Pearson's Hard Soft Acid Base Theory - AdiChemistry

Hard and Soft Acids and Bases (HSAB) Theory is a qualitative concept introduced by Ralph Pearson to explain the stability of metal complexes and the mechanisms of their reactions. However it is possible to quantify this concept based on Klopman's FMO analysis using interactions between HOMO and LUMO.

According to this theory, the Lewis acids and bases can be further divided into hard or soft or border line types.

Hard Lewis acids are characterized by small ionic radii, high positive charge, strongly solvated, empty orbitals in the valence shell and with high energy LUMOs.

Soft Lewis acids are characterized by large ionic radii, low positive charge, completely filled atomic orbitals and with low energy LUMOs.

Hard Lewis bases are characterized by small ionic radii, strongly solvated, highly electronegative, weakly polarizable and with high energy HOMOs.

Soft Lewis bases are characterized by large ionic radii, intermediate electronegativity, highly polarizable and with low energy HOMOs.

The Border line Lewis acids and bases have intermediate properties.

Remember that it is not necessary for Lewis acid or base to possess all the properties to be classified as hard or soft or borderline.

In short, Hard acids and bases are small and non-polarizable, whereas Soft acids and bases are larger and more polarizable. 

More at:

https://www.adichemistry.com/inorganic/cochem/hsab/hard-soft-acid-base-theory.html

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Elements and compounds

What is an element?

According to modern view, in simpler terms, an element can be defined as a substance made up of atoms of same atomic number.

For example, hydrogen gas, H₂ is an element since the atoms in it have the same atomic number i.e. 1. 

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http://www.adichemistry.com/general/chemistry-basics/elem/elements-compounds.html

A chemical compound consists of two are more different types of atoms. For example, water is a compound made up of hydrogen and oxygen. Its formula is H₂O i.e., there are two hydrogen atoms per every oxygen atom.

Electrolysis at adichemistry

Electrolysis involves dissociation (lysis) of an electrolyte by using a direct electric current.  In this process, electrical energy is used to carryout a non-spontaneous redox chemical reaction. 

The setup used to serve this purpose is known as electrolytic cell.

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http://www.adichemistry.com/physical/electrochemistry/electrolysis/electrolysis.html

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HYDROGEN BONDING

The electrostatic force of attraction between partially positively charged hydrogen in a polar molecule and an electronegative atom is called hydrogen bond.
It is represented by dotted line as shown below.

H--------X

where X is an electronegative atom.
Characteristics of Hydrogen bond 
1) The nature of H-bonding is purely electrostatic.
2) The hydrogen atom must be covalently bonded to a highly electronegative atom in order to acquire sufficient positive charge to participate in hydrogen bonding.
3) The electronegative atom should possess high charge density so as to attract positively charged hydrogen atom effectively. Thus H-bonding is observed with smaller and highly electronegative atoms such as F, O and N.
4) The hydrogen bond is stronger than van der Waal's forces of attraction. However it is weaker than the covalent bond and ionic bond.
5) The strength of H- bond is directly proportional to the electronegativity of electronegative atom. Thus the strengths of hydrogen bonds with different electronegative atoms varies as follows:
H------F > H------O > H-------N

Read more at: HYDROGEN BONDING