0The s-block elements are the 14 elements contained within these columns. All of the s-block elements are unified by the fact that their valence electrons (outermost electrons) are in an s orbital. The s orbital is spherical and can be occupied by a maximum of two electrons. Elements in column 1 have one electron in the s orbital, and elements in column 2 (plus helium) have two electrons in the s orbital.
The s-block elements include hydrogen (H), helium (He), lithium (Li), beryllium (Be), sodium (Na), magnesium (Mg), potassium (K), calcium (Ca), rubidium (Rb), strontium (Sr), cesium (Cs), barium (Ba), francium (Fr) and radium (Ra). The periodic table shows exactly where these elements are within the s-block.
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In a modern periodic table of elements Group 1 and Group 2 are called the S-block elements. It is observed that the physical and chemical properties of these elements changes in a particular trend as the atomic number of the elements increases. Changes in the various properties across the group are as mentioned below:
1. Atomic and ionic radii: When a modern periodic table is observed it is seen that the size of the alkali metals is larger compared to other elements in a particular period. As the atomic number increases the no. of electrons increases along with addition of shells. On moving down the group the atomic number increases. As a result the atomic and ionic radius of the alkali metals increases.2. Ionization enthalpy: As we go down the group the size of the atoms increases due to which the attraction between the nucleus and the electrons in the outer most shell decreases. As a result the ionization enthalpy decreases. The ionization enthalpy of the alkali metals is comparatively lesser than other elements.
2. Ionization enthalpy: As we go down the group the size of the atoms increases due to which the attraction between the nucleus and the electrons in the outer most shell decreases. As a result the ionization enthalpy decreases. The ionization enthalpy of the alkali metals is comparatively lesser than other elements.
3. Hydration enthalpy: As the ionic sizes of the elements increases the hydration enthalpy decreases. Smaller the size of the ion the hydration enthalpy is high as the atom has the capacity to accommodated larger number of water molecules around it due to high charge/radius ratio and hence gets hydrated.
4. Physical properties:
• The density of the alkali metals increases down the group. Exception: the density of potassium is less than the density of sodium.
• The alkali metals have a low melting and boiling point due to the weak metallic bonding.
• Alkali metals and its respective salts have the capability to impart colour to the oxidizing flame due to the heat generated from the flame which excites the valence electrons from one energy level to another energy level. This helps in the detection of alkali metals during the flame test.
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Chemical properties:-
- Action of Air:
Alkali metals lose their lustre very easily upon exposure to air. To prevent the alkali metals from tarnishing, they have to be preserved under paraffin oil or kerosene.
Lithium readily forms lithium oxide when burnt with oxygen. Sodium forms peroxide while other alkali metals i.e. K, Rb and Cs form superoxide. - Reaction with water:
Alkali metals react with water and form hydroxides along with liberation of hydrogen. This can be given as
2M + 2H2O --- => 2MOH + H2
Where M represents any alkali metal, MOH represents the corresponding hydroxide such as lithium hydroxide.
As we know, the reactivity with water increases as we move down the group evolving more amount of hydrogen. As a result, the hydrogen liberated in the reactions involving the alkali metals K, Rb and Cs catches fire immediately. - Reaction with hydrogen:
Alkali metals react with hydrogen and form ionic hydrides of the form M+ H-. These metal hydrides in turn react with water to give back the hydrogen. These two reactions are as given below:
2M + H2 --- => 2MH
MH + H2O --- => MOH + H2
All metal halides are easily soluble in water, good conductors of electricity in fused state and have high melting and boiling. Due to high lattice energy alkali metal halides are highly stable. - Reaction with halogens:
Alkali metals react with halogens very easily forming halides. The reactivity of alkali metals with halogen increases Li to Cs. - Solubility in liquid Ammonia:
All alkali metals are readily soluble in liquid ammonia. When the concentration of the metal is less in the alkali-metal ammonia solutions, light blue colour solution is formed. As the concentration of the metal increases, the solution turns to copper blue in colour. After that the metals does not dissolve in the solution and solution is said to be saturated with the metal.
The metal ammonia solution exhibits certain properties such as conductivity, paramagnetism and stability due to presence of metal cations and ammoniated electrons. - Reducing property:
It is the ability of the metal to lose electrons. As the ionization energies of the alkali metals decreases from Li to Cs, the reducing property increases from Li to Cs. However, Li shows an anomalous behaviour and is the strongest reducing agent.
The reducing power of alkali metals is expressed as oxidation potential. Thus, the oxidation potential increases as the reducing power of an element increases. Lithium, a strong reducing agent has highest oxidation potential. - Ability to form alloys:
Alloys are mixtures of metals. Alkali metals can form alloys with other elements in the same period or with metals in other groups. Alkali metals form amalgams very easily by dissolving in mercury. - Ability to form Complexes:
Alkali metals do not have the ability to form complexes. This is due to large size, low nuclear charge and poor ability to attract electrons. These properties are opposite to the requisites for formation of complexes. As lithium is smallest element in the group, it has the ability to form certain complexes. The complex forming ability decreases as we move down the group.
