Neil's Bohr'sTheory:

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Neil Bohr in 1913, gave his idea of line spectra.

The main postulates are:

1. Electrons revolve round the nucleus in fixed orbits or energy levels.
2. The electrons moving in a fixed orbit have fixed energy.
3. The electrons in an atom move only in certain energy levels, so an electron in allowed energy state will not radiate energy and there fore will not fall in the nucleus.
4. An electron absorbs energy to go to higher orbit and radiates energy as a light only when electron jumps from higher energy level to lower energy level.
5. The quantity of energy radiated is in discrete quantity, called Quanta. A quantum of energy is directly proportional to the frequency of the radiation.

where............... h = Plank's constant.
..........................v= is frequency of the radiation.
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Dalton's Atomic Theory:

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Introduction:

In 1808, Dalton suggested the fundamental ideas of atomic theory which explains chemical nature of matter and existence of atoms. It is called as Dalton's Atomic Theory.

Postulates:

The main postulates are as;

1. All elements are made up of small indivisible particles called Atoms.
2. Compounds are formed when atoms of more than one element combine in simpler whole number ratio.
3. A chemical reaction is re-arrangement of atoms, but atoms themselves are not changed means atoms are neither created nor destroyed.
4. As time passed, new experimental facts led to modification of Dalton's atomic theory. i.e.
5. Atom is further composed of smaller particles called proton electron and neutron.

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Arrhenius Concept of Acids and Bases:

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According to Arrhenius concept all substances which give ions when dissolved in water are called acids while those which ionize in water to furnish ions are called bases.

Some acids and bases ionize almost completely in solutions and are called strong acids and bases. Others are dissociated to a limited extent in solutions and are termed weak acids and bases. HCl, , , , etc., are examples of strong acids and NaOH, KOH, NOH are strong bases. Every hydrogen compound cannot be regarded as an acid, e.g., is not an acid. Similarly, , , etc., have OH groups but they are not bases.

(i) Utility of Arrhenius concept :

The Arrhenius concept of acids and bases was able to explain a number of phenomenon like neutralization, salt hydrolysis, strength of acids and bases etc.


(ii) Limitations of Arrhenius concept:

1. For the acidic or basic properties, the presence of water is absolutely necessary. Dry HCl shall not act as an acid. HCl is regarded as an acid only when dissolved in water and not in any other solvent.
2. The concept does not explain acidic and basic character of substances in non-aqueous solvents.
3. The neutralization process is limited to those reactions which can occur in aqueous solutions only, although reactions involving salt formation do occur in absence of solvent.
4. It cannot explain the acidic character of certain salts such as AlCl3 in aqueous solution.

Valence Shell Electron Pair Repulsion Theory (VSEPR):

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The basic concept of the theory was suggested by Sidgwick and Powell (1940). It provides useful idea for predicting shapes and geometries of molecules. The concept tells that, the arrangement of bonds around the central atom depends upon the repulsion’s operating between electron pairs(bonded or non bonded) around the central atom. Gillespie and Nyholm developed this concept as VSEPR theory.


The Main Postulates of VSEPR Theory are:

1. For polyatomic molecules containing 3 or more atoms, one of the atoms is called the central atom to which other atoms are linked.
2. The geometry of a molecule depends upon the total number of valence shell electron pairs (bonded or not bonded) present around the central atom and their repulsion due to relative sizes and shapes.
3. If the central atom is surrounded by bond pairs only. It gives the symmetrical shape to the molecule.
4. If the central atom is surrounded by lone pairs (lp) as well as bond pairs (bp) of then the molecule has a distorted geometry.
5. The relative order of repulsion between electron pairs is as follows : lp – lp > lp – bp > bp – bp.
6. A lone pair is concentrated around the central atom while a bond pair is pulled out between two bonded atoms. As such repulsion becomes greater when a lone pair is involved.

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Valence Bond Theory (VBT):

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It was developed by Hitler and London in 1927 and modified by Pauling and Slater in 1931.

