THE s-block ELEMENTS Chapter outline Group 1 Elements: Alkali Metals General Characteristics: Physical State Anomalous behaviour of lithium Chemical Properties Reactivity with H & Reaction with H O 2 2 Reactivity with halogens Solutions in liquid ammonia 1
Which atoms come under IA group of S block elements. How to write the electronic configuration of elements. Electron filling in atomic orbital s. Knowledge on some general properties such as density, electro negativity, melting and boiling points. What do you mean by chemical properties. In this chapter we will discuss about Group 1 elements (Alkali metals). We study about Occurrence and General Characteristics of Alkali metals and Characteristics. A complete view about electronic configuration, ionisation enthalpy, density, electro negativity, melting and boiling points, flame coloration and oxidation state of IA group elements. PREREQUISITES LEARNING OBJECTIVES Concept of anomalous behavior of lithium and diagonal relationship of lithium. Few of the Chemical properties of alkali metals. 2
THE s- BLOCK ELEMENTS (ALKALI METALS) GROUP 1 ELEMENTS: ALKALI METALS Introduction tu ts.c om Alkali metals are Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs) and Francium (Fr). Sometimes hydrogen is also included in the group. Lithium Potassium Rubidium ci en ce Sodium Caesium Francium w w w.s Except hydrogen all other elements of the group are referred as alkali metals because their oxides form strong alkalies when dissolved in water. H2 (gas) Li2O H 2O 3
Occurance All these elements are hily reactive. So they do not occur in the free state. They always occur in the combined state. Li - 3 Na - 11 They are all widely distributed in nature in earth s crust. Sodium and Potassium are the most abundant of the alkali metals. They occur mostly as their halides. Example: 1s 2 2s 1 1s 2 2s 2 2p 6 3s 1 Common Salt - NaCl Sylvine - KCl Carnallite - KCl MgCl 2 6H 2 O Earth Crust NaOH Na 2 CO 3 Oxygen: 46.4% Ca: 4.15% Silicon: 28.2% Al: 8.32% Fe: 5.63% K 2 Cr 2 O 7 combined states Na: 4.15% Mg: 4.15% K: 2.09% Ti: 0.57% {H: 0.14% NaCl KCl KCl MgCl 2 6H 2 O 4
GENERAL CHARACTERISTICS Physical state i. All the alkali metals are silvery white (except Caesium, which is golden yellow) solids. Silvery lustre is due to hily mobile electrons of the metallic lattice. Caesium golden yellow ii. Softness also increases from Li to Cs due to weaking of interatiomic attractions. iii.all are soft metals and can be cut with knife. When freshly cut, they have brit lustre which quickly tarnishes due to surface oxidation. surface oxidation softness metals iv. At normal temperature, all the alkali metals adopt Body Centered Cubic (bcc) type of lattice with a co-ordination number of 8. v. At hier temperature (above 156 K) Li metal crystallizes as BCC crystal, like other alkali metals and on cooling below 72K it begins to transform in FCC structure. At very hi temperature, Lithium forms a hexagonal close packed structure with co-ordination number 12. 5
BCC General Electronic configuration: [(n-1)s 2 p 6 ] ns 1 (n is valence shell) FCC Structure BCC Crystal ELECTRONIC CONFIGURATION They are S-block elements as last electron enters in S-sub shell. Element Atomic Number Electronic Configuration Li 3 1s 2 2s 1 Na 11 1s 2 2s 2 2p 6 3s 1 K 19 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 Rb 37 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 5s 1 Cs 55 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 5s 2 5p 6 6s 1 Fr 87 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 4f 14 5s 2 5p 6 5d 10 6s 2 6p 6 7s 1 6
