ALKALI METALS

 ALKALI METAL 

Alkali Metal are  lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr)

1.Physical state of alkali metal

•  One electron in outermost shell & General formula ns¹. 
•  Francium is radioactive element. 
• All are silvery white. 
• Light soft, malleable and ductile metals with metallic lustre. 
•  Alkali metals are paramagnetic, diamagnetic and colourless in form of ions.

2.Atomic size of alkali metal 

• Biggest in their respective period (except noble gas element) 
• Size increases from Li to Fr due to addition of an extra shell.  Li < Na < K < Rb < Cs < Fr

3. Softness of alkali metal 

• Alkali metals are soft because of-

 (i) Large atomic size

 (ii) BCC crystal structure (HCP in Li)

 (iii) Loose packing (68% packing efficiency) 

(iv) Weak metallic bond 

• Cs is the softest metal in s-block

4. Melting point and boiling point of alkali metal 

• Weak interatomic bonds are due to their large atomic radii and presence of only one valence electron hence melting point and boiling point are low. 
• Decreasing order of melting point and boiling point of alkali metal  is Li > Na > K > Rb > Cs 
• With the increase in the size of metal atom, the repulsion of the non-bonding electrons increases and therefore melting point and boiling point decreases from Li to Cs.

5. Electro positive character or Metallic character of alkali metal 

   Electropositive ∝ 1/ Ionisation potential

Due to their larger size electron can easily be removed to form M+ ion. Electro positive property increases from Li to Cs. 

6. Flame test  of alkali metal 

Alkali metals and their salts gives characteristic colour to bunsen flame. The flame energy causes an excitation of the outer most electron which on dropping back to ground state emits absorbed energy as visible light.

Example :

 Li-Crimson red 

Na-Golden yellow 

K-Violet 

Rb-Red violet 

Cs-Blue

7. Reaction of alkali metal with NH₃

•  2Li + NH₃   ⟶ Li₂NH (Lithimide) 

2Na + 2NH₃  ⟶ 2NaNH₂ + H₂ ↑ (Sodamide) 

•  Solubility in liquid ammonia 

 (i) All the alkali metals dissolves in NH₃ (liq.) and produces blue solution. 

(ii) This blue solution conducts electricity and possesses strong reducing power, due to the presence of ammoniated electrons. 

Na(s) + (x+y) NH₃ ⟶[Na(NH₃ ) _x ] + + [e(NH₃ ) _y ] – ammoniated electron 

(iii) This dilute solution is paramagnetic in nature

 8. Photo electric effect of alkali metal 

• Atomic size of K, Rb and Cs is quite large, so their ionisation potential is very low.
• Due to very low ionisation potential their valence shell electrons gets excited even by absorbing visible light. That's why Cs is used in photo cells.

9. Standard oxidation potential of alkali metal 

• All the alkali metals have high +ve values of standard oxidation potential (tendency of releasing electrons in water or self ionic solutions).
• So these are good reducing agent, having upper most positions in the electro chemical series. 
• Li has highest standerd oxidation potential (+3.05 eV) due to its high hydration energy. Such that it converts into. Li+ ion by loosing one electron.

10. Hydration energy ( Heat of Hydration ) of alkali metal 

• Alkali metals salts are generally soluble in water due to hydration of cations by water molecules.
• Smaller the cation, greater is the degree of its hydration. 

11. Reducing property of alkali metal 

• Since alkali metals have high standard oxidation potential, so these are strongest reductants. 
• Reducing property increases down the group in gaseous or molten state

 Li+ < Na+ < K+ < Rb+ < Cs+ 

• But in aqueous solution order is -

 Li+ > K+ ~ Rb+ > Cs+ > Na+

12. Reaction of alkali metal with air 

• Alkali metals gets turnish in air due to the formation of oxide at their surface hence they are kept in kerosene or paraffin oil. 
• These elements reacts with moist air to form carbonates 

4Na + O₂ ⟶ 2Na₂O 
Na₂O + H₂O ⟶ 2NaOH (moist) 
2NaOH + CO₂ ⟶ Na₂ CO₃ + H₂ O (in air) 
In dry air only Li gives nitride and oxide both while other elements gives only oxides.

