UNIT 10 THE s-BLOCK ELEMENTS
GROUP-1 ELEMENTS: ALKALI METALS
Electric configuration: All alkali metals have one valence electron, 1s2.
Atomic and ionic radii: The alkali metal atoms have the largest sizes in a particular period of the periodic table.
Ionization Enthalpy: The ionization enthalpies of the alkali metals are considerable low and decrease down the group.
Hydration Enthalpy: The hydration enthalpies of alkali metal ions decrease as we move down the group.
Physical Properties: All the alkali metals are silvery white, soft and light metals. These elements have low density which increases down the group. The melting and boiling point of alkali metals are low.
Chemical Properties:
- Reactivity toward air: The alkali metals tarnish in dry air due to the formation of their oxides which in turn react with moisture to form hydroxides.
- Reactivity toward water: Alkali metals react with water to form hydroxide and dihydrogen.
- Reactivity towards dihydrogen: Alkali metals react with dihydrogen to form hydride.
- Reactivity towards halogens: Alkali metals react vigorously with halogens to form ionic halides.
- Reducing nature: The alkali metals are strong reducing agents.
- Solutions in liquid ammonia: Alkali metals dissolve in liquid ammonia giving deep blue solutions which are conducting in nature.
Uses:
- Lithium is used to make useful alloys. It is also used in thermonuclear reactions and to make electrochemical cells.
- Liquid sodium metal is used as coolant in fast breeder nuclear reactor.
- Potassium chloride is used as a fertilizer.
- Caesium is used in devising photoelectric cells.
GROUP-2 ELEMENTS: ALKALINE EARTH METALS
Electronic configuration: These elements have two electrons in the s-orbital of the valence shell. Their general configuration is ns2.
Atomic and ionic radii: The atomic and ionic radii of the alkaline earth metals are smaller than those of the corresponding alkali metals in the same periods.
Ionization enthalpies: Alkaline earth metals have low ionization enthalpies due to fairly large size of the atoms.
Hydration enthalpies: The hydration enthalpies of alkaline earth metals ions decrease with increase in ionic size down the group.
Physical properties:
- The alkaline earth metals are silvery white, lustrous and relatively soft but harder than the alkali metals.
- The melting and boiling points of these metals are higher than the corresponding alkali metals.
Chemical properties:
- Reactivity towards air and water: These metals burn in air to give their oxides and nitrides. They also react with water to form hydroxides.
- Reaction with halogens: These metals combine with halogens to form halides.
- Reactivity towards hydrogen: These metals react with hydrogen to form their hydrides.
- Reactivity towards acids: These metals react with acids liberating dihydrogen.
- Reducing nature: Alkaline earth metals are strong reducing agents.
- Solutions in liquid ammonia: These metals dissolve in liquid ammonia to give deep blue black solution forming ammoniated ions.
Uses:
- Beryllium is used in the manufacture of alloys.
- Magnesium alloys are used in aircraft construction.
- Calcium is used in the extraction of metal from oxides which are difficult to reduce with carbon.
- Radium salts are used in radiotherapy.