Group 1A atoms are the largest in their horizontal periods in the periodic table. When the outermost electron is removed to give a positive ion, the size decreases considerably. There are two reasons for this Na + i) The positive charge on the nucleus is now acting on lesser number of electrons, i.e., attraction increases which brings contraction in size. ii) Atomic as well as ionic size increases from Lithium to Francium due to the presence of one extra shell of electrons. Radius 400 360 320 280 240 200 160 120 80 Li Na K Rb Ca Fr Atomic & Ionic radii of gp.ia metals IONISATION ENTHALPY Na Atomic radii Ionic radii (m + ) Due to their large size, the outermost electron is far from the nucleus and can easily be removed. Their ionization energies or ionization enthalpies are relatively low. Thus, the metals have a great tendency to lose the electron to change into ions. These metals are hily electropositive in nature. As the ionization enthalpy decreases from Lithium to Caesium, the electropositive character increases, i.e., metallic character increases. The reactivity of these metals increases from Lithium to Cesium. 7
Ionisation Enthalpy KJ mol -1 600 500 400 300 Li Na K Rb Cs Ionisation energies of gp.ia metals e - does not easily remove The densities of alkali metals are quite low as compared to other metals. Lithium, Sodium and Li Potassium are liter than water. On moving from Lithium to Caesium, density of alkali metals increases however density of Potassium is less than Sodium (Exception). DENSITY Li Na K Rb Cs Na Lithium e - easily remove Sodium Potassium Rubidium Caesium 0 0.5 1.0 1.5 2.0 2.5 Density (g/cm 3 ) 8
Explanation Low densities of alkali metals are due to their large atomic radii. Also due to large atomic size, their atoms are less closely packed in their crystal lattice. On moving down, both atomic mass and atomic volume increase but increase in atomic volume can t compensate increase in atomic mass. Exceptionally the lower density of Potassium than Sodium is due to abnormal increase in size of Potassium. Atomic and Ionic Radii 1A Density g ml -1 2.0 1.0 Li + Be 2+ 0 Li Na K Rb Cs Densities of gp.ia metals 1.52 0.60 1.11 0.31 Na + Mg 2+ Al 3+ 1.86 2.31 K + 1.60 2A Ca 2+ 1.43 2.44 0.95 1.33 Rb + 1.48 0.65 3A 0.50 Ga 3+ 1.97 0.99 1.22 0.62 2.15 Sr 2+ In 3+ 1.13 1.62 0.81 9
ELECTRO NEGATIVITY These metals are hily electropositive and thereby posses low values of electro negativities. On moving down in the group from Lithium to Cesium, electro negativity decreases. Explanation 1 0.8 0.6 0.4 0.2 0 Li Na K Rb Cs Electronegativities In alkali metals, the atoms are held together by metallic bonds and stronger is the metallic bond, hier is the m.pt. and b.pt. As the size of atom increases, the metallic bond becomes weak and that is why the lattice energy of metallic crystal lattice is relatively low. Metal m.pt.(k) b.pt.(k) Li Na K Rb Cs Fr 453.75 370.95 336.85 312.65 301.5 1615.0 1156.1 1032.1 961.1 944.1 Explanation MELTING AND BOILING POINTS FLAME COLORATION When heat energy is supplied to alkali metal atom or ion in salt, the electronic excitation occurs in which electron jumps to hier energy level. When this excited electron de-excites to ground state. The energy is emitted in the form of electromagnetic radiation which lies in visible region there by imparting color to the flame. 10
Potassium.c Sodium Caesium w w w.s ci en Rubidium ce tu ts Lithium om THE s- BLOCK ELEMENTS (ALKALI METALS) Excitation & de-excitation of an electron 11
