CBSE Class 11 Chemistry The S Block Element Notes Set A

The s- Block Element Important points

♦ Groups (1 & 2) belong to the s-block of the Periodic Table.

♦ Group 1 consists of : lithium, sodium, potassium, rubidium, caesium and francium and collectively known as the alkali metals.

♦ Group 2 include : beryllium, magnesium,calcium, strontium, barium and radium. Except Beryllium they are known as alkaline

♦ Physical properties-

a) Large atomic radii: The atomic radii of alkali metals are the largest in their respective periods. These increase as we travel down the group.

b) Large ionic radii: The ionic radii increase as we move down the group due to the addition of a new energy shell with each succeeding element.

c) Low ionization enthalpy: The ionization enthalpies decrease as we move down the group.The ionization enthalpies of the alkali metals are the lowest due to loosely held s- electron.

d) Hydration enthalpy: It decreases with the increase in ionic radii.The hydration enthalpy of Li ion is the maximum and the hydration enthalpy of Cs ion is the minimum.

e) Oxidation state: The alkali metals exhibit oxidation state of +1 in their compounds and are strongly electropositive in character. The electropositive character increases from Li to Cs.

f) Metallic character: The metallic character increases down the group.

g) Melting point and boiling point:: The m p and b p of alkali metals are very low and decrease with increase in atomic number.

h) Nature of bonds formed: These metals form ionic bonds. The ionic character increases as we down the group.

i) Flame colouration: All the alkali metals impart a charactersistic colour to the flame.

j) Photoelectric effect: Alkali metals (except Li) exhibits photoelectric effect.

♦ Chemical features of alkali metals:

a) Reducing character: As the ionization enthalpies of the alkali metals decrease down the group their reducing character or reactivity in the gaseous state increases down the group. i.e., Li < Na < K < Rb < Cs .

b) Reaction with dihydrogen: Alkali metals react with dry hydrogen at about 673 K to form crystalline hydrides which are ionic in nature and have high melting points.

2 M + H₂  —Heat→2M + H^- 

c) Oxides and hydroxides: Alkali metals when burnt in air form different compounds, for example the alkali metals on reaction with limited quantity of oxygen form normal oxides ( M2O) M= Li, Na, K, Rb, Cs ISSUED BY K V - 

d) Reaction with halogens: The members of the family combine with halogen to form corresponding halides which are ionic crystalline solids. Reactivity of alkali metls with particular halogen increases from Li to Cs.

e) Reaction with water: Alkali metals react with water and other compounds containing acidic hydrogen atoms such as hydrogen halides, acetylene etc. to liberate hydrogen gas.

f) Solubility in liquid ammonia: All alkali metals dissolve in liquid ammonia giving deep blue solutions which are conducting in nature.

g) Reaction with sulphur and phosphorus: Alkali metals react with sulphur and phosphorus on heating to form sulphides and phosphides respectively.

♦ Diagonal relationship between Li and Al

Li resembles Mg mainly due to similarity in sizes of their atoms and ions. The main points of similarity are:

i) Both are quite hard.

ii) Both LiOH and Mg(OH)₂ are weak bases.

iii) Carbonates of both on heating decompose to produce oxides and carbondioxide.

iv) Both react with nitrogen to give ionic nitrides.

v) Nitrates of both decompose on heating to give oxides.

vi) Both Li and Mg do not form solid bicarbonates.

vii) Because of covalent character LiCl and MgCl₂ are soluble in ethanol.

viii) The hydroxides, bicarbonates and fluorides of both Li and Mg are sparingly soluble in water.

♦ Biological importance of Na and K

i) Sodium ions participate in the transmission of nerve signals.

ii) Sodium ions also regulate flow of water across the cell membranes and in transport of sugars and amino acids into the cells.

iii) Potassium ions are the most abundant cations within cell fluids, where they activate many enzymes, participate in oxidation of glucose to produce ATP.

iv) Potassium ions in combination with sodium ions are responsible for transmission of nerve signals.

v) The functional features of nerve cells depend upon the sodium potassium ion gradient that is established in the cell.

♦ Group 2 elements: Alkaline earth metals

(a) Atomic radii : The atomic radii of alkaline earth metals are fairly large though smaller than the corresponding alkali metals and they increase down the group. This is because on moving down the group, atomic radii increase primarily due to the addition of an extra shell of electrons in each succeeding element.

(b) Ionic radii: the atoms of these elements form divalent ions which show the same trend of increase in their size down the group.

(c) Ionization enthalpy: The alkaline earth metals have fairly low Ionizations enthalpies though greater than those of the corresponding elements of group 1 and this value decreases down the group.

(d) Hydration enthalpy: the Hydration enthalpies of alkaline earth metal ion decrease as the size of the metal ion increases down the Group Be²⁺ >Mg²⁺ >Ca²⁺ >Sr²⁺ >Ba²⁺

(e) Oxidation State: All the members of the family exhibit +2 oxidation state in their compounded and the form divalent cations (M2+)

(f) Electro negativity : The electro negativity values of alkaline earth metals are quite close to those of alkali metals, though slightly more.

(g) Metallic Character : Alkaline earth metals have stronger metallic bonds as compared to the alkali metals present in the same period.

(h) Melting and boiling point : The melting and Boiling points of these metals are higher than those of alkali metals present in the same period. (i) Colouration to the flame : With the exceptio9n of beryllium and magnesium, the rest of the elements impart characters in colour to the same flame. For example, Be      Mg     Ca     Sr                     Ba                    Ra

                                                                                                             -        -   Brick Red      Crimson Grassy     Green Crimson

J) Complex formation: Generally the members do not form complexes. However, smaller ions ( Be & Mg Ions) form complexes with the electron donor species

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