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S- Block Elements 37 DGT MH –CET 11th CHEMISTRY Study Material 1 s- Block Elements 37 10. s- Block Elements 10.0 : Prominent Scientists Scientists Contributions Sir Humphry Davy (1778 - 1829) i. Remembered for his discoveries of several alkali and (English chemist) alkaline earth metals. ii. Contributed in the discoveries of the elemental nature of chlorine and iodine. iii. Invented Davy lamp which allowed miners to enter mines. iv. Pioneer in the field of electrolysis using the battery to split up common compounds and prepared many new elements. v. Discovered magnesium, boron, barium and potassium from caustic potash. vi. Isolated elements by electrolysis like potassium (which was first metal isolated by him), sodium (which was isolated by him by electrolyzing molten sodium hydroxide). Arthur H. Crompton (1892 - 1962) i. Proved wave particle duality through his research on X-rays. (American physicist) ii. Did original work on sodium vapour lamp. iii. Developed instrumentation of an aircraft. Received iv. Nobel prize in physics in 1927. 10.1 : General introduction Q.1. State modern periodic law. Ans: Modern periodic law states that, "the physical and chemical properties of the elements are periodic function of their atomic numbers ". Q.2. Why are the elements in the long form of periodic table called s-block, p-block, d-block or f-block elements? Ans: i) The long form of the periodic table is based on the modem periodic law. ii) It is divided into four blocks (s, p, d, f) based on the subshell (s, p, d, f) into which the last/differentiating electron enters. iii) If the electron enters the s-subshell, the element is in s-block and so it is called s-block element. Similarly, the corresponding elements in their respective blocks are therefore called p-block, d-block, f-block elements. Q.3. What are s-block elements? Explain the position of s-block elements in periodic table. Ans: i) In the long form of periodic table, elements in which the last electron in an atom enters the outermost 's' orbital are called 's' block elements. ii) Since the s-orbital can accommodate a maximum of 2 electrons, it consists of two groups namely group-I (alkali metals) and group-2 (alkaline earth metals). iii) Atoms of group-I (with one valence electron) and group-2 (with two valence electrons) are characterized by the general electronic configuration: ns' and ns2 respectively. iv) s-block elements are placed on the extreme left hand side of the long form of the periodic table. v) They are collectively called as normal elements or representative elements. MH - CET Chemistry - XI DGT Group - Tuitions (Feed Concepts) XIth – XIIth | JEE | CET | NEET | Call : 9920154035 / 8169861448 DGT MH –CET 11th CHEMISTRY Study Material 2 s- Block Elements 38 Q.4. Why are the elements of group-1 called alkali metals? Ans: i) Hydrogen (Z = 1), Lithium (Z = 3), Sodium (Z = 11), Potassium (Z = 19), Rubidium (Z = 37), Caesium (Z = 55) and Francium (Z = 87) together constitute group-I. ii) Oxides and hydroxides of group-l elements on combining with water form water soluble bases i.e., alkali. Hence, elements of group-I are called as 'alkali metals' . eg. Na 2 O + H 2 O 2NaOH iii) The word 'alkali' is derived from arabic word 'alquis' meaning 'plant ashes' which are rich in the carbonates of sodium and potassium. Q.5. As per recent trend hydrogen is placed at the top of the periodic table and not in the group 1, why? Ans: i) Hydrogen has atomic number 1 and electronic configuration ls 1 . ii) It resembles alkali metals having the electronic configuration ns', with respect to the formation of unipositive ions and formation of oxides, halides and sulphides. iii) Alkali metals have low ionisation enthalpy, thus, in terms of ionization enthalpy, hydrogen (which has a very high ionisation enthalpy) resembles more with halogens. iv) Hydrogen is gaseous while all other elements of group 1 are solids. v) Also, hydrogen does not possess metallic character under normal conditions unlike the alkali metals which exhibit metallic character. vi) Hydrogen also resembles halogens. – eg. It forms anion H like halides, diatomic molecule H 2 , covalent compounds, etc. vii) Hydrogen is very less reactive as compared to halogens. viii) Thus, hydrogen resembles both the alkali metals and the halogens, though in case of certain properties, it also differs from them. ix) This