Metal Li Na K Rb Cs Colour Crimson red Yellow Violet Red voilet Blue l / nm 670.8 589.2 766.5 780.0 455.5 All the alkali metals form unipositive (M + ) ion easily by losing ns 1 electron and thus show +1 oxidation state in its compounds. Due to absence of unpaired electron. (M + ) ions are diamagnetic and colorless. Reasons for its abnormal behaviour are mainly due to the following reasons: 1) Lithium atom and its ion has very small size. Lithium ion on account of its small size exerts hi polarising effect on anions in its compounds. Lithium 2) Lithium atom has hiest ionisation enthalpy and low electropositive character. 3) Nonavailability of d-orbitals in its valence shell. OXIDATION STATE ANOMALOUS BEHAVIOUR OF LITHIUM Hiest ionisation enthalpy (I.E) = 519.82 Li - 1s 2 2s 1 1s 2 2s 1 2p 3d Nonavailable d-orbitals Some of the important abnormal characters of lithium are given below 1) It is more harder and liter than other alkali metals. 12
2) It is not affected by air easily and does not lose its luster even on melting. 3) It reacts slowly with water to liberate hydrogen. 4) It does not react with oxygen below 0 C. When burnt in air or oxygen, it forms only monoxide, Li 2 O is much less basic than oxides of other alkali metals. 5) Lithium is the only alkali metal which directly reacts with nitrogen to form Li 3 N 6) Lithium hydroxide decomposes at red heat to form is much less basic than oxides of other alkali metals don t compose. 2LiOH Li 2 O + H 2 O 7) Lithium bicarbonate is known in solution but not in solid state while the bicarbonates of their alkali metals are known in solid state. Lithium bicarbonate Harder 6Li + N 2 Li 3 N 8) Li 2 CO 3 is less stable, as it decomposes on heating. Sodium chloride Harder Liter Li 2 CO 3 Li 2 O + CO 2 13
9) LiNO 3 on heating gives a mixture of NO 2 and O 2 while nitrates of rest of alkali metals yield only oxygen. 4LiNO 3 2LiO 2 + 4NO 2 + O 2 2Na NO 3 2Na NO 2 + O 2 10) Lithium fluoride, lithium phosphate lithium oxalate, lithium carbonate, etc., are sparingly soluble in water. The corresponding salts of other alkali metals are freely soluble in water. 11) LiCl is hily deliquescent and soluble in alcohol and pyridine. LiCl separates out from aqueous solution as hydrated crystals LiCl 2H 2 O. where as chlorides of other alkali metals are insoluble in organic solvents and do not form hydrated crystals under ordinary temperatures. The similarity between Lithium and magnesium is particularly striking and arises because of their similar size. Atomic radii Li = 152 pm, Mg = 160 pm, Ionic radii Water lithium bicarbonate Li 2 CO 3 Li + = 76 pm, Mg +2 = 72 pm and similar polarizing power. Group1 Group2 Group13 Group14 Period 2 Li Be B Na 2 CO 3 Sodium bicarbonate DIAGONAL RELATIONSHIP OF LITHIUM Period 3 Mg Al Si 14
Lithium shows similarity to Magnesium in the following respects i) Lithium is slow to react with water: Magnesium decomposes water only in the hot condition. 2Li + 2H 2 O 2LiOH + H 2 Mg + 2 H 2 O Mg(OH) 2 + H 2 ii) Lithium as well as magnesium combines directly with N 2 forming nitride like Li 3 N and Mg 3 N 2 6Li + N 2 Li 3 N iii) Both Lithium and Magnesium give only monoxides Li 2 O, MgO. iv) Lithium chloride is deliquescent like MgCl 2 LiCl undergoes hydrolysis to a smaller extent in hot water in a similar way to MgCl 2. v) Due to their covalent nature, the halides lithium and magnesium are soluble in organic solvents. vi) Both Li + and Mg +2 are hily hydrated. vii) The carbonates, phosphates, and fluorides of both Lithium and Magnesium are sparingly soluble in water. The alkali metals are hily reactive due to their large size and low ionization enthalpy. Reactivity towards air The alkali metals tarnish in dry air due to the formation of their oxides which in turn react with moisture to form hydroxides. They burn vigorously in oxygen forming oxides. Lithium forms monoxide, sodium forms peroxide, the other metals form superoxides. The superoxide O 2 - ion is stable only in the presence of large cations such as Potassium, Rubidium, Caesium. 2- CHEMICAL PROPERTIES 1 2Li + 2 O 2 Li 2 O (Lithium Oxide) 2Na + O 2 Na 2 O 2 (Sodium peroxide) M + O 2 MO 2 (Superoxide) 2-2- x x x xx xx Oxide Peroxide Superoxide 15
