atomic and Ionic Radii of Elements

Elements’ atomic and ionic radii increase in a straight line down the column. Furthermore, in the corresponding period, every alkali metal has the largest radii of any element.

Increasing order of Atomic and Ionic Radius: Li ˂ Na ˂ K ˂ Rb ˂ Cs and Li⁺ ˂ Na⁺ ˂ K⁺ ˂ Rb⁺ ˂ Cs⁺

• Density of Alkali Metals

Alkali elements have the lowest density despite having the largest radius and volume. As a result, they are extremely soft and can be cut with a knife. The elements lithium, sodium, and potassium are all lighter than water. Among alkali metals, potassium has the lowest density.

• Electropositive Metallic Character and Ionization Energy

All alkali metals are electropositive univalent metals. The small lithium atom will require the most ionization energy to remove the valence electron. The valence electron becomes shielded by the inner electrons as the atomic size increases, making it easier to remove with less energy. As a result, as the atomic number increases, so does the ionization energy.

Increasing order of Ionization Energy: Li > Na > K > Rb > Cs

• Solubility or Hydration of Alkali Metal Ions

The most soluble is lithium-ion, and as size increases, so does solubility, with cesium ion being the least water-soluble alkali metal ion. The ionic nature and size of a substance influence its solubility in water. Smaller ions have a higher charge density and can be dissolved by more water molecules. This results in a higher enthalpy of hydration and more stable hydrated ions.

Solubility of Li⁺ > Na+ > K⁺ > Rb⁺ > Cs⁺

• Flame Coloration

The energy required for an electronic transition between available energy levels in s-block elements falls in the visible spectrum region. As a result, when heated, they emit a distinctive colour to the flame that is reflective of their emission or absorption spectrum and can be used to identify them.

Chemical Properties of Alkali Metals

• Hydrides

At higher temperatures, alkali metals react with hydrogen to form metallic hydrides. Hydrides ions are released by metallic hydrides.

2M + H₂ → 2MH → M⁺ + H^–

• Nitrides

Nitrides can be formed when alkali metals react with even atmospheric nitrogen.

6M + N₂ → 2M₃N

• Oxides

Alkali metals tarnish their gleaming nature when they react with atmospheric oxygen. Oxides are formed when they react with oxygen. However, the nature of the oxides formed differs. They each have a different oxidation state for oxygen. Smaller lithium atoms form a normal oxide, while sodium atoms form peroxides and larger atoms form superoxides. Because alkali metals react in the air with nitrogen, oxygen, and water, they are always stored in kerosene.

4Li + O₂ → 2Li₂O (oxides)

2Na + O₂ → Na₂O (Peroxide)

Ce + O₂ → CeO₂ (Superoxide)

Extraction of Alkali Metals

The conventional extraction method is inapplicable to the extraction of alkali metals. Since these metals have the highest electropositivity, displacement and electrolysis are not applicable. Furthermore, high electrode potential limits the ability to reduce agents such as carbon to reduce them. 

Hydrogen ions are preferentially reduced to gaseous hydrogen during aqueous solution electrolysis over sodium ions. As a result, sodium and potassium can only be obtained by electrolysis of the fused salts of sodium hydroxide and sodium chloride. Alkali metals combine to form alloys, amalgams, and amalgams with other metals.

Anomalous Behaviour of Lithium

Because of its small size, high ionization energy, and strongest electropositive and polarizing properties, lithium differs from other alkali metals in that it has a more covalent nature. Because of its small size, higher solubility, and highest electrode potential, lithium has the strongest reducing character.

Diagonal Relationship of Lithium with Magnesium

1. Lithium from the alkali metal group is more similar to magnesium from the alkaline earth metal group.
2. Magnesium and lithium are harder metals with higher melting points.
3. Both slowly react with water, releasing hydrogen.
4. Water hydrolyzes both nitrides, releasing ammonia.
5. Both produce normal oxides, which are less water-soluble.
6. Both produce carbide, which when hydrolyzed yields acetylene.
7. Lithium and magnesium bicarbonates are only stable in solution and not in solid form.

Applications of Alkali Metals

1. Lithium, sodium, and potassium have numerous applications, whereas rubidium and caesium are particularly useful in academic settings.
2. Lithium is commonly used in lithium-ion batteries, and lithium oxide can aid in the processing of silica.
3. Lithium chloride is a brazing alloy used for aluminium parts.
4. Metallic lithium is combined with magnesium and aluminium to create extremely tough and light alloys.
5. Sodium chloride, which is used as table salt.
6. Soap is made from sodium salts of fatty acids.
7. Pure sodium metal has numerous applications, including use in sodium-vapour lamps, which produce very efficient light compared to other types of lighting and can aid in the smoothing of other metals’ surfaces.
8. Potassium compounds are frequently used as fertilisers because potassium is an essential nutrient for plants.
9. Potassium hydroxide is a very strong base that is used to regulate the pH of a variety of substances.

Sample Questions

Question 1: Why are Group I elements known as the most electropositive element?

Answer:

These elements have the most electropositive metals due to the loosely held s-electron in the outermost valence shell. They easily lose electrons, resulting in monovalent M⁺ ions.

Question 2: Why are melting and boiling points of alkali metals low?

Answer:

Since the alkali metals have only one valence electron, their melting and boiling points are low, indicating weak metallic bonding.

Question 3: Why is lithium kept under kerosene oil?

Answer:

Because of their high reactivity to air and water, lithium is typically stored in kerosene oil.

Question 4: Why do alkali metal hydroxides are made the strongest bases?

Answer:

They make the strongest base as they dissolve freely in water and generate a lot of heat due to intense hydration, alkali metal hydroxides are the most powerful of all bases.

Question 5: Why are lithium compounds soluble in organic solvents?

Answer:

They are soluble in organic solvents because of their high polarising power, lithium compounds have an increased covalent character, which accounts for their solubility in organic solvents.