A chemical bond the force of attraction that holds the atoms or ions together in a substance is called a chemical bond, and these notes included theories of electronic theory, valence shell electron pair repulsion theory, and shapes of molecular, hybridization.
Electronic Theory of Valence
In 1916 G.N. Lewis gave this electronic theory of valence to explain why atoms join to form a bond.Â
Why chemical bond is formed?Â
According to the electronic theory of valence, during the formation of a chemical bond, the atoms interact by losing, gaining, or sharing electrons to get a stable electronic configuration of Nobel gases.
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Explanation of electronic theory
The electrons present in the valence shell of an atom are called valence electrons. In chemical bonding, only the valence electrons are involved. Lewis used the idea of an electronic configuration for the explanation of bonding between atoms. According to him, atoms tend to gain, lose, or share electron weight they eight valence electrons (octet rule) or two electrons (duplet rule).
Octet Rule:- The tendency for atoms to have eight electrons in the valence shell is known as the octet rule.
Duplet Rule:- The tendency for atoms to have two electrons in the valence shell is known as the duplet rule the bond formed due to the sharing of electrons is called a covalent bond.
Single Covalent Bond Example
The bond which is formed by sharing one pair of electrons is called a single covalent bond.
Double covalent bond example
The covalent bond which is formed by sharing of two electrons is called a double covalent bond.
Valence shell Electron Pair Repulsion Theory (VSEPR)
Nyhavn and Gillespie developed a model known as valence shell electron repulsion theory (VSEPR) for the prediction of the shape of molecules. According to this theory, the geometry of a molecule depends on the number of bonding and non-bonding electrons around a central atom.
The main point of VSEPR Theory
(i) In polyatomic molecules one is called the central atom to which other atoms are linked. Â
(ii) The suitable arrangement of electron pairs around the central atom is one in which they remain of maximum distance from each other to minimize repulsion.
(iii) The overall geometry of a molecule depends on both lone and bonding pairs of electrons around the central atom.
(iv) The lone pairs of electrons occupy more space than the bond pairs.
(v) The magnitude of the repulsion between various electron pairs in a given molecule decreases in the following order.
Lone pair- lone pair > lone pair-bond > bond pair-bond pair. These repulsions are called Vander Waals repulsions.
(vi) Multiple bonds (double and triple bonds) behave as a single electron pair bond.
(vii) In the same molecules deviations from the expected bond angle are due to the presence of lone pair electrons.
Resonance Definition and Explanation
The phenomenon in which two or more structures can be written for a substance that involves the identical position of atoms is called resonance.
Molecules having localized bonding
For those molecules or ions that have localized bonding, it is possible to draw a single Lewis structure, which gives information about the molecule or ion. For example, the following molecules have localized bonding electrons.
Molecules having delocalized bonding
However, the molecules or ions which have delocalized bonding can not be represented by a single Lewis structure. For such molecules or ions, two or more structures can be written. None of these structures describe all the properties of a molecule or ion.
Resonance Hybrid
The actual structure of the molecules is known as a resonance hybrid of various possible resonance structures or contributing forms of canonical structures a double-headed arrow ( ↔ ) is used to indicate the phenomenon of resonance in various resonating structures.
Condition for resonance
The position of the nuclei(atom) in each resonance structure must be the same. The resonance structure differs from each other only by the position of electrons. The number of unpaired electrons in each resonance structure must be the same.
Importance of Resonance
The resonance phenomenon explains different properties and facts about the molecule which can not be explained by a single Lewis structure.Â
Resonance stabilizes the resonance hybrid structure of a molecule. The bond energy of the resonance hybrid is Lewis than the calculated value.
Resonance explains the stability and energy of benzene molecules.
Click here for Valence bond theory (VBT)
Sigma Covalent bond DefinitionÂ
The bond which is formed by the head-to-head or linear overlap of partially filled atomic orbitals is called the sigma covalent bond. A covalent bond that is formed between two atoms by the overlap of partially filled atomics along their axes (head-to-head or linear overlap) is called the sigma covalent bond.
Sigma covalent bond is formed by the overlapping of different orbitals (atomic orbitals and hybridized orbitals). However, we will discuss the formation of the sigma bond by atomic orbitals. it includes s-s, s-p, and p-p overlap.
Hybridization
The phenomenon of mixing atomic orbitals of different shapes and energy to produce new orbitals of the same shape energy and equivalent in number is called hybridization. The new orbitals which are produced during this process are called hybrid orbitals.
Characteristics of hybrid orbitals
Hybrid orbitals have the same shape energy and symmetry however they have a different orientation in space.
Type of Hybridization
There are different types of hybridization depending on the orbitals. However, we will discuss only sp2, sp3, and sp hybridization.
sp3 Hybridization
The mixing of one s and three p-orbitals from four equivalent sp3-hybrid orbitals of the same shape and energy is called sp3-hybridization.
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