Unit 10
The s-block elements
After studying this unit, you will be able to
• describe the general charact-eristics of the alkali metals and their compounds;
• explain the general characteristics of the alkaline earth metals and their compounds;
• describe the manufacture, properties and uses of industrially important sodium and calcium compounds including Portland cement;
• appreciate the biological significance of sodium, potassium, magnesium and calcium.
NEETprep Audio Note:
Among the alkali metals sodium and potassium are abundant and lithium, rubidium and caesium have much lower abundances (Table 10.1). Francium is highly radioactive; its longest-lived isotope 223Fr has a half-life of only 21 minutes. Of the alkaline earth metals calcium and magnesium rank fifth and sixth in abundance respectively in the earth’s crust. Strontium and barium have much lower abundances. Beryllium is rare and radium is the rarest of all comprising only 10–10 per cent of igneous rocks† (Table 10.2, page 299).
NEETprep Audio Note:
* The thin, rocky outer layer of the Earth is crust. † A type of rock formed from magma (molten rock) that has cooled and hardened.
Lithium and beryllium, the first elements of Group 1 and Group 2 respectively exhibit some properties which are different from those of the other members of the respective group. In these anomalous properties they resemble the second element of the following group. Thus, lithium shows similarities to magnesium and beryllium to aluminium in many of their properties. This type of diagonal similarity is commonly referred to as diagonal relationship in the periodic table. The diagonal relationship is due to the similarity in ionic sizes and /or charge/radius ratio of the elements. Monovalent sodium and potassium ions and divalent magnesium and calcium ions are found in large proportions in biological fluids. These ions perform important biological functions such as maintenance of ion balance and nerve impulse conduction.
10.1 GROUP 1 ELEMENTS: ALKALI METALS
NEETprep Audio Note:
10.1.1 Electronic Configuration
All the alkali metals have one valence electron, ns1 (Table 10.1) outside the noble gas core. The loosely held s-electron in the outermost valence shell of these elements makes them the most electropositive metals. They readily lose electron to give monovalent M+ ions. Hence they are never found in free state in nature.
10.1.2 Atomic and Ionic Radii
The alkali metal atoms have the largest sizes in a particular period of the periodic table. With increase in atomic number, the atom becomes larger. The monovalent ions (M+) are smaller than the parent atom. The atomic and ionic radii of alkali metals increase on moving down the group i.e., they increase in size while going from Li to Cs.
10.1.3 Ionization Enthalpy
The ionization enthalpies of the alkali metals are considerably low and decrease down the group from Li to Cs. This is because the effect of increasing size outweighs the increasing nuclear charge, and the outermost electron is very well screened from the nuclear charge.
10.1.4 Hydration Enthalpy
The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes.
Li+> Na+ > K+ > Rb+ > Cs+
Li+ has maximum degree of hydration and for this reason lithium salts are mostly hydrated, e.g., LiCl· 2H2O
10.1.5 Physical Properties
NEETprep Audio Note:
Table 10.1 Atomic and Physical Properties of the Alkali Metals
*ppm (part per million), ** percentage by weight; † Lithosphere: The Earth’s outer layer: its crust and part of the upper mantle
10.1.6 Chemical Properties
NEETprep Audio Note:
(i) Reactivity towards air: The alkali metals tarnish in dry air due to the formation of their oxides which in turn react with moisture to form hydroxides. They burn vigorously in oxygen forming oxides. Lithium forms monoxide, sodium forms peroxide, the other metals form superoxides. The superoxide O2– ion is stable only in the presence of large cations such as K, Rb, Cs.
(M = K, Rb, Cs)
In all these oxides the oxidation state of the alkali metal is +1. Lithium shows exceptional behaviour in reacting directly with nitrogen of air to form the nitride, Li3N as well. Because of their high reactivity towards air and water, alkali metals are normally kept in kerosene oil.
Problem 10.1
What is the oxidation state of K in KO2? NEETprep Audio Note:
Solution
The superoxide species is represented as O2–; since the compound is neutral, therefore, the oxidation state of potassium
is +1.
NEETprep Audio Note (English):
(M = an alkali metal)
It may be noted that although lithium has most negative E value (Table 10.1), its reaction with water is less vigorous than that of sodium which has the least negative E value among the alkali metals. This behaviour of lithium is attributed to its small size and very high hydration energy. Other metals of the group react explosively with water.
They also react with proton donors such as alcohol, gaseous ammonia and alkynes.
NEETprep Audio Note:
NEETprep Audio Note (English):
(v) Reducing nature: The alkali metals are strong reducing agents, lithium being the most and sodium the least powerful
(Table 10.1). The standard electrode potential (E) which measures the reducing power represents the overall change :
With the small size of its ion, lithium has the highest hydration enthalpy which accounts for its high negative E value and its high reducing power.
Problem 10.2
The E for Cl2/Cl– is +1.36, for I2/I– is
+ 0.53, for Ag+ /Ag is +0.79, Na+ /Na is
–2.71 and for Li+ /Li is – 3.04. Arrange the following ionic species in decreasing order of reducing strength:
I–, Ag, Cl–, Li, Na NEETprep Audio Note:
Solution
The order is Li > Na > I– > Ag > Cl–
(vi) Solutions in 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 electron which absorbs energy in the visible region of light and thus imparts blue colour to the solution. The solutions are paramagnetic and on standing slowly liberate hydrogen resulting in the formation of amide.
(where ‘am’ denotes solution in ammonia.)
In concentrated solution, the blue colour changes to bronze colour and becomes diamagnetic.
10.1.7 Uses
NEETprep Audio Note: