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Friday, 10 February 2017

The s- Block Element I

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

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.
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
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.
2M + H2 -(Heat) --> 2M + H-
g) Reaction with sulphur and phosphorus: Alkali metals react with sulphur and phosphorus on heating to form sulphides and phosphides respectively.

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