S Block Elements Class 11
OCCURRENCE OF S BLOCK ELEMENTS
S – Block elements do not occur in nature in free state. Among the Alkali Metals sodium and potassium are abundant and lithium, rubidium and cesium have lower abundance. Among the alkaline earth metals magnesium and calcium are abundant in the earth crust. Radium is rarest of all.
GROUP – 1 ELEMENTS : ALKALI METALS
The general electronic configuration of group – 1 element is ns1. All alkali metals have one valance electron hence form monovalent M+ions, and are highly reactive. Group – 1 consists of following elements.
Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (S) and Francium (Fr).
The electronic configuration of group – 1 elements is as follows –
| Elements | Symbol | Atomic Number | Electronic configuration in condensed form |
| Hydrogen | H | 1 | 1s1 |
| Lithium | Li | 3 | [He] 2s1 |
| Sodium | Na | 11 | [Ne] 3s1 |
| Potassium | K | 19 | [Ar] 4s1 |
| Rubidium | Rb | 37 | [Kr] 5s1 |
| Cesium | Cs | 55 | [Xe] 6s1 |
| Francium | Fr | 87 | [Rn] 7s1 |
ANOMALOUS PROPERTIES OF LITHIUM
Amongst alkali metals, lithium has smallest size and highest polarizing power (charge / radius ratio) due to this reason lithium behaves different from other elements of group-1 (alkali metals). It shows following anomalous properties.
· Lithium is hard metal, its melting point and boiling point are highest among the group.
· Lithium is least reactive among all alkali metals and it is the strongest reducing agent.
· Lithium hydride is the stable amongst all the alkali metal hydrides.
· LiOH is weak base while hydroxides of other alkali metals behave as strong bases.
· Lithium nitrate when heated gives lithium monoxide(Li2O). While other alkali metal nitrates decompose to give corresponding nitrites.
· Lithium combines with ammonia to form lithium amide (Li2NH) while other alkali metals form amides of the general formula MNH2(where M = Na, K, Rb, Cs).
DIAGONAL RELATIONSHIP OF LITHIUM WITH MAGNESIUM
The element lithium of second row shows similar properties with its diagonally opposite member magnesium of third row. This is known as diagonal relationship. The diagonal relationship is due to the similarity in ionic sizes and polarizing power (i.e. charge/radius ratio) of lithium and magnesium. Also electronegativity of Li (1.00) and Mg (1.20) are not much different.
PERIODIC TRENDS OF ALKALI METALS
1. Atomic and ionic radius of S Block Elements
Alkali metals have the largest atomic and ionic radius in their respective periods. On moving down the group, the size goes on increasing due to the presence of extra shells hence the atomic and ionic radius also increases.
2. Ionization enthalpy or energy of S Block Elements
Ionization enthalpies of alkali metals are low and decrease down the group from Li to Cs. This is due to increase in atomic number, size, nuclear charge and increase in screening effect.
3. Electronegativity of S Block Elements
The electronegativity of the alkali metals is very low due to their electropositive character. The electronegativity decreases from Li to Cs as the electropositive character increases.
4. Melting and boiling point of S Block Elements
Alkali metals have very low melting and boiling points due to presence of weak intermetallic bonds. It decreases on moving down the group.
5. Oxidation States of S Block Elements
Due to the presence of only one electron in the valence shell, they exhibit only +1 oxidation state.
S BLOCK ELEMENTS CHEMICAL REACTIVITY
The alkali metals are highly reactive due to their large size and low ionization enthalpy.
1. Reactions of S-block elements with oxygen (air)
Alkali metals burns vigorously in oxygen to form oxides. Lithium forms monoxide (Li2O) and sodium forms peroxide, (Na2O2).
4Li + O2 → 2Li2O (Oxide)
2Na + O2 → Na2O2 (Peroxide)
The other elements form super-oxides.
M + O2 → MO2 (Superoxide)
(where, M = K, Rb, Cs)
2. Reaction of S-block elements with water
The alkali metals react with water to form corresponding hydroxides and evolve hydrogen gas (Dihydrogen). (M = Li, Na, K, Rb, Cs)
2M + 2H2O → 2MOH + H2 ↑
3. Reaction of S-block elements with hydrogen (Dihydrogen)
The alkali metals react with dry hydrogen at high temperature (673K) to form corresponding hydrides.
2M + H2 → 2MH
4. S-block elements Reaction with halogens
All the alkali metals react vigorously with halogen to form their respective ionic crystalline halides with general formula M+X- where M = Na, K, Rb, Cs and X = Cl, Br, I and F.
2M + X2 Δ → 2M+X-
SOME IMPORTANT COMPOUNDS OF SODIUM
1] Sodium carbonate (washing soda, Na2CO3)
Sodium carbonate is usually prepared by a process known as the ammonia–soda process or Solvay process. Raw materials are NaCl, NH3 and limestone (for CO2).
The process involves the following steps
Step – I
CO2 gas bubbled through a solution of NH3 to form NH4HCO3 i.e. ammonium hydrogen carbonate.
NH3 + H2O + CO2 → NH4HCO3
Step – II
Sodium hydrogen carbonate (NaHCO3) precipitates out because of the common ion effect caused due to the presence of excess NaCl.
NH4HCO3 + NaCl → NaHCO3 + NH4Cl
Step III
The precipitate NaHCO3 is then filtered of and ignited to get sodium carbonate (Na2CO3)
2NaHCO3 Δ → Na2CO3 + CO2 + H2O
Properties of Na2CO3
Sodium carbonate crystallizes from water as a decahydrate (sodium carbonate decahydrate, Na2CO3.10H2O, also known as washing soda).Above 373K, the monohydrate from becomes completely anhydrous and change to a white powder called soda ash.
Na2CO3.H2O above 373K → Na2CO3 + H2O
(Anhydrous)
Uses of Na2CO3
i. It is used in the manufacture of glass soap, borax and caustic soda.
ii. It is used in water softening in laundry and cleaning.
iii. It is used in paper, paints and textile industries.
2] Sodium hydroxide (caustic soda, NaOH)
Commercially sodium hydroxide is prepared by the electrolysis of sodium chloride in Costner – Kellner cell. It is also prepared by adding calcium hydroxide to the solution of sodium carbonate.
Na2CO3 + Ca(OH)2 →