unique behaviour of hydrogen makes its position in the periodic table controversial or anomalous. x) Therefore, hydrogen is best placed separately at the top of the periodic table. Q.6. Give reasons: Alkaline earth metals do not occur in free state. Ans: Alkaline earth metals have two valence electrons which can be lost easily during a reaction. Hence, alkaline MH - CET Chemistry - XI DGT Group - Tuitions (Feed Concepts) XIth – XIIth | JEE | CET | NEET | Call : 9920154035 / 8169861448 DGT MH –CET 11th CHEMISTRY Study Material 3 s- Block Elements 39 earth metals are highly reactive and do not occur in free state. Q.7. Write the electronic configuration of alkali and alkaline earth metals. Ans: i) In alkali and alkaline earth metals, the last electron (differentiating electron) enters the ns orbital of the atom. ii) Thus, the general electronic configuration of outermost shell of elements of group 1 and group 2 are ns 1 and ns 2 respectively, where 'n' represents the valence shell. iii) The elements belonging to group 1 of the periodic table and their electronic configurations are given below. (Except hydrogen, remaining elements are alkali metals). Period Element Symbol Atomic Electronic con- Electronic Valence shell Number figuration configuration electronic with respect to configuration nearest noble (ns 1 ) gases 1 Hydrogen H 1 1s 1 – 1s 1 2 Lithium Li 3 1s 2 2s 1 [He]2s 1 2s 1 3 Sodium Na 11 1s 2 2s 2 2p 6 3s 1 [Ne]3s 1 3s 1 4 Potassium K 19 1s 2 2s 2 2p 6 3s 2 3p 6 [Ar]4s 1 4s 1 5 Rubidium Rb 37 1s 2 2s 2 2p 6 3s 2 3p 6 [Kr]5s 1 5s 1 6 Caesium Cs 55 1s 2 2s 2 2p 6 3s 2 3p 6 [Xe]6s 1 6s 1 3d 10 4s 6 4p 6 4p 10 5s 2 5d 6 6s 1 7 Francium Fr 87 1s 2 2s 2 2p 6 3s 2 3p 6 [Rn]7s 1 7s 1 3d 10 4s 2 4p 6 4d 10 4f 14 5s 2 5p 6 5d 10 6s 2 6p 6 7s 1 iv) The elements belonging to group 2 of the periodic table and their electronic configuration are given below: Period Element Symbol Atomic Electronic con- Electronic Valence shell Number figuration configuration electronic with respect to configuration nearest noble (ns 1 ) gases 1---- -- 2 Beryllium Be 4 1s 2 2s 2 [He]2s 2 2s 2 3 Magnesium Mg 12 1s 2 2s 2 2p 6 3s 2 [Ne]3s 2 3s 2 4 Calcium Ca 20 1s 2 2s 2 2p 6 3s 2 3p 6 [Ar]4s 1 4s 2 5 Strontium Sr 38 1s 2 2s 2 2p 6 3s 2 3p 6 [Kr]5s 2 5s 2 3d 10 4s 2 4p 6 5s 2 6 Barium Ba 56 1s 2 2s 2 2p 6 3s 2 3p 6 [Xe]6s 1 6s 1 3d 10 4s 6 4p 6 4p 10 5s 2 5p 6 6s 2 7 Radium Ra 88 1s 2 2s 2 2p 6 3p 6 3d 10 [Rn]7s 1 7s 2 4s 2 4p 6 4d 10 4f 14 5s 2 5s 6 5d 10 6s 2 6p 6 7s 2 10.2 : Occurrence Q.8. Write a brief note on the occurrence of s-block elements. Ans: s-block elements of the periodic table consists of two groups: Group-l (Alkali metals) and Group-2 (Alkaline earth metals). i) Alkali Metals : MH - CET Chemistry - XI DGT Group - Tuitions (Feed Concepts) XIth – XIIth | JEE | CET | NEET | Call : 9920154035 / 8169861448 DGT MH –CET 11th CHEMISTRY Study Material 4 s- Block Elements 40 a) Among the alkali metals, sodium and potassium are abundant and lithium, rubidium and caesium have lower abundance. b) Sodium and potassium together make up over 4% by weight of the earth's crust. Sodium and potassium are seventh and eighth most abundant elements respectively by weight in the earth's crust. c) Rubidium and caesium are obtained as a byproduct of lithium processing. They have lower abundance. d) Francium ( 223 Fr) being radioactive does not occur appreciably in nature because its half life period is very small i.e. 21 minutes. ii) Alkaline Earth Metals : a) Beryllium is rare and found on surface deposits of beryl in pegmatite rocks. b) Magnesium is the sixth most abundant element in the earth's crust and also occurs in the sea water in the form of chlorides and sulphates upto the extent of 0.13%. c) Calcium is fifth most abundant element in the earth's crust. It is found in vast sedimentary rocks of CaCO 3 existing as whole mountain ranges of limestone, marble and chalk. d) Strontium and barium have much lower abundance. e) Radium is the rarest of all comprising only 10 –10 percent of igneous rocks. It occurs only in association with uranium having terrestrial abundance of approximately 10 –6 ppm. Note: Occurrence of s-block elements in lithosphere. Li Na K Rb Cs Fr 18 ppm 2.27% by 1.84% by 78–12 ppm 2–6 ppm 10 –18 ppm Be Mg Ca Sr Ba Ra 2ppm 2.76% by 4.6% by 384 ppm 390 ppm 10 –10 % by weight weight weight [ppm stands for parts per million.
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