CBSE Class 12th Chemistry Notes for Chapter 7 The P-Block Elements

                    UNIT 7 THE p-BLOCK ELEMENTS

 

• Groups 13 to 18 of the periodic table consist of p-block elements with their valence shell configuration ns² np^1-6.

 

• Group 15 consists of five elements: N, P, As, Sb, Bi which have general electronic configuration ns²np³.

 

• Nitrogen differs from other elements of this group due to small size, formation of pπ- pπ bonds with itself and with highly electronegative atom like O or C and non-availability of d orbitals to expand its valence shell.

 

• Elements of group 15 show gradiation in properties. They react with oxygen, hydrogen and halogens. They exhibit two important oxidation states, +3 and +5, but +3 oxidation state is favoured by heavier elements due to ‘inert pair effect’.

 

• Dinitrogen can be prepared in laboratory as well as on industrial scale. It forms oxides in various oxidation states as N₂O, NO, N₂O₃, NO₂, N₂O₄ and N₂O These oxides have resonating structures and have multiple bonds.

 

• Haber’s process: This process is used to prepare ammonia on a large scale.

 

• HNO₃ (nitric acid) is an important industrial chemical. It is a strong monobasic acid and is a powerful oxidizing agent.

 

• Phosphorus exists as P₄ in elemental form. It exists in several allotropic forms. It forms hydride, PH₃ which is a highly poisonous gas. It forms two types of halide as PX₃ and PX₅. PCl₃ is prepared by the reaction of white phosphorus with dry chlorine while PCl₅ is prepared by the reaction of phosphorus with SO₂Cl₂. Phosphorus forms a number of oxoacids. The oxoacids which have P-H bonds are good reducing agents.

 

• The group 16 elements have general electronic configuration ns²np⁴. They show maximum oxidation state +6.

 

• Dioxygen is prepared in the laboratory by heating KClO₃ in the presence of MnO₂. It forms a number of oxides with metals. Allotropic form of oxygen is O₃ which is a highly strong oxidizing agent.

 

• Sulphur forms a number of allotropes. Of these, α- and β- forms of sulphur are the most important. Sulphur combines with oxygen to give oxides such as SO₂ and SO₃. SO₂ is prepared by the direct union of sulphur with oxygen and is used in the manufacture of H₂SO₄.

 

• H₂SO₄ is prepared by contact process. It is a dehydrating and oxidizing agent.

 

• Group 17 consists of the following elements: F, Cl, Br, I and At. These elements are extremely reactive and as such they are found in combined state only. The common oxidation state of these elements is -1.

 

• They form oxides, hydrogen halides, interhalogen compounds and oxoacids.

 

• Chlorine is obtained by the reaction of HCl with KMnO₄.

 

• Halogens combine with one another to form interhalogen compounds.

 

 

 

• Group 18 consists of the noble gases. They have ns²np⁶ valence shell electronic configuration except He which has 1s². All gases except Rn occur in the atmosphere. Rn is obtained as the decay product of ²²⁶

 

• Due to complete octet of outermost shell, they have fewer tendencies to form compounds. The best characterized compounds are those of xenon with fluorine and oxygen only under certain conditions. These gases have several uses like Argon is used to provide inert atmosphere, helium is used in filling balloons for meteorological observations and neon is used in discharge tubes and fluorescent bulbs.