Group 1 Elements alkali Metals Occurrence, Physical and Chemical Properties, Reactivity and Compounds

Group 1 Elements alkali Metals Occurrence, Physical and Chemical Properties, Reactivity and Compounds

Group I-A Elements

Elements of Group I-A are called alkali metals due to the alkaline nature of their oxides and hydroxides.

Namely, Lithium (Li) , Sodium (Na) , Potassium (K), Rubidium (Rb) , Cesium (Cs) and Francium (Fr) are group 1A elements of the periodic table.

They belongs to s-block of the periodic table due to one one valence electron in s-subshell (ns1).

Occurrence of group 1A elements:

Both alkali metals and alkaline earth metals are very reactive so they do not exist free in nature and exist in a combined state.

Sodium:

Its compounds constitute about 3% of the earth’s crust. The most important ore of sodium is Rock salt or Halite (NaCl). Its large deposits are found throughout the world. In Pakistan, its deposit is found in khewra, and bahaderkhel (Karak). It is also found in seawater.

Other ores of sodium are Chile saltpeter (NaNO₃), Natron (Na₂CO₃.H₂O), Trona (Na₂CO₃.2NaHCO₃.2H₂O) and Borax (Tincal) [Na₂B₄O₇.10H₂O].

Potassium:

Its important ores are Feldspar (KAlSi3O8), Sylvite (KCl) and Carnallite (KCl.MgCl2.6H2O). Deposits of these ores are found in Germany, New Mexico and Searles Lake in California. 

General Physical Properties of Alkali metals (group IA):

(i)  Softness: They are soft and their softness increases down the group. Sodium is so soft that it can be cut even by a blunt knife.

(ii)  Luster: They have a silvery luster when freshly cut.

(iii)  Nature: They are malleable.

(iv)  Melting boiling points: They have low melting and boiling points and which decrease down the group. 

(v)  Electrical conductance: They are good conductors of heat and electricity.

(vi)  Atomic radii: Their atomic radii increase down the group due to the addition of more and more shells.

(vii) Electropositivity: They are highly electropositive and electropositivity increases down the group.

(viii) Electronegativity: Their electronegativities are very low. Their electronegativities decrease down the group. 

(ix)  Ionization Energies: Their ionization energies are very low. So they can easily lose their electrons. Ionization energy decreases down the group. 

(x)    Density: Their densities increases down the group, however, fluctuation occurs at potassium.

(xi)    Reducing ability: They are good reducing agents.

(xii)   Flame test: They give characteristic colors to Bunsen burner flame. When they are heated in the flame their electrons jump to higher orbits. Then on dropping back they emit visible light of different colors.

Element

Li Na K Rb Cs

Color of flame

Crimson red Golden yellow Lilac/violet violet violet

Procedure:

Paste of salt is made in concontrated HCl then a wire of platinum is dipped in the paste and is then brought to the blue flame of Bunsen burner. The color of the flame is observed and identification is made.

Chemical Properties:

Alkali metals are very reactive.

Reaction with water:

Alkali metals are very strong reducing agents so they react vigorously with water and reduce it to hydrogen gas and themselves oxidize to metal hydroxide. 

           2Na + 2H₂O → 2NaOH + H₂

The reaction is highly exothermic so the hydrogen produced catches fire. Vigorousness of reaction increases down the group.

   Lithium shows a slow reaction. Sodium shows a vigorous reaction, fizzing and skating about on the water.          

Potassium shows a more vigorous reaction. It cracks and pops as hydrogen burns. Rubidium and cesium explode violently in contact with water.

Reaction with oxygen:

Alkali metals are very reactive with air. Reactivity increases down the group. So Li, Na and K are stored in kerosene oil while Cs and Rb are stored in a sealed glass tube to prevent air to contact. 

Usually, Lithium produces normal oxide

4Li + O₂ → 2Li₂O (oxidation number of oxygen is -2)

Sodium often produces peroxide

2Na + O₂ → Na₂O₂ (oxidation number of oxygen is -1)

Potassium, Rubidium and Cesium produce superoxide

K + O₂ → KO₂ (oxidation number of oxygen is -1/2)

Reaction with Nitrogen:

Only Lithium reacts with the nitrogen of the air and produces lithium nitride.

