Chemical Bonding and Molecular Structure class 11

 Chemical Bonding and Molecular Structure class 11

Chemical Bonding and Molecular Structure

Define a chemical bond.

The attractive force which holds various constituents (atoms, ions etc.) together in different chemical species is called a chemical bond. 

Give the main feature of Lewis approach of chemical bonding.

Lewis postulated that atoms achieve the stable octet when they are linked by chemical bonds. He assured that atoms are positively charged centre and the outer shell that could accommodate a maximum of eight electrons. These electrons occupy the corners of a cube which surrounds the centre. Lewis introduced simple notations to represent valence electrons in an atom called Lewis symbol.

Give the main feature of Kossel’s explanation of chemical bonding. 

Kossel in relation to chemical bonding drew attention to the following facts –

 (i) In the periodic table, the highly electronegative halogens and the highly electropositive alkali metals are separated by the noble gases. 

(ii) In the formation of a negative ion from a halogen atom and a positive ion from an alkali metal, atom is associated with a gain and loss of an electron by the respective atoms. 

(iii) The negative and positive ions so formed attain stable noble gas electronic configurations. The noble gases have particularly eight electrons, ns2 np6. 

(iv) The –ve and +ve ions are stabilized by electrostatic attraction.  

Define electrovalent bond.

The bond formed, as a result of the electrostatic attraction between the positive and negative ions are termed as the electrovalent bond. 

Give the octet rule in short.

The atoms tend to adjust the arrangement of their electrons in such a way that they (except H and He) achieve eight electrons in their outermost shell. This is known as the octet rule.

Write the significance of octet rule

Octet rule signifies –

(i) It is useful for understanding the structures of most of the organic compounds. 

(ii) It mainly applies to the second period elements of the periodic table.  

Define an ionic bonding.

An ionic bond (or electrovalent bond) is formed by a complete transfer of one or more of outer most electrons from the atom of a metal to that of a non – metal. 

What changes are observed in atoms undergoing ionic bonding? 

Due to the electron transfer the following changes occurs – (i) Both the atoms acquire stable noble gas configuration. 

(ii) The atom that loses electrons becomes +vely charged called cation whereas that gains electrons becomes –vely charged called anion. 

(iii) Cation and anion are held together by the coulombic forces of attraction to form an ionic bond.  

Mention the factors that influence the formation of an Ionic bond.

Ionic bond formation mainly depends upon three factors – (i) Low ionization energy – elements with low ionization enthalpy have greater tendency to form an ionic bonds. 

(ii) High electron gain enthalpy – high negative value of electron gain enthalpy favours ionic bond. (iii) Lattice energy – high lattice energy value favours ionic bond formation. 

Define bond order.

Bond order is defined as number of bonds between two atoms in a molecule. 

What type of bond is formed when atoms have high difference of electornegativity? 

Electrovalent or ionic bond.

Out of covalent and hydrogen bonds, which is stronger.

Covalent bond. 

Define covalent radius. 

The covalent radius is measured approximately as the radius of an atom’s core which is in contact with the core of an adjacent atom in a bonded situation.

Define dipole moment. 

Dipole moment is defined as the product of the magnitude of the charge and the distance between the centers of positive and negative charge. 

Give the mathematical expression of dipole moment.

Mathematically dipole moment is expressed as dipole moment (M) = charge (Q) x distance of separation (r). Dipole moment is usually expressed in Debye units (D).

On which factor does dipole moment depend in case of polyatomic molecules. 

The dipole moment of the polyatomic molecule depends on individual dipole moments of bonds and also on the spatial arrangement of various bonds in the molecule. 

Give the main features of VSEPR Theory.

The main postulates of VSEPR theory are as follows : (i) The shape of a molecule depends upon the number of valence shell electron pairs around the central atom.

 (ii) Pairs of electrons in the valence shell repel one another since their electron clouds are negatively charged. 

(iii) There pairs of electrons tends to occupy such position in space that minimize repulsion and thus maximize distance between them. 

(iv) The valence shell is taken as a sphere with the electron pairs localizing on the sphere at maximum distance from one another. 

(v) A multiple bond is treated as it is a single electron pair and two or three electron pairs of a multiple bond is treated as super pair. 

(vi) When two or more resonance structures can represent a molecule, the VSEPR nodal is applicable to any such structure.  

What’s difference between lone pair and bonded pair of electrons?

Lone pair electrons do not take part in bond formation whereas bond pair electrons take part in bond formation. 

