CBSE Class 11 Chemistry Chemical Bonding And Molecular Structure Notes Set B

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4.Chemical bonding and Molecular Structure

Some Important Points and Terms of the Chapter

1.  Lewis dot structures are shorthand to represent the valence electrons of an atom. The structures are written as the element symbol surrounded by dots that represent the valence electrons. 

2.  Covalent Bonds-  The bond formed between two atoms by mutual sharing of electrons between them so as to complete their octets or duplets.   When two atoms share one electron pair they are said to be joined by a single covalent bond.e.g H2If two atoms share two electron pairs of electrons, the covalent bond between them is called a double bond. e.g O2If two atoms share three electron pairs of electrons, the covalent bond between them is called a double bond. e.g N2

3.   Octet Rule- Kossel and Lewis in 1916 developed an important theory of chemical combination between atoms known as electronic theory of chemical bonding. According to this, atoms can combine either by transfer of valence electrons from one atom to another or by sharing of valence electrons in order to attain their octet. This is known as octet rule. 

4.  Limitations of octet rule:-

a)  Incomplete octet of the central atom: In some compounds the number of electrons surrounding the central atom is less than eight. This is especially the case with elements having less than four valence electrons.   Examples-  LiCl ,BeCl2 , BCl3

b)  Odd-electron molecules:  In molecules with an odd number of electrons like nitric oxide, NO and  nitrogen dioxide, the octet rule is not satisfied for all the atoms.   

c)  The expanded octet : Elements in and beyond the third period of the periodic table have, apart from 3s and 3p orbitals, 3d orbitals also available for bonding. In a number of compounds of these elements there are more than eight valence electrons around the central atom. This is termed as the expanded octet. Some of examples of such compounds are: PF5, SF6. 

d)  This theory does not account for the shape of molecules.

5.  Electrovalent bond or Ionic Bond: The chemical bond as result of transfer of electron from one atom(electropositive) to another atom (electronegative).Ionic bonds will be formed more easily between elements with comparatively low ionization enthalpies and elements with comparatively high negative value of electron gain enthalpy. Most ionic compounds have cations derived from metallic elements and anions from non-metallic elements.

7.  Bond length is defined as the equilibrium distance between the nuclei of two bonded atoms in a molecule. 

8.  Bond Angle:It is defined as the angle between the orbital containing bonding electron pairs around the central atom in a molecule/complex ion. It gives some idea regarding the distribution of orbital around the central atom in a molecule/complex ion and hence it helps us in determining its shape

9.  Bond enthalpy: It is defined as the amount of energy required to break one mole of bonds of a particular type between two atoms in a gaseous state. The unit of bond enthalpy is kJ mol–1

10.  Bond Order : The Bond Order is given by the number of bonds between the two atoms in a molecule. E.g.: Bond Order of O2= 2. With increase in bond order, bond enthalpy increases and bond length decreases. 

11. Resonance:   According to the concept of resonance, whenever a single Lewis structure cannot describe a molecule accurately, a number of structures with similar energy, positions of nuclei, bonding and the non- bonding pairs of electrons are taken as the canonical structures of the hybrid which describes the molecule accurately

12.  Polarity of bonds: In case of heteronuclear molecules like HCl, the shared pair of electron between the two atoms gets displaced more towards chlorine  since the electronegativity of chlorine is far greater than that of hydrogen. The resultant covalent bond is called a polar covalent bond.

13.   Dipole moment: As a result of polarization, the molecule possesses the dipole moment  which can be defined as the product of charge and the distance between the centers of positive and negative charge. It is usually designated by a Greek letter ‗µ‘. Mathematically, it is expressed as follows:

Dipole moment (µ ) = charge (Q) X distance of separation (r)

Please refer to attached file for CBSE Class 11 Chemistry-Chemical bonding and Molecular Structure