S- block elements and their groups.

S-block elements.

The elements in which the last electrons enters in the S-orbital of valence shell i,e., the ns orbital are called s-block elements.It comprises total two groups.

Group IA (1) Alkali metal

Group IIA (2) Alkali earth metal.

Group IA (1) Alkali metal.

The electronic configuration and the properties of alkali metal are described below.

[I] Electronic configuration.

General valance shell electronic configuration-ns¹

Fr= Radioactive element.

The element is called alkali metal because oxide and hydroxide of these elements produce strong alkali in aqueous solution.

[II] Properties.

Atomic (Ionic radius)

Alkali metal has largest atomic or ionic radius in their corresponding period in the periodic table. Atomic radius increases on descending from Li to Cs.

Ionisation energy.(I.E)∝1/r

Since these elements have a largest atomic radius, they have lowest ionisation energy in their corresponding period in the periodic table. Ionisation energy decreases on decrease the group from the top to bottom.

First ionisation of alkali metal is sufficiently low. However, the 2^nd ionisation energy is extremely high. This is because after removing an electron, the metal cation (M) attain the most stable noble gas configuration.The second electron has to be removed from the closed shell (---) cell heaving octane number which requires extremely high energy, even high than its nearest noble gas. That’s why alkali metal for monovalent cation (M⁺) and always exhibit oxidation state. Formation of the divalent cation (M⁺⁺) is impossible under normal condition.

Metallic/ Electropositive character.

$$Li^+------Cr^+$$

The tendency to loose an electron and to form cation is called metallic property. All alkali metal have single electron on this valence shell , ns¹. So, these elements have strong tendency to loose valance electron and to attain most stable noble gas configuration.That why all the elements in these group exhibit metallic property.

$$M→M^++e^-$$

Alkali metal has the largest atomic size and lowest ionisation energy. Hence, the most electropositive elements in the periodic table.

Hydration energy.

$$M^+(g)+nH_2O→ [M(H_2O)n]^++Energy\, released\, called \,hydration\, energy$$

The amount of the energy released when 1 mole of the gaseous cation is dissolved in water to produce hydrated cation is called hydration energy.

Hydration energy∝ Degree of hydration (n)∝ Charge density = Charge⁄radius

Hydration energy depends on a degree of hydration which intern depends on charge density of ion. Small cation has high charge density, the high degree of hydration, and high hydration energy. It means hydration energy of ions is inversely proportional to radius.i,e-cation with small size have high hydration energy.

Ionic radius decreased on descending from top to bottom. Hence, the hydration energy decreases on descending from Li to Cs.That’why Li⁺ have high hydration energy and most extensively hydrated amongst the alkali metal.

Application.

Hydration Ionic mobility and the electrical conductivity.

Smaller ion moves fast in solution than the larger ion. Hence solution of Li salt should exhibit high ionic mobility and high electrical conductivity than the solution of Cs salt. But the ionic mobility and the conductivity measurements show the trend in opposite direction.

$$Expected \,order=Li^+>Na^+>K^+>Rb^+>Cs^+$$

$$Actual\, order=Li^+<Na^+<K^+<Rb^+<Cs^+$$

The reason for this apparent anomaly is that ions are hydrated in solution to the different extent. ^Li+ has very small size. Hence, a large number of water molecule accumulated around the Li ⁺ion. This makes the radius of hydrated Li⁺ ion very large. Consequently, it moves slowly in solution (low ionic mobility) and conducts electricity less readily.

On the other hand, the Cs ⁺has very large size (low Charge density). Hence, it is poorly hydrated by water molecules. This makes the radius of the hydrated Cs ⁺ion smaller than the hydrated Li⁺ ion and they conduct electricity more readily.

Group IIA (2) Alkali earth metal.

[I] Electronic configuration.

General electronic configuration ns²

[II] Properties.

Atomic radius.

The Atomic radius of these elements is smaller than those of alkali metals. However, these elements have largest atomic size than those other elements in periodic table. Atomic radius increases on the descending group from Be to Ba.

Ionisation energy.

The ionisation energy of these elements is higher than those of alkali metal.However, these elements have lower ionisation energy than other elements in periodic table.

The sum of first and second ionisation energy of these elements are low, but the third ionisation elements are low, but the third ionisation energy is extremely high. This is because after removing of two electrons from the valence shell metal cation (M²⁺) attain the most stable noble gas configuration.The third electrons have to be removed from a closed cell (shell with outer configuration) which require extremely high energy.

That'why these elements always form the divalent cation and exhibit oxidation state +2. Formation of trivalent cation (M³⁺) is impossible under normal condition.

Important question.

First ionisation energy is always lower than second ionisation energy. However, alkali earth metal form divalent cation (compound) rather than the monovalent cation or compound.Explain why this is so.

Second ionisation energy is always greater than first ionisation energy. Therefore, it seems that alkali earth metal should form monovalent cation or compound. However, these elements form divalent compound rather than monovalent compound. This anomaly can be explained by the fact that monovalent compounds have lower lattice energy than that of divalent compounds.Since compounds with high lattice energy flavour formation of the divalent compound but not monovalent.

Reference;

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.