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Atomic Structure

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Published in: Chemistry
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Atomic Structure

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  1. S-BLOCK ELEMENTS ALKALI METALS : All elements of group 1 are typical metals. These are referred to as alkali metals since they react with water to form highly alkaline substances. Francium is a radioactive element and its longest isotope has a half-life of 21 minutes. So very little is known about this element but in its properties it resembles with Cs and therefore has been placed in group l. MELTING POINT AND BOILING POINT All these elements are soft and have low melting point. The melting points invariably decrease in moving down the group from Li to Cs. The boiling point also decreases in the same order as the melting point. IONIZATION ENERGY The first ionisation energies for the atoms in this group are appreciably lower than those for any other group in the periodic table. On descending the group from Li to Cs, the size of the atoms increases, the outermost electrons become less strongly held and so the ionisation energy decreases. ELECTRONEGATIVITY The electronegativity values for the elements in this group are very small, in fact the smallest values of any element. Thus, when these elements react with other elements to form compounds, a large electronegativity difference between the two atoms is probable and ionic bond is formed. PHOTOELECTRIC EFFECT AND COLOURATION TO THE FLAME Electrons may also be quite readily excited to a higher energy level, for example in the flame test. To perform this test, a sample of the metal chloride, or any salt of the metal moistened with concentrated HCI, is heated on a platinum or nichrome wire in a Bunsen burner flame. In general, chlorides are more volatile than most of other solids. Thus, the colour of the flame is crimson red in the case of Li, yellow in the case of Na, violet in the case of K and almost same in the case of Rb and Cs. COLOUR OF COMPOUNDS Colour arises because the energy absorbed or emitted in electronic transitions corresponds to a wavelength in the visible region. The compounds are typically white. Any transitions, which do occur, will be of high energy, will appear in the ultraviolet region rather than in the visible region and will be invisible to the human eye. LATTICE ENTHALPY OF ALKALI METAL COMPOUNDS Lattice enthalpies (ALHO) of alkali metal salts are very high. The lattice enthalpy decreases (in magnitude) down the group from Li to Cs. The change in enthalpy when one mole of any crystalline solid is completely separated into its constituent units (ions if the solid is ionic and molecule if the solid is molecular) in the gaseous form under standard conditions is called lattice enthalpy (AL HO). For example, the enthalpy change for the reaction, ArH = AL HO is called the lattice enthalpy of M+X (s). As per definition lattice enthalpy is a positive quantity. Compounds of alkali metals are ionic in nature. These compounds consist of cations and anions arranged in space around each other, which are held together by strong columbic forces. Therefore, lattice enthalpies of alkali metal salts are very high. Lattice enthalpy of any salt depends directly upon the product of the charges on the cation and anion and inversely on the sum of ionic radii (r + + L). So, for a salt of particular type, the lattice enthalpy will be lower for bigger ions. That is why, the magnitude of the lattice enthalpy decreases in going from Li to Cs. Example 1 : Solution: The alkali metals have low densities. Explain. The alkali metals have low densities due to their large atomic sizes. In fact, Li, Na and K are even lighter than water.
  2. CHEMICAL PROPERTIES ACTION OF AIR 4Li + 02 21_i20 lithium monoxide 2 N a + 02 Na202 sodium peroxide M (= K, Rb, Cs) + 02 MO 2 superoxide SOLUTION OF METALS IN LIQUID AMMONIA If a small amount of an alkali metal is dissolved in liquefied ammonia, the latter becomes light blue in colour. If more alkali metal is added in the ammonia the later becomes dark blue colour. If more alkali metal is added to the ammonia, a point is reached when a bronze coloured phase separates out and floats on the blue solution. Further addition of alkali metal results in the disappearance of blue solution and its complete conversion to bronze solution. Evaporation of ammonia from the bronze solution allows one to recover unchanged alkali metal. This unusual behaviour has fascinated chemists since its discovery in 1864. The interpretation is as follows: The blue solutions exhibit the following characteristics: Its colour, which is independent of the metal. Its density, when is similar to that at pure NH3. Its conductivity, which is in the same range as those of other electrolytes in NH3. Its paramagnetism indicating unpaired electrons. Its reversible nature. Its strong reducing nature. This has been interpreted in terms of ionisation of alkali metal to form alkali metal cations and electrons which are solvated by ammonia. dissolvein (x+Y)NH3 The dissociation into cation and electron accounts for the electrical conductivity. The dilute solutions thus consist of free e s (thus showing reducing behaviour); such solutions are metastable and when catalysed give hydrogen and amide. MANUFACTURE OF SODIUM CARBONATE Sodium carbonate is manufactured by the Solvay or Ammonia-soda process. Principle When carbon dioxide is passed into a concentrated solution of brine saturated with ammonia, ammonium bicarbonate is produced, NH3 C02 NH40H H 20 H 20 H2C03 NH40H H2C03 NH4HC03 + H20 ammonium bicarbonate The ammonium bicarbonate then reacts with common salt forming sodium bicarbonate, NH4HC03 + NaCl NaHC03 + NH4Cl sodium bicarbonate Sodium bicarbonate being slightly soluble (in presence of sodium ions) gets precipitated. The precipitated sodium bicarbonate is removed by filtration and changed into sodium carbonate by heating. 2NaHC03 Na2C03 + 1420 C02
