CHEMICAL BONDING AND MOLECULAR STRUCTURE 1 Flashcards
describe chemical bond
l A force that acts between two or more atoms to hold them together as a stable molecule.
l It is union of two or more atoms involving redistribution of e–
among them.
l This process accompanied by decrease in energy.
l Decrease in energy a Strength of the bond.
l Therefore molecules are more stable than atoms.
what are two types of bonds
(i) strong bonds
interatomic, bond energy= 200kJmol-1
ionic/covalent/coordinate/metallic
(ii) weak bonds
intermolecular,bond energy= 2.42kJmol-1
hydrogen bond/ van der waals
what are the causes for atoms to comine
(1) tendency to lower energy
Attraction a 1/energy a Stability.
(e) Bond formation is an exothermic process (DH = –ve)
(2) tendency to gain octet configuration
Octet rule can be accomplished by
(i) equal sharing of electrons (ii) Unequal sharing of electrons (iii) Transfer of electrons
exceptions of octet rule
Pseudo inert gas configuration :-
Cations which contains 18 electrons in outermost orbit
Ex. Ga+3, Cu+, Ag+, Zn+2, Cd+2, Sn 4+
Electronic configuration of Ga - 1s22s22p93s23p94s24d104p1
Electronic configuration of Ga+3- 1s22s22p63s23p63d10
relation b/w electronegativity and bond strength
Electronegativity difference a nature of ionic bond.
conditions for formation of ionic bond
(a) Ionisation Energy
-Lesser Ionisation energy ∝ Greater tendency to form cation.
(b) Electron Gain Enthalpy
-Higher electron affinity ∝ Greater tendency to form anion
(c)Lattice Energy
Amount of energy released when one mole of crystal lattice is formed from its constituents ions or amount of energy required to break one mole of crystal lattice into its constituents ion
is known as lattice energy.
Higher lattice energy ∝ Greater will be the stability or strength of ionic compound.
(d) Large Electronegativity Difference
what are the properties of ionic cpds
(a) Ionic bond is non directional ( ie equal in all directions).The force of attraction is equal in all direction so ionic bond is non-
directional. A definite three dimensional structure is formed called
crystal lattice.
(b)
Ionic compound do not have molecular formula. It has only empirical formula.
e.g. NaCl is empirical formula of sodium chloride
(c) Physical state –
Ionic compounds are hard, crystalline and brittle due to strong electrostatic force of attraction.
Brittleness, When ionic crystal is hit, a layer of ions get shifted. Then the like charged ions repel each other and hence get seperated.
(d)*Ionic compounds have high boiling point and melting point due to
strong electrostatic force of attraction among oppositely charged
ions.
(e)It depends on ionic mobility.
* In solid state - No free ions - Bad conductor of electricity.
*In fused state or aqueous solution Due to free ions - Good conductor of
electricity.
*Conductivity order Solid state < fused state < Aqueous solution
g) Solubility
*Highly soluble in Polar solvents like water.
* Less soluble in non polar solvents like benzene.
(g) Ionic reaction –
(a) Ionic compounds shows ionic reaction and covalent compounds shows - molecular reaction.
(b) Ionic reactions are faster than molecular reaction because of free ions.
explain hydration and hydration energy?
Whenever an ionic compound is dissolved in water. The opposite pole of water (H+d–O–d) will attract the opposite
ion of ionic compound. Due to this some amount of energy is released. If this released energy is more than
lattice energy, the ionic compound will dissociate into ions.
These ions get surrounded by water molecules .
This whole process is known as hydration and released energy is hydration energy and if the solvent is other
than water then this process is known as solvation and released energy is solvation energy.
what is dielectric constant
*It is the property of medium to dissociate itself into poles/ions so that
opposite ion of ionic compound may be neutralised.
what is isomorphism
They have similar crystal structure
Conditions –
(1) Two compounds are said to be isomorphous if they have similar no. of electrons as well as similar number
of ions i.e. similar configuration of their cation and anion.
Example –
NaF, MgO
(2) Crystal having same number of water of crystallisation.
Example - Green vitriol FeSO4.7H2O, Epsom salt MgSO4.7H2O, White vitriol ZnSO4.7H2O are isomorphous
but blue vitriol CuSO4.5H2O is not isomorphous.
what is polarisation
When a cation approaches an anion closely the positive charge of a cation attract the electron cloud of the anion
towards itself, due to the electrostatic force of attraction between them.
At the same time cation also repel the positively charge nucleus of anion.
Due to this combined effect, cloud of anion is bulged or elongated towards the cation. This is called distortion,
deformation or Polarisation of the anion by the cation and anion is called Polarised.
what is polarisation power
The ability of cation to polarise a nearby anion is called Polarisation power of cation.
Polarisation power of a cation is usually called ionic potential or charge density.
Ionic potential f (phi) =
Charge on cation
Size of cation
what is polarisability
Ability of an anion to get polarised by the cation.
Polarisation of anion causes some sharing of electron between the ions so ionic bond acquires certain covalent
character.
Polarisation direclty prop Covalent character
Magnitude of polarisation depends upon number of factors, suggested by Fajan and are known as Fajan’s rule.
which anions are difficult to polaris
Oxide or fluoride ions are very difficult to polarize due to their small
size and high electronegativity
*Only a few cations like Li+, Be2+, Al3+ , etc. can polarize them to some
extent.
explain metal decomposition
Metal carbonate → metal oxide + CO2
*For IA, Only Li2CO3 decomposes, Na to Cs carbonates do not
decomposes
Li2CO3 → Li2O + CO2
*For IIA, All carbonates decomposes.
CaCO3 → CaO + CO2