1. Orbitals having unpaired electrons of anti spin overlaps with each other.
2. After overlapping a new localized bond orbital is formed which has maximum probability of finding electrons.
3. To form a covalent bond, two atoms must come close to each other so that orbitals of one overlaps with the other.
4. Greater is the overlapping, lesser will be the bond length, more will be attraction and more will be bond energy and the stability of bond will also be high.
5. Covalent bond is formed due to electrostatic attraction between radii and the accumulated electrons cloud and by attraction between spins of anti spin electrons.
6. More closer the valence shells are to the nucleus, more will be the overlapping and the bond energy will also be high.
7. Between two sub shells of same energy level, the sub shell more directionally concentrated shows more overlapping. Bond energy : 2s – 2s <>
8. The extent of overlapping depends upon: Nature of orbitals involved in overlapping, and nature of overlapping.
9. s-orbitals are spherically symmetrical and thus show only head on overlapping. On the other hand, p-orbitals are directionally concentrated and thus show either head on overlapping or lateral overlapping.Overlapping of different type gives sigma (σ) and pi (π) bond.

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Molicular Orbital Theory or MOT:

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Molecular orbital theory was given by Hand and Mullikan in 1932.

The main ideas of this theory are,

1. Molecular orbitals are the energy states of a molecule in which the electrons of the molecule are filled just as atomic orbitals are the energy states of an atom in which the electrons of the atom are filled.
2. When two atomic orbitals combine or overlap, they lose their identity and form new orbitals. The new orbitals thus formed are called molecular orbitals.
3. In terms of probability distribution, a molecular orbital gives the electron probability distribution around a group of nuclei just as an atomic orbital gives the electron probability distribution around the single nucleus.
4. Only those atomic orbitals can combine to form molecular orbitals which have comparable energies and proper orientation.
5. The number of molecular orbitals formed is equal to the number of combining atomic orbitals.
6. When two atomic orbitals combine, they form two new orbitals called bonding molecular orbital and anti bonding molecular orbital.
7. The bonding molecular orbital has lower energy and hence greater stability than the corresponding anti bonding molecular orbital.
8. The shapes of the molecular orbitals formed depend upon the type of combining atomic orbitals.
9. The bonding molecular orbitals are represented by σ, π etc, whereas the corresponding anti bonding molecular orbitals are represented by σ*, π* etc.
10. The filling of molecular orbitals in a molecule takes place in accordance with Aufbau principle, Paulo's exclusion principle and Hand's rule.The general order of increasing energy among the molecular orbitals formed by the elements of second period and hydrogen and their general electronic configurations are given below.
11. Electrons are filled in the increasing energy of the MO which is in order.

(a) σ1s, σ*1s, σ2s, σ*2s, σ2px, π*2py, σ*2px, π2pz, π*2pz

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• Bond order ∝ Stability of molecule ∝ Dissociation energy ∝ 1/Bond length .
• If all the electrons in a molecule are paired then the substance is a diamagnetic on the other hand if there are unpaired electrons in the molecule, then the substance is paramagnetic. More the number of unpaired electron in the molecule greater is the paramagnetism of the substance.

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Third Law of Thermodynamics:

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This law was first formulated by German chemist Walther Nernst in 1906. According to this law,

“The entropy of all perfectly crystalline solids is zero at the absolute zero temperature. Since entropy is a measure of disorder, it can be interpretated that at absolute zero, a perfectly crystalline solid has a perfect order of its constituent particles.”

The most important application of the third law of thermodynamics is that it helps in the calculation of absolute entropies of the substance at any temperature T.

Where is the heat capacity of the substance at constant pressure and is supposed to remain constant in the range of 0 to T.

Limitation of Law:

1. Glassy solids even at has entropy greater than zero.
2. Solids having mixtures of isotopes do not have zero entropy at . For example, entropy of solid chlorine is not zero at .
3. Crystals of , etc. do not have perfect order even at thus their entropy is not equal to zero.

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