13. Reaction of alkali metal with Oxygen 

• Oxide ion [O_2-] : 

Li forms only Li₂ O (Lithium oxide). 

• Peroxide [O₂] -² : 

Na reacts with O₂ to form peroxide (Na₂O₂ ). 

• Super oxide [O₂ ‐] :

 K, Rb and Cs forms MO₂ type oxides (super oxides) in excess of O₂ . So super oxides are paramagnetic and coloured. 

 Their stability order is – Normaloxide > Peroxide > Superoxide 

14. Reaction of alkali metal with water 

• Alkali metals react vigorously with water forming hydroxides with the liberation of H2 . 

2M + 2H2O ⟶ 2MOH + H2 

•  Reactivity with water increases from Li to Cs. 

Li ⟶ least reactive towards water 

Na ⟶ reacts vigorously

K ⟶ reacts producing a flame

Rb, Cs⟶ reacts explosively. 

• These metals also reacts with alcohol gives alkoxide and H₂ . 

2Li + 2C₂ H₅ OH ⟶ 2C₂H₅O-Li+ + H₂ 

•  Monoxides gives strongly alkaline solution with water 

M₂ O + H₂ O ⟶ 2MOH

15. Reaction of alkali metals with Halides 

•  Alkali metals reacts directly with halogen to form MX 

(M – alkalimetal, X – Halide ion)

• Ionic properties of MX increases from LiCl to CsCl 
• LiCl is covalent in nature (due to polarisation of Cl– ion by small Li+ ion). hence it hydrolyses with water while rest are ionic so do not hydrolyse. 
• K, Rb and Cs halides reacts with more halogens to gives polyhalides. 

KI + I₂ ⟶KI₃ ⟶

K+   + I₃-

 CsBr + Br₂ ⟶CsBr₃⟶Cs+ + Br₃ –

16. Carbonates of alkali metals 

• All the alkali metals forms M₂CO₃ type carbonates. 
• Except Li₂CO₃ , all the carbonates are stable towards heat 

Li₂CO₃     ⟶ (heat)  Li₂O + CO₂ 

•  Thermal stability of carbonates ∝1/ф(Ionic potential) Order of stability is – 

Cs₂CO₃ > Rb₂CO₃ > K₂CO₃ > Na₂CO₃ > Li₂CO₃  

17.  Nitrates of alkali metal 

• Alkali metals forms MNO₃ type nitrates (M – alkali metal) 
• Stability increases from LiNO₃ to CsNO₃ . LiNO₃ decompoes into Lithium oxide & NO₂ on heating. 

4LiNO₃ ⟶(heat + oxide) 2Li₂ O + 4NO₂ + O₂ 

• Other nitrates, on heating to give nitrite and oxygen.

MNO₃⟶ (heat) 2MNO₂ + O₂

18. Nitrides of alkali metals

 Only Li reacts directly with N₂ to form nitride which gives NH₃ on reacting with water. 

6Li + N₂ ⟶2Li₃N 

 Li₃N + 3H₂O ⟶ 3LiOH + NH₃ ↑

  19. Formation of amalgam with alkali metals 

• Alkali metals gives amalgam with Hg. 
• These metals reacts with other metals to give mixed metals (alloys)

20. Sulphates for alkali metals 

• Alkali metals forms M₂SO₄ type sulphates.
• All alkali metal sulphates are ionic. Ionic properties increases from Li to Cs. 

Li₂SO₄ < Na₂SO₄ < K₂SO₄ < Rb₂SO₄ < Cs₂SO₄ 

• Li₂SO₄ Least soluble in water. 
• These sulphates on burning with C forms sulphides 

M₂SO₄ + 4C  →M₂S + 4CO 

• Except lithium, sulphates of IA group reacts with sulphates of trivalent metals like Fe+3, Cr+3, Al+3 etc. gives double salts called alum.

21. Reaction of alkali metals with acids

• Reacts vigaorously with acids 

2M + H₂SO₄ ⟶ M₂SO₄ +H₂ ↑