Properties of 's' block elements

Properties of 's' block elements :

• s-block consist of two groups, group I and II.
• General electronic configuration of s-block is ns1-2  .
• Group I elements are called as Alkali metals.
• Group II elements are called as Alkaline earth metals.
• They act as good reducing agents.
• They have high electropositivity as compared to p-block and d-block.
• Because of high electropositivity, sodium is kept in kerosine.
• Lithium is the most hardest element in the group I.
• Lithium shows diagonal relationship with magnesium.
• Lithium and Magnesium both are deliquescent or hygroscopic (absorbs water molecules from atmosphere) in nature.
• Lithium and Sodium forms oxide, Potassium forms peroxide and Rubidium,Cesium forms superoxide along with peroxide and oxide on reacting with oxygen.
• Lithium and Berillium forms covalent bond on reacting with halogens whereas rest of the elements forms ionic bond.

                             

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Inert paired effect

Inert paired effect : It is the non participation of s-pair of electron in the bond formation due to poor screening or shielding of d and f orbitals.

Consequence of inert paired effect :

1) Due to inert paired effect lower oxidation state of elements of group of "p" block becomes more stable.

exam:-In group 13, +1 oxidation state becomes more stable in Indium and Thallium due to inert paired effect.

2) Due to inert paired effect f-block shows lanthanoid and actinoid contraction.

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Molecular orbital theory (MOT)

Molecular orbital theory :Postulates of this theory are--

• Atoms combine to form molecules and their atomic orbitals also combines to form molecular orbitals.
• Just like electrons are present in an atomic orbitals of an atom similarly in a molecule electrons are present in its molecular orbitals.
• The atomic orbitals of same energy and similar symmetry combines to form molecular orbitals.
• Atomic orbital is mono centric due to one nucleus centre whereas, molecular orbital s are poly centric.
• The number of molecular orbitals forms are equal in number of combining orbitals.
• Two atomic orbitals combine to form one as BMO (bonding molecular orbital) and other as AMO (anti bonding molecular orbital).
• BMO has more stability and less energy than AMO.
• Electrons in molecular orbitals are filled according to Auf-bau's principle, Hund's rule and Pauli's exclusion principle.

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Fajan rule

Fajan's rule : 1) Greater the charge on anion or cation, greater is the polarasibility of anion and greater is the polarising power of cation.

2) Smaller is the size of cation, greater is its polarising power.

3) Greater is the size of anion, greater is its polarasibility.

4) Greater polarising power and greater polarasibility induces higher non polar character in an ionic bond.  

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Spectrum

Spectrum : The collection of waves of different frequency or wavelength in increasing or decreasing order of their wavelength or frequency is called as spectrum.

Depending on nature waves are of two type:

a) Continuous spectrum : A spectrum in which different regions are not separated by dark lines are called as continuous spectrum.

b) Line spectrum : The spectrum in which different regions are separated by dark fine lines are called as line spectrum.

Line spectrum are also of two type:

1) Atom emission spectrum : It is obtained from the radiation  emitted from atom in its excited state (higher energy state of atom).

The radiation of excited atom are made to fall on prism so that a line emission spectrum of an atom is obtained.

2) Atom absorption spectrum :It is obtained from an atom in its ground state (lower most energy state).

When a white light is passed through an atom and it is taken up to prism after passing through an atom then it splits up and form a line absorption spectrum of  an atom.

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Gel (gelation)

Gels : A gel is jelly-like colloidal system in which a liquid is dispersed in a solid medium. The process of gel formation is known as gelation.

Gel represents a liquid-solid system i.e., a liquid is dispersed in a solid. Gelation may be considered as partial coagulation of a sol. 

The coagulating solution particles first aggregate to form a long thread-like chain. These chains are then interlocked to form a solid framework. The liquid dispersion medium is caged in the cavities of this structure. The resulting semi-solid porous mass has gel structure.

Classification of Gel :-

1) Elastic Gels :These are those gels which posses the property of elasticity. They change their shape on applying force and return to original shape when the force is withdrawn. 

exam:- Gelatin, starch and soaps are substance which forms elastic gels.

2) Non-Elastic Gels :These are those gels  which are rigid.They are irreversible. When hydrated becomes glassy or changes into a powder which on addition of water and followed by warming does not change back into the original gel.

exam:- Silica gel.

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Emulsion

Emulsions : When a liquid forms a colloidal solution by the dispersion in another liquid is called an emulsion.

Therefore, a colloidal solution in which both dispersed phase and dispersion medium are liquid is called an Emulsion.

exam:- when olive oil and water are mixed then oil gets dispersed in water in the form of small droplets thus forming an emulsion.

Types of emulsion :-

A)Oil in water :In this small droplets of oil are dispersed in water.

exam:- milk(liquid fat in water), vanishing cream.

B) Water in oil :In this emulsion, the droplets of  water are dispersed in the oil.

exam:- cream, cold cream.

Preparation of emulsion :

When two immiscible liquids are stirred vigorously, an emulsion is formed. This emulsion is unstable. To obtain the stable emulsion a third substance, known as emulsifying agent is added.

                                                                                         https://youtu.be/O3D2belxKBs