The normal oxides of Li and Na are colourless and diamagnetic, but surprising K 2 O is pale yellow, Rb 2 O is brit yellow and Cs 2 O is orange. Sodium peroxide is yellow, probably due to presence of small amount of superoxide. Superoxide ion O 2 - has three electron bond with one unpaired electron. Brit yellow Colourless Pale yellow S No. Type of Oxide Formula of the Oxide and its colour Orange 1 MonoOxide Li 2 O Na 2 O K 2 O Rb 2 O Cs 2 O (M - ) 2 (O 2- ) (White) (White) (White) (Pole Yellow) (Orange Red) 2 Peroxide --- Na 2 O 2 K 2 O 2 Rb 2 O 2 Cs 2 O 2 (M + ) 2 (O - ) 3 Superoxide --- --- KO 2 RbO 2 CsO 2 (M + ) (O - ) 2 (Orange) (Brown) (Orange) Explanation The formation of various oxides is explained as. 1 2 O 2 O 2 - O 2 2-2O 2 - The normal oxides M 2 O react with water to form hydroxides. O 2 M 2 O + H 2 O 2MOH The basic nature of the oxides (M 2 O) increases gradually on moving down in the group. CsOH > RbOH > KOH > NaOH > LiOH 16
REACTIVITY WITH H 2 Alkali metals react with H 2 directly at 300-600 C and form hydrides. The reaction can be written as follows: M + H 2 2MH Where, M = Li, Na, K, Rb, or Cs. These hydrides are ionic in nature. Their ionic nature increases with the metallic nature of alkali metals. CsH > RbH > KH > LiH The thermal stability of the hydrides decreases with increase in ionic nature. The electrolysis of the fused hydride (LiH) yields hydrogen at anode [other hydrides decompose before melting]. The hydrides react with water liberating hydrogen. MH + H 2 O 2MOH + H 2 All the alkali metals react with water to form metal hydroxide and H 2. Reaction is hily exothermic. Sometimes evolved H 2 may catch fire. Order of reactivity: REACTIVITY WITH WATER 2M+ 2 H 2 O 2MOH + H 2 + Heat Lithium < Sodium < Potassium < Rubidium < Caesium Lithium decomposes water very slowly at 25 C. Sodium melts on the surface of the water and molten metal skates about vigorously and may catch fire Potassium, Rubidium and Caesium always catches fire. REACTIVITY WITH HALOGENS However lithium halides are somewhat covalent. It is because of the hi polarisation capability of lithium ion. The melting and boiling points always follow the trend: Fluoride > Chloride > Bromide > Iodide 17
All these halides are soluble in water. The low solubility of LiF in water is due to its hi lattice enthalpy where as the low solubility of CsI is due to smaller hydration enthalpy of its two ions. The blue colour of the solution is due to the ammoniated electron which absorbs energy in the visible region of lit and thus imparts blue colour to the solution. In the presence of catalyst and impurities, When dry ammonia is passed over hot metal, amides are formed. Finally towards the end of this chapter we have come across the following Occurrence and General Characteristics of Alkali metals and Characteristics. General electronic configuration and some other important properties of IA group elements. Concept of anomalous behavior of lithium and diagonal relationship of lithium. mportant Chemical properties of alkali metals. SOLUTIONS IN LIQUID AMMONIA SUMMARY Hydride, oxides and hydroxides of alkaline metals and their reactivity towards different elements. 18