NEET Chemistry Notes s-Block Elements – General Introduction of s-Block Elements
General Introduction of s-Block Elements
General Introduction of s-Block Elements
The components in the long form of the periodic table have been partitioned into four blocks, in particular s, p, d and /-blocks. The elements of group I and II have their last electron in s-orbitals thus, they are called s-block elements.
Group 1 Elements: Alkali Metals
Lithium (Z =3), sodium (Z = 11), potassium (Z =19), rubidium (Z =37), caesium (Z =55) and francium (Z =87) are the elements of LA (or 1) group of the periodic table. These elements because of the highly alkaline nature of their water soluble hydroxides are known as alkali metals.
Occurrence of Alkali Metals
Sodium ((7th) and potassium (8 th) ) are the most abundant alkali metals found in earth’s crust. Rubidium and caesium are found in very less amount.
Francium being radioactive is found only in trace amount.
The general electronic configuration of alkali metals is [noble gas] ns1 [n = 2 to 7]
Trends in the Variation in Properties
- Alkali metals are most electropositive and on moving down the group the electropositive character increases. Therefore, they readily lose electron to give monovalent, M+ ions.
- The atomic and ionic radii are largest in the respective period and increases as we move down the group.
- Low densities of alkali metals are due to their large size.
- The ionisation enthalpies of alkali metals are considerably low and decrease down the group from Li to Cs.
- Hydration enthalpy of alkali metal ions decreases with increase in ionic size
Therefore, Li+ has maximum degree of hydration and due to this reason lithium salts are mostly hydrated, e.g. LiCl 2H2O
- Mobility of their cations in aqueous medium is directly proportional to the size of cation due to poor hydration
- Alkali metals give flame test. The colour of the flame depends upon the wavelength of radiation emitted. Caesium and potassium are used as electrodes in photoelectric cell due to their low ionisation enthalpy (photoelectric effect).
- Alkali metals are strong reducing agent and down the group, their reducing nature increases.
- Alkali metal compounds are most ionic in nature and down the group, ionic nature increases.
- Alkali metal oxides are most basic in nature and down the group, basic character increases.
- Metallic character of alkali metals increases on moving down the group.
- Reactivity with Oxygen
Metal peroxides are diamagnetic and sodium peroxide is widely used as an oxidising agent in inorganic ‘ chemistry. The superoxides are paramagnetic in nature because the presence of three electron bonds will one unpaired electron.
- Reaction with Water
Alkali metals are easily and readily oxidised by water. The reaction with water is so exothermic. The vigorous nature of the reaction of water with alkali metals increases from Li to Cs.
Be is amphoteric, as it reacts with acid as well as with base.
- Reaction with Hydrogen
All the group 2 element except beryllium form hydrides, MH2 by direct combination with hydrogen.
Beryllium and magnesium hydrides are covalent and polymeric.
- Reaction with Halogen
Alkali metals readily react with halogens to form MX type of halides. These are mainly ionic compounds having NaCl type of structure.
LiCl, LiBr and Lil all exist as trihydrates.
The hydration and lattice energies of LiF are -1034 and -1039 kj/mol respectively.
- With Liquid Ammonia
The alkali metals dissolve in liquid ammonia giving deep blue solutions which are conducting in nature.
The blue colour of the solution is due to the ammoniated electrons. The solution are paramagnetic and on standing, slowly liberate hydrogen resulting in the formation of amide.
- Lithium metal is used to make usefull alloys, e.g. with lead to make ‘white metal’ bearing for motor engines, aircraft parts.
- Sodium is used to make a Na/Pti alloy needed to make PbEt4 and PbMe4. Three organolead compound were earlier used as anti-knock additives to petrol, but now a-days vehicles are lead-free petrol.
- Liquid sodium metal is used as a coolant in fast breeder nuclear reactors.
- Potassium hydroxide is used in the manufacture of soft soap.
- Caesium is used in devising photoelectric cells.