Electronic theory of valency and bonding

Electronic theory of valency and bonding

This theory explains about the cause of chemical reaction/combination on the basis of valence shell electron (i.e electron present in the outermost orbit) & tendency to lose or gain an electron. The outermost shell of an atom is called valence shell &electron present in it is called valence electron. The valence electron of an atom is denoted by Lewis dot symbol.For E.g.

In chemical reaction/combination very atom tries to adjust it’s electronic configuration like nearest noble gas by losing r gaining or sharing of the electron. This rule is called octet rule.

Postulates of electronic theory of valency

• The electron present in the outermost shell is valence electron & the corresponding shell is called valence shell.
• Combining capacity of each & every atom is determined by the electron present in the valence shell.
• Atoms containing 8 electrons in the valence shell are called octet (except helium) & all other atom try to adjust their electronic configuration like nearest noble gas. Because of this tendency chemical reaction occurs. This is called octet rule.
• The number of electrons gained or lost or shared during the chemical reaction is called valency of that electron.

Types of bond

1. Ionic bond or Electrovalent bond

The bond formed by gaining or losing or sharing valence electron with the another reacting atom is called ionic bond or electrovalent bond. The compound having the ionic bond is called the ionic compound. The force of attraction is the electrostatic force.

E.g. NaCl , CaCl₂ , AlF₃

factors affecting ionic bond

• Ionization potential
• Electron affinity
• Lattice energy

Characteristics of Ionic bond

• Generally crystalline in nature
• Generally soluble in nature due to ionization of compound
• Good conductor of electricity in molten state or aqueous solution due to formation of ion
• High melting and boiling point due to electrostatic force between the oppositely charged ions
• Non-directional in nature

Formation of Ionic Compound

1. Formation of NaCl

1. Formation of CaCl₂

Covalent bond

The bond formed by sharing of the electron between reacting atoms is called the covalent bond. Generally, non-metals form this type of bond. This bond is denoted by (-). The compounds having covalent bond are called covalent compounds.

Characteristics of Covalent bond

• Found in all 3 state i.e. solid , liquid, gas
• Generally insoluble in water & soluble in organic solvent like benzene
• The melting point and boiling point are generally low due to the weak force of attraction between molecules.
• Generally bad conductor of electricity cause they weakly ionize.
• Directional in nature

Formation of Covalent Bond

1. Formation of H₂ molecule

Formation of O₂ molecule

1. Formation of CH₄ molecule

Coordinate covalent bond

The bond formed of sharing of the electron but shares electron in pairs & thus formed the bond is called Coordinate covalent bond. The bond is formed between electron deficient & electron excess species. This is denoted by an arrow pointing from donor to receptor of the electron.

Formation of coordinate covalent bond

1. Formation of ozone (O₃)

1. Formation of H₂SO₄

Exception

1. Only duplet formed between combining atoms

1. Where Centre atom contains only 6 electrons in a combined atom

• Where Centre atom contains more than 8 electron

Ionic character in Covalent bond

When covalent bond forms then the bonding pair of the electron does not lie exactly midway between the reacting atoms but the pair of electron lies ear toward the atom which has more electronegativity. Since, the electrons shift towards it the atom having more electronegativity bears partial negative charge and the atom having less electronegativity possess partial positive charge. Such molecules are called polar molecules & the bond is called the polar covalent bond.

For e.g. In HCl, chlorine has more electronegativity than hydrogen so shared electron shift towards chlorine and hence chlorine bears partial negative charge whereas hydrogen bears partial positive charge.

Dipole moment

It is defined as the product of the magnitude of charge developed in an atom of a polarized molecule & the distance between the combing atom. It is denoted by . Its unit is Debye (D).

If net dipole moment of the molecule is zero then molecule will be non-polar & possess regular geometry & if dipole moment is not equal to zero then molecule will be polar as well as it has the irregular geometry.

e.g.

Structure of BeF₂

In the case of BeF₂ molecule , the dipole moment of one beryllium fluorine is cancelled by another beryllium fluorine bond . Hence, the molecule is non-polar and possess linear molecule.

Structure of H₂O

The dipole moment of H₂O is 1.84 & dipole along H-O is 1.5D. In molecule, there is two H-O bonds not opposing straight to one another which result in distortion of the linear structure of H­₂O molecule. Two H-O bonds of H₂O molecules are inclined to each other & bears an angular V-shape.

Application of dipole moment

• It helps to identify the polar and non-polar molecule. If dipole moment is 0 molecules will be non-polar & if it doesn’t equal 0 then molecules will be polar.
• It helps to explain the geometry of some simple molecule. If molecules don't possess dipole moment the geometry is regular & if molecule possesses dipole moment then the geometry is regular. In the case of BeF₂ it doesn’t possess dipole moment & hence has regular linear geometry but in the case of H₂O molecule, it possesses dipole moment which causes distortion of geometry & bears V-shaped angular structure.
• It helps to calculate percentage of ionic character

Special type of bond

• Hydrogen bond

This is defined as the bond formed by the electrostatic force of attracting between polarized hydrogen of one molecule which is already bonded to highly electronegative element.

Types of hydrogen bonding

• Intermolecular hydrogen bonding

If hydrogen bond exists between more than two molecules then hydrogen bonding is said to be intermolecular hydrogen bonding.

• Intramolecular hydrogen bonding

If hydrogen bond exists within a molecule than it is called intramolecular hydrogen bonding.

• Metallic bond

It is a special type of bond that exist only in metal. It is the force of attraction between positively charged kernel & sea of mobile electron which is responsible for binding metal ions together. Metallic bonding is responsible for malleability &ductability.

• Vander wall force

The force of attraction between electrically neutral molecule that collides with each other. It is formed by the temporary attraction between electron rich species and electron deficient species.

Resonance

The electronic phenomenon in which a molecule shows more than 2 lewis structure due to delocalization of π electron.

Reference

poulse, tracy. Introduction to chemistry. u.s.a: flexbook, 2010.

Pathak, Sita Karki. The Text Book of Chemistry. kathmandu: Vidhyarthi Pustak Bhandar, 2012.