How would you attribute the structure of PH3 molecule using VSEPR model? 

Phosphorus atom has 5 electrons in its outermost orbit. H – atoms contribute one electron each to make in all 8 electron around P – atom. Thus 4 pairs of electrons would be distributed in a tetrahedral manner around the central atom. Three pairs from three P – H bonds while the fourth pair remains unused. Due to repulsion between the bp and lp, the shape is not of tetrahedral but trigonal pyramidal molecule.

How is VBT different from Lewis concept?

In Lewis concept, bond formation is explained in terms of sharing of electron pairs and the Octet rule whereas in VBT bond formation is described in terms of hybridization and overlap of the orbitals.

S – orbital does not show any preference for direction. Why? 

S – Orbital does not show any preference for direction because it is spherically symmetrical.

 What is sigma bond?

A covalent bond formed due to the overlap of orbitals of the two atoms along the line going the two nuclei (orbital axis) is called sigma (σ) bond.

What is pi – bond? 

A covalent bond formed between the two atoms due to the sideways overlap of their p – orbitals is called a pi (π ) bond. 

Why is σ– bond stronger than π - bond? 

Orbitals can overlap to a greater extent in a σ - bond due to axial orientation, so σ - bond is strong. Whereas, in a pi – bond sideways overlapping is not to an appreciable extent due to the presence of σ - bond which restricts the distance between the involved atoms. 

How many σ – and π - bond are there in a molecule of C2H4 (ethene )? 

In a molecule of ethene, there are 5 σ - bonds (one between C-C , and four between C-H and one π - bond. 

How many σ - and π - bonds are there in a molecule of CH2 = CH – CH = CH2 ? 

There are 9 σ - bonds (three between C – C and 6 between C – H) and 2 π - bonds.

What type of bond exists in multiple bond (double / triple)? 

pi (π) – bond is always present in molecules containing multiple bond

What are the different types of σ - bond formation? 

What are the different types of σ - bond formation? 

What type of bond are formed due to orbital overlap? 

Covalent bonds are formed due to the overlap of certain orbitals that are oriented favourably in the space. 

How do covalent bonds form due to orbital overlapping? 

According to orbital overlap concept, the formation of a covalent bond between two atoms results by pairing of electrons present in the valence shell having opposite spins.

What is zero over lap? 

 The unsymmetrical overlap of orbitals results in zero overlap i-e; between px-s and pxpy orbital 

Define hybridisation. 

Hybridisation is defined as the process of intermixing of the orbitals of slightly different energies so as to redistribute their energies, resulting in the formation of new set of orbitals of equivalent energies and shape. 

Give the features of hybridisation. 

The main features of hybridization are (i) The number of hybrid orbitals is equal to number of the atomic orbitals that get hybridized. 

(ii) The hybridized orbitals are always equivalent in energy and shape. 

(iii) The hybrid orbitals are more effective in forming stable bonds than the pure atomic orbitals. 

(iv) The hybrid orbitals orient in a manner to minimize repulsion resulting in a particular geometrical shape.

What are the important consolations for hybridisation?

(i) The orbitals present in the valence shell of the atom are hybridised.

 (ii) The orbitals undergoing hybridization should have almost the same energy. 

(iii) It is not essential that electrons get promoted prior to hybridization. 

(iv) It is necessary that only half filled orbitals participate in hybridisation even filled orbitlals can take part. 

. Out of p-orbital and sp-hybrid orbital which has greater directional character and Why?

Sp-hybrid orbital has greater directional character than p-orbital. Because in case of p-orbitals, the two lobes are equal in size and equal electron density is distributed whereas in Sp-hybrid orbital, electron density is greater on one side. 

Define bonding molecular orbital. 

The molecular orbital formed by the addition of atomic orbitals is called bonding molecular orbital. 

 σ =Ψ +Ψ A B

Why are bonding molecular orbitals more stable than antibonding molecular orbitals? 

Bonding molecular orbital has lower energy and hence greater stability than the corresponding antibonding molecular orbital. 

Define hydrogen bonding 

Hydrogen bond can be defined as the attractive force which binds hydrogen atom of one molecule with the electronegative atom (F, O or N) of another molecule. 

What are the types of H-bonding? Which of them is stronger? 

(i) Inter-molecular H-bonding 

(ii) Intra molecular H-bonding. Inter molecular H-bonding is stronger than intramolecular H-bonding.