Study of Representative
Elements (spdf blocks)
Revision Notes on s-Block Elements:
Alkali Metals (Group I elements of modern periodic table):
*ppm (parts per million)
** percentage by weight
Physical Properties of Alkali Metals:
• These are soft in nature and can be cut with the help of knife except the lithium.
• The atoms of alkali metals have the largest size in their respective periods.
• The first ionization energy of the alkali metals are the lowest as compared to the elements in the other group.
• The alkali metals show +1 oxidation state.
• The alkali metals have low values of reduction potential (as shown in table-I) and therefore have a strong tendency to lose electrons and act as good reducing agents.
• The melting and boiling points of alkali metals are very low because the intermetallic bonds in them are quite weak.
• ll the alkali metals form ionic (electrovalent) compounds.
• The alkali metals are good conductors of heat and electricity.
• Alkali metals (except Li) exhibit photoelectric effect
• The alkali metals and their salts impart a characteristic colour to flame
Hydroxides of Alkali Metals:
a) All the alkali metals, their oxides, peroxides and superoxides readily dissolve in water to produce corresponding hydroxides which are strong alkalies.
• 2Na + 2H2O → 2NaOH + H2
• Na2O + 2H2O 2NaOH
• Na2O2 + 2H2O → 2NaOH + H2O2
• 2KO2 + 2H2O → 2KOH + H2O2 + O2
b) The basic strength of these hydroxides increases as we move down the group Li to Cs.
c) All these hydroxides are highly soluble in water and thermally stable except lithium hydroxide.
d) Alkali metals hydroxides being strongly basic react with all acids forming salts.
• NaOH + HCI → NacI + H2O
• 2NaOH + H2 SO4 → Na2SO4 + 2H2O
Halides of Alkali metals:
M2O + 2HX → 2MX + H2O
MOH + HX → MX + H2O
M2CO3 + 2HX → 2MX + CO2 + H2O (M = Li, Na, K, Rb or Cs)
(X = F, Cl, Br or I)
a) Standard enthalpies of formation in (kJ/mol-1)
b) Covalent Character:
• Small cation and large anion favors covalency.
• Order: LiCl > NaCl > KCl > RbCl > CsCl & . LiI > LiBr > LiCl > LiF
• Greater the charge on the cation greater is its polarizing power and hence larger is the covalent character: Na+CI- < Mg+2CI2 < AI+3 CI3
• Greater the charge on the anion, more easily it gets polarized thereby imparting more covalent character to the compound formed eg covalent character increase in the order.
NaCI < Na2SO4 < Na3PO4
c) Lattice Energies: Amount of energy required to separate one mole of solid ionic compound into its gaseous ions.Greater the lattice energy, higher is the melting point of the alkali metals halide and lower is its solubility in water
d) Hydration Energy: Amount of energy released when one mole of gaseous ions combine with water to form hydrated ions.
• M+ (g) + aq → M+ (aq) + hydration energy
• X- (g) + aq → X- (aq) + hydration energy
Higher the hydration energy of the ions greater is the solubility of the compound in water.
The solubility of the most of alkali metal halides except those of fluorides decreases on descending the group since the decrease in hydration energy is more than the corresponding decrease in the lattice energy.
Due to high hydration energy of Li+ ion, Lithium halides are soluble in water except LiF which is sparingly soluble due to its high lattice energy.
For the same alkali metal the melting point decreases in the order
fluoride > chloride > bromide > iodide
For the same halide ion, the melting point of lithium halides are lower than those of the corresponding sodium halides and thereafter they decrease as we move down the group from Na to Cs.
The low melting point of LiCl (887 K) as compared to NaCl is probably because LiCl is covalent in nature and NaCl is ionic.
Anomalous Behavior of Lithium and diagonal relationship with Magnesium:
Li has anomalous properties due to
• Very small size
• High polarizing Power
Lithium show diagonal relationship with magnesium because both elements have almost same polarizing power.
• The melting point and boiling point of lithium are comparatively high.
• Lithium is much harder than the other alkali metals. Magnesium is also hard metal.
• Lithium reacts with oxygen least readily to form normal oxide whereas other alkali metals form peroxides and superoxides.
• LiOH like Mg (OH)2 is weak base. Hydroxides of other alkali metals are strong bases.
• Due to their appreciable covalent nature, the halides and alkyls of lithum and magnesium are soluble in organic solvents.
• Unlike elements of group 1 but like magnesium. Lithium forms nitride with nitrogen.6Li + N2 → 2Li3N
• LiCl is deliquescent and crystallizes as a hydrate, LiCI2H2O. Other alkali metals do not form hydrates. also forms hydrate, MgCI2.8H2O .
• Unlike other alkali metals lithium reacts directly with carbon to form an ionic carbide.Magnesium also forms a similar carbide.
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