chemical bonding and shape of molecules
KEY CONCEPTS :
1. Kossel-Lewis Approach to chemical Bonding & Formal Charge
2. Bond Parameters
3. VSEPR Theory
4. Resonance
5. Valence Bond Theory
6. Hybridisation
7. Molecular Orbital Theory
Concept 1. Kossel -Lewis Approach to Chemical Bonding & Formal Charge
WALTHER KOSSEL G.N. LEWIS G.N. Lewis, an American chemist introduced simple notations ( LEWIS SYMBOLS) to represent valence electrons in an atom. G. N. Lewis and Kossel studied the electronic configuration of noble gases and observed that the inertness of noble gases is due to their complete octet or duplet in case of Helium which has 2 electrons in its last shell and gave a generalization which states, "the atoms of different elements combine with each other in order to complete their octets or duplets ( in case of H, Li and Be ) to attain stable electronic configuration". Significance of Lewis Symbols
: The number of dots around the symbol represents the number of valence electrons
. This number of valence electrons helps to calculate the common or group valence of the element. . www.tiwariacademy.com Focus on free education www.tiwariacademy.in According to Kossel – Lewis approach, the atoms of different elements take part in chemical combination in order to complete their octet (to have eight electrons in the outermost valence shell) or duplet (to have two valence electrons in some cases such as H, Li, Be etc, or to attain the nearest noble gas configuration. This is known as OCTET RULE
. Ionic Bond: Ionic bond is a bond is formed by the complete transference of a certain number of electrons from one atom to another atom, so that both acquire stable electronic configuration". Elements of group 1 and group 2 on combining with halogens, oxygen and sulphur generally form ionic bonds Conditions necessary for the formation of an ionic bond
1. Atoms forming positive (Cat) ions should have: (Usually Metals) Low Ionization Energy Low electron affinity Low electro negativity High Lattice Energy
2. Atoms forming negative (An) ions should have (Usually Non-Metals) High Electron affinity High Electro
negativity High ionization energy High Lattice energy An ionic or electrovalent bond has the following characteristics:
An ionic bond is formed due to the columbic attraction between the positively and negatively charged ions. An ionic bond is non-directional i.e., the strength of interaction between two ions depend upon distance, but
d but not on the direction.
An ionic bond gets broken when the substance is dissolved in a polar solvent or when the substance is melted. www.tiwariacademy.com Focus on free education www.tiwariacademy.in Covalent Bond A covalent bond is formed when the atoms of same or different elements combine by mutual sharing of electrons". The compound, thus formed, is known as a Covalent Compound. Such chemical bonds are formed by sharing of electrons, eg. formation of O2 molecule. Conditions necessary for the formation of covalent bonds: A bond formed by mutual sharing of electrons
Formed between two or more non-metals – difference in electro negativity should not be high. A single bond represents 2 electrons
They satisfy the octet rule by sharing of electrons
These molecules have a definite shapeocus .
Polar Covalent Bond A bond in which electrons are shared between elements having a difference of electro negativity of between 0.5 & 2.0. Dipole Moment Dipole moment is defined as the product of the magnitude of charge on anyone of the atoms and the distance between them. Dipole moment is represented by a Greek letter 'µ'. It can be expressed mathematically, as: µ = e × d Where, e = charge on anyone of the atoms d = distance between the atoms. Its unit in CGS system is debye (D). www.tiwariacademy.com Focus on free education www.tiwariacademy.in Formal Charge The formal charge is a hypothetical charge assigned from the dot structure. The formal charges is the electrical charge difference between the valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis Structure. Formal Charge = [Number of valence electrons on atom] – [non-bonded electrons + number of bonds]. For Example BH4
The number of valence electrons for boron is 3.
The number of non-bonded electrons is zero.
The number of bonds around boron is 4. So formal charge = 3 – (0 + 4) = 3 – 4 = –1 Limitations of Octet Rule
(a) The rule failed to predict the shape and relative stability of molecules.
(b) It is based upon the inert nature of noble gases. However, some noble gases like xenon and krypton form compounds such as XeF2 , KrF2 etc
. (c) The octet rule cannot be applied to the elements in and beyond the third period of the periodic table. The elements present in these periods have more than eight valence electrons around the central atom. For example: PF5 , SF6 , etc
. (d) The octet rule is not satisfied for all atoms in a molecule having an odd number of electrons. For example, NO and NO2 do not satisfy the octet rule
. (e) This rule cannot be applied to those compounds in which the number of electrons surrounding the central atom is less than eight. For example, LiCl, BeH2, AlCl3 etc. do not obey the octet rule.
BOND LENGTH Bond length is the average distance between the centres of the nuclei of two bonded atoms in a molecule. It is expressed in Angstrom units (Å) or picometers (pm). 1Å = 10-10m and 1pm = 10-12 m. It is determined with the help of X-rays diffraction and other spectroscopic methods. Bond length depends upon the following factors: Bond Multiplicity Bond length decreases with increase in bond multiplicity. CC bond length is shorter than C=C bond which in turn is shorter than C-C. Size of the Atom The bond length increases with increase in the size of the atom. From the above values it is clear that the bond lengths for a given family increase with increase in atomic number. For example : C-C < Si-Si < Ge-Ge This is because with the increase in size of the atom, the distance of the electrons from the nucleus increases successively with the addition of a new shell. Therefore the average distance between the bonding nuclei (bond length) increase
. BOND ANGLE
It is defined as the average angle between the orbitals of the central atom containing the bonding electron pairs in the molecule. It is expressed in degree/minute/second. This gives an idea about the distribution of orbitals around the central atom in a molecule. Therefore bond angle determines the shape of a molecule. For example, the H-O-H bond angle in H2O is 104.5° and H-N-H bond angle is NH3 107°. BOND PARAMETERS
n BOND DISSOCIATION ENTHALPY It is defined as the enthalpy change involved to break one mole of bonds of a particular type between the atoms of a molecule in the gaseous state. It is expressed in terms of kJ mol-1. When a bond is formed between the atoms, energy is released and the bonded atoms have lesser energy than the separated individual atoms. Then, same amount of energy will be needed to form the bond. This energy is called the bond dissociation energy and is a measure of bond strength. Larger the bond dissociation energy, stronger will be the bond in the molecule. Bond Dissociation Enthalpy depends upon
: Size of Bonded Atoms
Bond Length
Bond Polarity