JEEMAIN.GURU
ALKALI METAL
1. PH YSICAL STATE
(a) One electron in outermost shell & General formula ns 1.
(b) Francium is radioactive element.
(c) All are silvery white
(d) Light soft, malleable and ductile metals with metallic lustre.
(e) Alkali metals are paramagnetic, diamagnetic and colourless in form of ions.
2. ATOMIC SIZE
(a) Biggest in their respective period
(except noble gas element)
(b) Size increases from Li to Fr due to addition of an extra shell.
Li < Na < K < Rb < Cs < Fr
3. SOFTNESS
(a) 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
(b) Cs is the softest metal in s-block
1 1
A tomic size softn ess
stren gth of m etallic bond M elting & Boiling poin t
4. MELTING POINT AND BOILING POINT
(a) 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.
(b) Decreasing order of melting point and boiling point is
Li > Na > K > Rb > Cs
(c) 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 CHAR ACTER OR METALLIC CHAR ACTER
Electropositivity 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.
,JEEMAIN.GURU
6. FLAME TEST
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
Ex. Li-Crimson red Na-Golden yellow K -V i o le t
Rb-Red violet Cs-Blue
7. REACTION WITH NH 3
(a) 2Li + NH3 Li2NH (Lithimide)
2Na + 2NH3 2NaNH2 + H2
(Sodamide)
(b) 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
(a) Atomic size of K, Rb and Cs is quite large, so their ionisation potential is very low
(b) 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
(a) All the alkali metals have high +ve values of standard oxidation potential (tendency of releasing electrons
in water or self ionic solutions)
(b) So these are good reducing agent, having upper most positions in the electro chemical series.
(c) 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.
Order of standard oxidation potential of s - block element
Li > K > Ba > Sr > C a > Na > M g > Be
Hydration energy Charge density on ion
10. HYDR ATION ENERGY (HE AT OF HYDR ATION)
(a) Alkali metals salts are generally soluble in water due to hydration of cations by water molecules.
(b) Smaller the cation, greater is the degree of its hydration.
,JEEMAIN.GURU
(c) Li + Na+ K+ Rb + Cs+
* Degree of hydration decreasing
* Hydration energy decreasing
* Hydrated ion size decreasing
* Ionic conductance increasing
11. REDUCING PROPERTY
(a) Since alkali metals have high standard oxidation potential, so these are strongest reductants.
(b) Reducing property increases down the group in gaseous or molten state
Li+ < Na+ < K+ < Rb+ < Cs+
(c) But in aqueous solution order is -
Li+ > K+ ~ Rb + > Cs+ > Na+
12. REACTION WITH AIR
(a) Alkali metals gets turnish in air due to the formation of oxide at their surface hence they are kept in
kerosene or paraffin oil.
(b) These elements reacts with moist air to form carbonates
4Na + O2 2Na2O
Na2O + H2O 2NaOH
(moist)
2NaOH + CO2 Na2CO3 + H2O
(in air)
In dry air only Li gives nitride and oxide both while other elements gives only oxides.
13. REACTION WITH OXYGEN
Oxide ion [O 2– ] :
Li forms only Li2O (Lithium oxide).
Peroxide [O 2 ] — 2 :
Na reacts with O2 to form peroxide (Na2O2).
Super oxide [O 2 – ] :
K, Rb and Cs forms MO2 type oxides (super oxides) in excess of O2. So super oxides are paramagnetic and
coloured.
O2 O2 O2
M MO
2 MO
2 2 MO
2 2
Oxide perioxide super
oxide
(Li2O) (Na2O2) (KO2, RbO2, CsO2)
Their stability order is –
Normaloxide > Peroxide > Superoxide
ALKALI METAL
1. PH YSICAL STATE
(a) One electron in outermost shell & General formula ns 1.
(b) Francium is radioactive element.
(c) All are silvery white
(d) Light soft, malleable and ductile metals with metallic lustre.
(e) Alkali metals are paramagnetic, diamagnetic and colourless in form of ions.
2. ATOMIC SIZE
(a) Biggest in their respective period
(except noble gas element)
(b) Size increases from Li to Fr due to addition of an extra shell.
Li < Na < K < Rb < Cs < Fr
3. SOFTNESS
(a) 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
(b) Cs is the softest metal in s-block
1 1
A tomic size softn ess
stren gth of m etallic bond M elting & Boiling poin t
4. MELTING POINT AND BOILING POINT
(a) 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.
(b) Decreasing order of melting point and boiling point is
Li > Na > K > Rb > Cs
(c) 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 CHAR ACTER OR METALLIC CHAR ACTER
Electropositivity 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.
,JEEMAIN.GURU
6. FLAME TEST
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
Ex. Li-Crimson red Na-Golden yellow K -V i o le t
Rb-Red violet Cs-Blue
7. REACTION WITH NH 3
(a) 2Li + NH3 Li2NH (Lithimide)
2Na + 2NH3 2NaNH2 + H2
(Sodamide)
(b) 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
(a) Atomic size of K, Rb and Cs is quite large, so their ionisation potential is very low
(b) 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
(a) All the alkali metals have high +ve values of standard oxidation potential (tendency of releasing electrons
in water or self ionic solutions)
(b) So these are good reducing agent, having upper most positions in the electro chemical series.
(c) 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.
Order of standard oxidation potential of s - block element
Li > K > Ba > Sr > C a > Na > M g > Be
Hydration energy Charge density on ion
10. HYDR ATION ENERGY (HE AT OF HYDR ATION)
(a) Alkali metals salts are generally soluble in water due to hydration of cations by water molecules.
(b) Smaller the cation, greater is the degree of its hydration.
,JEEMAIN.GURU
(c) Li + Na+ K+ Rb + Cs+
* Degree of hydration decreasing
* Hydration energy decreasing
* Hydrated ion size decreasing
* Ionic conductance increasing
11. REDUCING PROPERTY
(a) Since alkali metals have high standard oxidation potential, so these are strongest reductants.
(b) Reducing property increases down the group in gaseous or molten state
Li+ < Na+ < K+ < Rb+ < Cs+
(c) But in aqueous solution order is -
Li+ > K+ ~ Rb + > Cs+ > Na+
12. REACTION WITH AIR
(a) Alkali metals gets turnish in air due to the formation of oxide at their surface hence they are kept in
kerosene or paraffin oil.
(b) These elements reacts with moist air to form carbonates
4Na + O2 2Na2O
Na2O + H2O 2NaOH
(moist)
2NaOH + CO2 Na2CO3 + H2O
(in air)
In dry air only Li gives nitride and oxide both while other elements gives only oxides.
13. REACTION WITH OXYGEN
Oxide ion [O 2– ] :
Li forms only Li2O (Lithium oxide).
Peroxide [O 2 ] — 2 :
Na reacts with O2 to form peroxide (Na2O2).
Super oxide [O 2 – ] :
K, Rb and Cs forms MO2 type oxides (super oxides) in excess of O2. So super oxides are paramagnetic and
coloured.
O2 O2 O2
M MO
2 MO
2 2 MO
2 2
Oxide perioxide super
oxide
(Li2O) (Na2O2) (KO2, RbO2, CsO2)
Their stability order is –
Normaloxide > Peroxide > Superoxide