  3. The mother liquor remaining after the precipitation of sodium bicarbonate contains ammonium chloride. This is then heated by steam with milk of lime to regenerate ammonia, which can be used as one of the raw materials. 2NH4Cl CaC12 Saturating the ammoniating tank + 21420 + 2NH3 In this tank, ammonia gas mixed with a little carbon dioxide gas (from ammonia recovery tower) is bubbled through a 20% sodium chloride solution (brine). Impurities of calcium and magnesium salts present in brine are precipitated as carbonates or hydroxides. H3 + C02 + 1—120 (N CaC12 + (NH4)2C03 + 2NH4Cl MgC12 2NH40H 2NH4Cl MgC12 (NH4)2C03 2NH4Cl The ammonia that escapes absorption in the saturating tank is absorbed by ammonia absorption tower fitted at the top of the saturating tank. Carbonating tower In this tower carbonation of ammoniacal brine is carried out on the principle of counter-current. The clear ammoniacal brine solution is pumped to the top of the tower which flows downward and meets a current of carbon dioxide (obtained from a lime kiln) introduced from the bottom of the tower at a pressure of 1-2 atmosphere. As a result of reaction shown below, ammonium chloride and crystals of sodium bicarbonate are formed. NH3 + C02 + 1-120 2N 1--120 + C02 (NH4)2C03 + H20 + C02 NH4HC03 + NaCl NH4HC03 (NH4)2C03 2NH4HC03 NaHC03 + NH4Cl white crystal These crystals remain suspended in the mother liquor giving rise to thick milky liquid. Example 2 : Solution: What happens when (give chemical equation only): (i) sodium is exposed to moist air (ii) sodium reacts with water (i) 2Na + —02 + + C02 —>Na2C03.H20 (i i) + 21—120 + 1—12 + Heat ALKALINE EARTH METALS : REDUCING AGENTS The high negative value of standard electrode potential indicate that in aqueous solution these elements are good reducing agents quite comparable to alkali metals and this is due to their great hydration energies. The high negative EO values of these elements mean that all react vigorously with water also. COLORATION TO THE FLAME EXCEPT Be AND Mg The chlorides of these elements produce characteristic flames due to easy excitation of electrons to higher energy levels. CONDUCTORS OF HEAT AND ELECTRICITY All alkaline earth metals are good conductor of heat and electricity. SIZE OF ATOMS AND IONS Group Il atoms are large, but are smaller than the corresponding group I elements as the extra charge on the nucleus draws the orbital electrons in. Similarly the ions are large, but are smaller than those of group l, especially because the removal of two orbital electrons increases the effective nuclear charge even further. Thus, these elements have higher densities than group l, metals.
  4. SOLUBILITY AND LATTICE ENERGY The solubility of most salts decreases with increased atomic weight, though the usual trend is reversed with the fluorides and hydroxides in this group. Solubility depends on the lattice energy of the solid and the hydration energy of the ions. With most compounds, on descending the group, the hydration energy decreases more rapidly than the lattice energy; hence the compounds become less soluble as the metal gets larger. However, with fluorides and hydroxides the lattice energy decreases more rapidly than the hydration energy and so their solubility increases on descending the group. SOLUTIONS OF THE METALS IN LIQUID AMMONIA These metals, all dissolve in liquid ammonia as do the group I metals. Dilute solutions are bright blue in colour due to the spectrum from the solvated electron. These solutions decompose very slowly, forming amides and evolving hydrogen, but the reaction is accelerated by many transition metals and their compounds. 2NH3 + + H 2 Evaporation of the ammonia from solutions of group I metals yields the metal, but with group Il metals evaporation of ammonia gives hexammoniates of the metals. These slowly decompose to give amides. + 4NH3 + Concentrated solutions of the metals in ammonia are bronze coloured, due to the formation of metal clusters. OXIDES AND PEROXIDES Action of oxygen/air All the metals of this group (except Be and Mg) are easily oxidised by the atmospheric oxygen. Barium readily inflames in air. All alkaline earth metals have affinity towards oxygen. heat 2M(Be, Mg, ca) + 02 monoxide heat peroxide • M(Ba, Sr) Nature of oxides + 02 Beo is amphoteric, MgO is weakly basic, CaO is more basic while SrO and Bao are extremely basic. Solubility Beo and MgO are insoluble in water, while the other oxides react with water to give corresponding hydroxides of the type viz, Beo and MgO are insoluble in water due to their large lattice enthalpies. SULPHATES The sulphates of alkaline earth metals are less soluble than the sulphate of corresponding alkali metal. The solubility of the sulphates of alkaline earth metals decreases in going down the group. The lattice enthalpies of alkaline earth metal sulphates are higher than those of the alkali metal sulphates. This is why the sulphates of alkaline earth metals are less soluble than those of alkali metals. HALIDES The anhydrous halides are polymeric. Beryllium chloride vapour contains BeC12 and (BeC12)2, but the solid is polymerised. a-Be-Cl (a) Vapour sp hybridized 900 Cl 900 Be 900 a-Be Be-a (b) (c)
  5. Both BeC12 and A12C16 are covalent and have a bridged polymeric structure. Both these chlorides are soluble in organic solvents and act as strong Lewis acid. a-Be CARBIDES Be-Cl The carbides of beryllium (Be) and aluminium (Al) react with water to give Be2C + 2H20 6H20 2BeO + CH4 2A1203 + 3CH4 Example 3 : Solution: Example 4 : Solution: Complete the following reactions: + + 51-420 +61-120 Differentiate between (i) quick-lime (ii) slaked lime and, (iii) lime water (iii) Quick lime is CaO and is obtained by heating CaC03 in a kiln at 1273 K. CaC03 —4—5 CaO + C02 Slaked lime is calcium hydroxide, CaO + Lime water is a solution of in water.

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