6Li + N₂ → 2Li₃N 

Reaction with chlorine:

Sodium burns in chlorine with orange flame. Other alkali metals also burn in chlorine and produce white solid metal chloride. 2Na + Cl₂ → 2NaCl

       2K + Cl₂ → 2KCl

Compounds of Alkali Metals (Group I-A):

Oxides:

Alkali metals in direct reaction with oxygen produce their oxides. Oxides of group I-A are basic.

Reaction with water:

Normal oxides on reaction with water produce metal hydroxide.

Li₂O + H₂O → 2LiOH 

Na₂O + H₂O → 2NaOH                     

Peroxides on reaction with water produce metal hydroxide and hydrogen peroxide.

Na₂O₂ + 2H₂O → 2NaOH + H₂O₂ 

Super oxides on reaction with water produce metal hydroxide and hydrogen peroxide and oxygen. 

2KO₂ + 2H₂O → 2KOH + H₂O₂ + O₂

Reaction with dilute acids:

Normal oxides on reaction with dilute acid produce metal chloride and water.

Li₂O + 2HCl → 2LiCl + H₂O 

Na₂O + 2HCl → 2NaCl + H₂O         

Peroxides on reaction with dilute acid produce metal chloride and hydrogen peroxide.

Na₂O₂ + 2HCl → 2NaCl + H2O₂ 

Super oxides on reaction with dilute acid produce metal chloride and hydrogen peroxide and oxygen. 

2KO₂ + 2HCl → 2KCl + H₂O₂ + O₂

• Potassium super oxide (KO2) is used in breathing equipments of mountaineers for getting oxygen from carbon dioxide.
• 4KO₂ + 2CO₂ → 2K₂CO₃ + 3O₂  

Nitrates:

Effect of Heat on Nitrates:

Nitrates of alkali metals on heating decompose and produce metal nitrites and oxygen. 2NaNO3 → 2NaNO2 + O2

However, the nitrate of lithium on heating decomposes to produce metal oxide, nitrogen dioxide (Brown color gas) and oxygen.

4LiNO₃ → 2Li₂O + 4NO₂ +O₂

 Carbonates:

Effect of Heat on Carbonates:

Carbonates of alkali metals are thermally stable and do not decompose on heating. However, Lithium Carbonate decomposes on heating.

Li₂CO₃ → Li₂O + CO₂

• Carbonates of group I-A are more stable than that of group II-A. Cations of the first group have less charge so less charge density. Therefore, their polarizing power will be low so less covalent character and hence their carbonates are more stable.

Bicarbonates:

Effect of Heat on Hydrogen Carbonates:

Hydrogen Carbonates or Bicarbonates of alkali metals are thermally unstable and so they decompose on heating. 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

The thermal stability of hydrogen carbonates (bicarbonates) group I-A and group II-A increases down the group. Thus LiHCO₃ decomposes easily and CsHCO₃ decomposes difficultly. 

Explanation:

A cation with high charge density causes significant polarization of anion. This creates some covalent character in the compound that decreases the stability and assists in its thermal decomposition. This is due to Fajans’ Rule.

Fajan’s Rule states:

“Small size cations having high charge density tends to make covalent compounds” 

As the size of the cation increases down the group its charge density decreases accordingly. Therefore, its polarizing power also decreases in the same order. This means covalent character decreases down the group and consequently ionic character increases. And, therefore, thermal stability increases down the group.   

• Hydrogen carbonates of group I-A are more stable than that of group II-A. Cations of the first group have less charge so less charge density. Therefore, their polarizing power will be low so less covalent character and hence their hydrogen carbonates are more stable.
• Sodium bicarbonate (NaHCO₃₎ is baking soda. It is used as a source of CO₂ in the baking process.

2NaHCO₃ Heat Na₂CO₃ + CO₂ + H₂O 

     It is also used in fire extinguishers.

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