Notes on Chemical Bonding
- Chemical bond is the attractive force which holds various constituents together in a molecule.
- There are three types of chemical bonds: Ionic Bond, Covalent Bond, Co-ordinate Bond.
Atoms form chemical bonds in order to complete their octet i.e. eight electrons in their valence shell.
- Pair of bonded electrons is by means of a ‘dash’ (-) usually called a ‘bond’.
- Lone pairs or ‘non-bonded’ electrons are represented by ‘dots’.
- Electrons present in the last shell of atoms are called valence electrons.
Exceptions to the Octet Rule:
- Species with odd number of electrons: NO, NO2,
- Incomplete octet for the central atom: LiCl, BeH2 and BCl3
- Expanded octet for the central atom: PF5, SF6 and H2SO4
- Formal charge is the difference between the number of valence electrons in an isolated atom and number of electrons assigned to that atoms in Lewis structure.
- Formal charge = [Total number of valence electrons in the free atom ) – (Total number of lone pairs of electrons) -1/2(Total number of shared electrons i.e. bonding electrons)]
- For molecules and ions showing resonance it is not possible to draw a single Lewis structure.
- All the properties of such species can only be explained by two or more Lewis structures. Example: Resonance of O3
- Formation of Ionic Bond:
Formation of ionic bond takes place between a metal and a non-metal by transfer of electron.
- Formation of gaseous cations
- A(g) + I.E. → A+ (g) + e
Formation of gaseous anions
X(g) + e → X- (g) + E.A
Packing of ions of opposite charges to form ionic solids
A+ (g) + X- (g) →AX (s) +Energy
Conditions required of formation of ionic bonds:
- Low I.E of cation.
- High E.A of anion.
- High lattice energy.
Covalent bond is formed between two non-metals by sharing of electrons.
- Electron pairs which participate in bonding are called bond pairs.
- Electron pairs which do not participate in bonding are called lone pairs.
There could be single, double or triple covalent bonds between two elements depending on the number of electrons being shared.
VSEPR (Valence Shell Electron Pair Repulsion) Theory:
The shape of the molecule is determined by repulsions between all of the electron pairs present in the valence shell.
- Order of the repulsion: Lone pair.Lone pair > Lone pair. Bond pair > Bond pair. Bond pair.
- Repulsion among the bond pairs is directly proportional to the bond order and electronegativity difference between the central atom and the other atoms.
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