Title S BLOCK.pdf ✅

 Digital www.allendigital.in [ 133 ] Introduction The s-block elements of the Periodic Table are those in which the last electron enters in the outermost s- orbital. As the s-orbital can accommodate only two electrons, two groups (1 & 2) belong to the s-block of the Periodic Table. Group 1 of the Periodic table consists of the elements: lithium, sodium, potassium, rubidium, cesium and francium. They are collectively known as the alkali metals. They are so called because they form hydroxides on reaction with water which are strongly alkaline in nature. The elements of Group 2 include Beryllium, Magnesium, Calcium, Strontium, Barium and Radium. These elements with the exception of beryllium are commonly known as the alkaline earth metals. They are so called because their oxides and hydroxides are alkaline in nature and these metal oxides are found in the earth's crust. Physical properties of s-Block Elements The atomic, physical and chemical properties of alkali metals are discussed below. Atomic and Physical Properties of the Alkaline Metals Physical properties of s-Block Elements ALKALI METALS ALKALINE EARTH METALS Physical state • One electron in outermost shell & General formula ns1 . • Francium is radioactive element. • All are silvery white • Light soft, malleable and ductile metals with metallic lustre. • Two electrons in outer most shell & General formula ns2 . • Radium is radioactive element. • All are silvery white. • These metals are harder than alkali metals. ALKALI METALS ALKALINE EARTH METALS Atomic size • Largest in their respective period (except noble gas element) • Size increases from Li to Cs due to addition of an extra shell. Li < Na < K < Rb < Cs Li Na K ↓ Rb ↓ Cs Be Mg Ca ↓ Sr ↓ Ba IA IIA • Smaller than IA group elements, since extra charge on nucleus attracts the electron cloud. • Size increases gradually from Be to Ba Be < Mg < Ca < Sr < Ba In s-block elements Be has smallest size, while Cs has largest size. s-Block 03
NEET : Chemistry [ 134 ] www.allendigital.in  Digital ALKALI METALS ALKALINE EARTH METALS Melting point and Boiling point • 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. • Decreasing order of melting point and boiling point is Li > Na > K > Rb > Cs • Metallic bond is stronger than IA group due to smaller atomic size and two electrons in valence shell hence melting point and boiling point are higher. • Decreasing order of melting point Be > Ca > Sr > Ba > Mg Boiling point Be > Ba > Ca > Sr > Mg Melting point & Boiling point  Strength of metallic bond  Number of valence shell e– ALKALI METALS ALKALINE EARTH METALS Ionisation energy (I.E.) • First ionisation energy (I.E.) is very less because of larger atomic size and only one electron in outer most shell. • Decreasing order of ionisation energy- Li > Na > K > Rb > Cs • Second ionisation energy of alkali metals is very high because by loosing one electron they achieve inert gas configuration. • First ionisation energy is higher than IA group because of smaller atomic size and completely filled s-orbital (stable electronic configuration) • Decreasing order of ionisation energy– Be > Mg > Ca > Sr > Ba • Second ionisation energy is lesser than IA group. ALKALI METALS ALKALINE EARTH METALS Oxidation state • The alkali metals shows only + 1 oxidation state. (difference between IE1 and IE2 > 16eV) • Alkaline earth metal shows +2. Oxidation state. (difference between IE1 and IE2 < 11eV) ALKALI METALS ALKALINE EARTH METALS Electro positive character or metallic character • Electropositivity  1/Ionisation energy Due to their larger size electron can easily be removed to form M+ ion. Electro positive property increases from Li to Cs. • Their atomic size is smaller than IA group so these are lesser electro positive than IA group. Electropositivity increases from Be to Ba ALKALI METALS ALKALINE EARTH METALS Density (D = M /V) • In a group atomic volume also increase along with atomic weight but atomic weight increases more than atomic volume, so density increases from Li to Cs Increasing order of density Li < K < Na < Rb < Cs • Density increases from Be to Ba Increasing order of density Ca < Mg < Be < Sr < Ba
s-Block  Digital www.allendigital.in [ 135 ] Exception : Density of K is less than Na. Why ? Ans. This is due to presence of vacant d- orbital in the inner shells of K (volume increases, density decreases) ALKALI METALS ALKALINE EARTH METALS Conductivity • Due to the presence of loosely held valence electrons which are free to move in a metal structure, these elements are good conductor of heat and electricity. • These are also good conductor of heat and electricity due to presence of two free electrons. Conductivity of IA < Conductivity of IIA ALKALI METALS ALKALINE EARTH METALS 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 a visible light Li - Crimson red; Na - Golden yellow; K-Violet; Rb - Red violet; Cs-Blue • Due to small size of Be & Mg outer most electrons are tightly bounded. So not excited to higher level, hence they do not give flame test. • Other elements gives characteristic colour to flame Ca-Brick red; Sr-Crimson red; Ba-apple green ALKALI METALS ALKALINE EARTH METALS Photo electric effect • Atomic size of K, Rb and Cs is quite large, so their ionisation energy is very low • Due to very low ionisation energy their valence shell electrons gets excited even by absorbing visible light. That's why Cs is used in photoelectric cells. • These elements do not show this property as their atomic size is small hence ionisation energy is higher than IA group. ALKALI METALS ALKALINE EARTH METALS Complex formation tendency • Only those elements can form complex compounds which have (a) Small cation size (b) High charge density (c) Vacant orbitals to accept electrons. • Only Li+ can form complex compound, due to its small size rest alkali metals have very less tendency to form complex compounds. • Less tendency to form complex compound, but due to small size of cations Be and Mg forms complex compounds like Be–(BeF4) –2 ; Mg – Chlorophyll
NEET : Chemistry [ 136 ] www.allendigital.in  Digital Chemical properties of s-block elements Reactivity • These elements are very reactive, so do not found in free state in nature. Reactivity  1/Ionisation potential order of reactivity – Li < Na < K < Rb < Cs • Less reactive than alkali metals. Order of reactivitity :- Be < Mg < Ca < Sr < Ba Reaction with air • Alkali metals gets tarnish in air due to the formation of oxide at their surface hence they are kept in kerosene or paraffin wax. • 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. • Except Be, these metals easily tarnished in air. • Beryllium in powdered form, burns brilliantly on ignition in air. • In moist air, except Be all the elements converts into carbonates. • In dry air all elements of II A give nitride and oxide both. Reaction with oxygen Oxide ion [O2–] : • Li forms mainly Li2O (Lithium oxide). Peroxide [O2 —2 ] : • Na reacts with O2 to form mainly peroxide (Na2O2). Super oxide [O2 – ] : • K, Rb and Cs forms MO2 type oxides (super oxides) in excess of oxygen. Super oxides are paramagnetic and coloured. ⎯⎯→ ⎯⎯→ ⎯⎯→ 2 2 2 2 O O O Oxide peroxide super oxide M M O M O MO (Li2O) (Na2O2) (KO2, RbO2, CsO2) • Stability order of different oxide of a metal is due to Lattice Energy Normal oxide > Peroxide > Superoxide • Alkaline earth metals reacts with O2 to form 'MO' type oxides (M = Be, Mg, Ca, Sr, Ba) • In IIA only Ca, Sr, Ba form peroxide. Ex. CaO2, SrO2, BaO2 • BeO shows amphoteric property. MgO → weak basic CaO, SrO & BaO → Strong basic • Basic properties increases from BeO to BaO. Reaction with hydrogen • Alkali metals combine with H2 forming ionic hydrides 2M + H2 → 2MH • Except Be all the alkaline metals forms MH2 type hydrides, (MgH2, CaH2, SrH2, BaH2) on heating directly with H2