- Chemical bond:–
=> 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.
- Octet Rule:–
=> Atoms form chemical bonds in order to complete their octet i.e. eight electrons in their valence shell.
- Lewis Structures:–
=> 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:–
=> 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)]
- Resonance:–
=> 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
- Ionic Bonding:–
- 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
- Ionization Energy
=> Formation of gaseous anions
=> X(g) + e → X- (g) + E.A
- Electron Affinity
=> Packing of ions of opposite charges to form ionic solids
=> A+ (g) + X- (g) →AX (s) +Energy
- Lattice energy:-
=> Conditions required of formation of ionic bonds:
a) Low I.E of cation.
b) High E.A of anion.
c) High lattice energy.
- Covalent Bonding:–
=> 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.