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MISCELLANEOUS CHEMICAL PROPERTIES
1. Periodicity of hydra acids :
(a) Acidic character of hydra acid increases from left to right in a period.
(b) Acidic character of hydra acid increases from top to bottom in a group.
2. Periodicity of oxy acids :
(a) Acidic character of oxy acid increases from left to right in a period.
(b) Acidic character of oxy acid decreases from top to bottom in a group.
3. Periodicity of nature of oxide :
(a) On moving from left to right in a period acidic nature of oxide generally increases.
e.g. CO2 < P2O3 < SO2 < ClO2 acidic nature
(b) On moving from top to bottom in a group acidic nature of oxide generally decreases.
4. Solubility of salt in water: Li2O < Na2O < K2O < Rb2O basic nature
(a) Hydration energy decreases along a group.
(b) Lattice energy decreases along a group.
5. Melting and Boiling Points :
The melting and boiling points of the elements also show periodic variation with atomic number.
Variation in a Period: On moving across the period among the representative elements, the melting and boiling-points first increase, reach a maximum value for elements of group 14 and thereafter start decreasing. The last element of each period i.e., a noble gas has minimum melting and boiling point.
Variation in a Group : In moving down the group, the melting and boiling points do show regular gradation, however, the pattern of variation is determent different groups. For examples, among the alkali metals the melting and boiling point decrease regularly in moving down the group. But among the halogens and noble gases, there is a regular increase in the melting and boiling points.
6. Electropositivity or Metallic Character Tendency of atoms of an element to lose electrons and form positive ion is known as electropositivity.
A more electropositive element has more metallic character
Variation in a period: from left to right electropositivity decreases. This is due to increase in ionization enthalpy along a period which, makes loss of electrons difficult.
For example, in the second period, Lithium and beryllium are metals. Boron is a semimetal whereas carbon. nitrogen, oxygen and fluorine are non-metals.
Variation in a group: from top to bottom electropositivity increase in metallic character (electropositive character) on moving down the group. In this group first two elements (Carbon and Silicon) are nonmetals, the third element (Germanium) is a metalloids whereas the next those elements (Tin and Lead) are metals.
Properties of elements of second period resemble with the element of third period. This type of relation between two periods is called diagonal relationship.
(i) Resemblance between Li and Mg :
(a) Unlike the other members of the group, lithium reacts with N2 to form a nitride in the same way as magnesium does.
(b) Lithium hydroxide, carbonate and nitrate decomposes on heating to give Li2O like Mg but other alkali hydroxides and carbonates do not have any effect on heating where as the nitrate decompose to give nitrite.
(c) Lithium hydroxide, carbonate and fluoride are much soluble than the corresponding sodium or potassium compounds. The solubilities are comparable to those of corresponding magnesium compound.
(ii) Resemblance between Be and Al :
(a) The ionic radius of Be2+ is nearly same as that for the Al3+.
(b) Like aluminium, beryllium is not readily attacked by acids because of the pressure of an oxide film.
(c) Beryllium dissolves in alkali to give the beryllate ion [B (OH)4]2– just as aluminium gives [Al(OH)6]3– .
(d) The oxides BeO and Al2O3 are hard high melting insoluble solids. The oxides as well as their hydroxides are amphoteric and dissolve in sodium hydroxide solution.
(e) Beryllium and aluminium form fluoro complex anion, BeF42– and AlF63– in solution, the other group II metals do not form stable fluoro complexes in solution.
(f) Beryllium chloride (BeCl2) is essentially covalent and has a bridged polymeric structure just as aluminium trichloride is covalent forming a bridged dimer, Al2Cl6. Both the chlorides are soluble in organic solvent and are strong Lewis acid.
(iii) Resemblance between B and Si :
(a) Boron and silicon form numerous hydride which spontaneously catch fire on exposure to air and are easily hydrolysed.
(b) Boron halide like silicon halides hydrolysed by water. Aluminium halides are only partially hydrolysed by water.
(c) Boron forms binary compounds with several metals known as borides just as silicon forms metal silicides some of the borides and silicides undergo hydrolysis to yield boron and silicon respectively.
(d) B2O3 and SiO2 are acidic in nature, Borates and silicates have tetrahedral BO4 and SiO4 structural units respectively. Boro silicates are known in which form can replace silicon in three dimensional lattice. However boron can also form planer BO3 unit.
(e) Both B and Al are semiconductors.
Bridge Elements : Typical elements of II period.
ACID-BASE CHARACTER OF OXIDES
SUMMARY OF PERIODICITY