S- BLOCK ELEMENTS

Introduction:

The long form of the periodic table is based upon the filling of a particular shell and has been accordingly divided into four blocks i.e. 's', 'p', 'd' and 'f' blocks. The 's' block elements involve the filling of 's' subshell. This block includes the elements of group 1 (or I A) and group 2 (or II A). They have one or two electrons in their outermost 's' subshell respectively.

S block

S block

Group 1 Group 2

Location of S- block in periodicity

General Features of the 's' Block

• Alkali and alkaline Earth metals are the most violently active of all the metals. These occur in the combined form with halide, sulphate, carbonate, silicate ions, etc. and are not found in the free state in nature, as these are readily oxidized.

 • A number of alkali and alkaline Earth metals are

found in abundance in the Earth's crust. Among these, calcium is the fifth most abundant element in the Earth's crust and hence the third most abundant metal after aluminium and iron. Vast sedimentary deposits of CaCO3 occur over large parts of the Earth's surface.

Examples:

• Lithium displays an anomalous behavior when compared to sodium and rest of the family members of the alkali metal family.  The difference between the first members from its succeeding members is highlighted with lithium as a representative example,  

• The first member of each group has the smallest size of atom and its ion in its group. The size goes on increasing as we go down a group.  

• The first member has largest ionization energy because of small atomic size: the ionization energy decreases down the group.  

Diagonal relationship:

Some elements of second period show similarities with elements of the third period present diagonally to each other, even though they belong to different groups. The similarity in properties of elements present diagonally is called diagonal relationship.

Division in s block

The s block element

Alkali Alkali earth metals

Alkali Metals

The six elements belonging to group 1 of the periodic table, namely lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr) are called alkali metals. They constitute alkali metals because they readily dissolve in water to form hydroxides, which are strongly alkaline in nature. They also form alkaline oxides. The element francium is radioactive.

Electronic configurations All the alkali metals have one electron in their

outermost 's' orbitals preceded by the noble gas configuration. Thus, the general configuration of alkali metals may be written as [Noble gas] ns1 where 'n' represents the valence shell. The electronic configurations of alkali metals are:

The electronic configurations of alkali metals  

General Characteristics of Alkali Metals

Atomic and ionic radii

Being the first elements of each period, alkali metals have the largest atomic and ionic radii in their respective periods. As we move within a period, the atomic radius and ionic radius tend to decrease due to increase in the effective nuclear charge. On moving down the group, there is increase in the number of shells and, therefore, atomic and ionic radii increase.

Ionization energies• Alkali metals have the lowest ionization energy in each period. Within

the group, as we go down, the ionization energies of alkali metals decrease due to their atomic size being the largest in their respective periods. In large atoms the valence electrons are loosely held by the nucleus and are easily lost, leading them to have low ionization energies and acquiring stable noble gas configurations. On moving down the group, the atomic size increases and the number of inner shells also increases, increasing the magnitude of screening effect and consequently, the ionization energy decreases down the group.  

• The second ionizations energies of alkali metals are very high. The removal of an electron from alkali metals causes the formation of monovalent cations having very stable electronic configurations (same as that of noble gases). Therefore, it becomes very difficult to remove the second electron from the stable noble gas configurations, giving very high second ionization energy values (IE2).

Chemical Characteristics of Alkali Metals

Alkali metals exhibit a high chemical reactivity because of their  

• low ionization energies  • low heat of atomization.   Since the value of ionization energy

decreases down the group (from Li to Cs) the reactivity of alkali metals increases from Li to Cs. All alkali metals are highly reactive towards the more electronegative elements such as oxygen and halogens. Some characteristic chemical properties of alkali metals are described.  

Action with Air

• All the alkali metals on exposure to air or oxygen burn vigorously, forming oxides on the surface of the metals. Lithium forms monoxide (Li2O), sodium forms the peroxide (Na2O2) and the other elements form superoxides.

Atomic and ionic radii

• Smaller than corresponding alkali metal due to nuclear charge.

• Increase down the group.

Ionization energy

First ionization energies are higher than first ionization energies of alkali metals due to increased nuclear charge and their small size.

Hydration enthalpy

• Hydration enthalpy decreases down the group.

• Hydrogenation enthalpy of alkaline earth metals is higher than that of alkaline metals due to their higher charge to size ratio

• Compounds of alkaline earth metals form hydrated salt like Mgcl2.6H20while compounds like Nacl do not form such hydrates.

Physical properties

• Flame colour of some elements are as follows:

1.Ca- brick red 2.Sr- crimson red 3.Ba- apple green

Chemical properties

• With Air They react with air to form nitrides(M3N2) and

oxides (MO).• With water They react with water to release hydrogen gas .As

we move down the group the electronic configuration decreases and hence the reactivity increases

• With halogen they react with halogens at elevated temperatures

forming halides Becl2 is prepared by heating BeO with cl2 in presence of coke.

Beo+ c+cl2 Becl2+co

Compiled by:-• Samaksh • Rakshit• Arjun• Siam