Henry Moseley was the scientist who first classified elements on the basis of atomic number. He stated that” the properties of elements are a periodic function of their atomic number”. Untill this all the scientists tried to classify elements on the basis of atomic mass.

This law states that the amount of heat evolved or absorbed in a process, including a chemical change is the same whether the process takes place in one or several steps.

Suppose in a process the system changes from state A to state B in one step and the heat exchanged in this change is q. Now suppose the system changes from state A to state B in three steps involving a change from A to C, C to D and finally from D to B. If q1, q2 and q­3 are the heats exchanged in the first, second and third step, respectively then according to Hess’s law

q₁ + q₂ + q₃ = q

Hess’s law is simply a corollary of the first law of thermodynamics. It implies that enthalpy change of a reaction depends on the initial and final state and is independent of the manner by which the change is brought about.

Surface energy is defined as the mechanical work required to increase the unit area of free surface under isothermal condition.
Surface tension is the property of any liquid by virtue of which tries to minimize its free surface area. Surface tension of a liquid is measured as the force acting per length on an imaginary line draw tangentially on the surface of the liquid.

Vapour pressure can be defined as pressure formed by the vapor of the liquid (or solid) over the surface of the liquid. This pressure is formed in a thermodynamic equilibrium state in a closed container at a certain temperature. Liquid’s evaporation rate is identified by the equilibrium vapor pressure. Vapour pressure increases with the temperature. The boiling point of the liquid is the point when the pressure exerted by surrounding equals to the pressure exerted by vapor.

Valence bond theory was proposed by Heitler and London in 1927 based on atomic orbitals and their configuration.

A covalent bond is formed when pure, valence atomic orbital of one atom overlaps with another pure, valence atomic orbital of another atom. During covalent bond formation energy releases that means stability increases.

Each of the overlapping orbitals contains the unpaired electron of opposite spin.

The electron pair is shared by both the atoms. The strongest bond is formed when the orbitals of the two atoms overlap to the maximum extent.

Based on the overlapping of orbitals, two types of covalent bonds are formed. These are known as sigma and pi bonds.

Sigma bonds are formed by the end-to-end overlap of atomic orbitals along the inter-nuclear axis known as a head-on or axial overlap.

End on over lapping is of three types, they are s-s overlapping, s-p overlapping and p-p overlapping.

The VSEPR theory is based on the premise that there is a repulsion between the pairs of valence electrons in all atoms, and that the atoms will always tend to arrange themselves in a manner in which this electron pair repulsion is minimalized. This arrangement of the atom determines the geometry of the resulting molecule.

The two primary developers of the VSEPR theory are Ronald Nyholm and Ronald Gillespie. This theory is also known as the Gillespie-Nyholm theory to honor these chemists.

In this theory, the repulsion between two electrons caused by the Pauli exclusion principle has greater importance than electrostatic repulsion in the determination of molecular geometry

Bhopal gas tragedy was caused by the leakage of methyl isocyanate gas ( CH₃ -N=C=O) from the Union Carbide India limited pesticide plant situated in Bhopal. 

• CCl₄ doesnot undergo hydrolysis by water because the carbon atom is small and is shielded by larger chlorine atoms.
• Carbon does not have 3d atomic orbitals that water can use to form co-ordinate bonds.
• In SiCl₄, the silicon atom is larger than the carbon atom and also has available 3d atomic orbitals for bonding, thus hydrolysis is possible.

(a) When heated, borax undergoes various transitions. It first loses water molecules and swells. Then, it turns into a transparent liquid, solidifying to form a glass-like material called borax bead.
Na₂B₄O₇.10H₂O → Na₂B₄O₇ + Heat → 2NaBO₂ + B₂O₃
(b) When boric acid is added to water, it accepts electrons from –OH ion. Boric acid is sparingly soluble in cold water however fairly soluble in hot water. boric acid behaves as a weak monobasic acid. It doesn't act as a proton-donor, i.e., protonic acid, however, behaves as a Lewis-acid, i.e., it accepts a pair of electrons.
B(OH)₃+2HOH→ [B(OH)₄]^- + H₃O⁺
(c) Al reacts with dilute NaOH to form sodium tetrahydroxoaluminate(III). Hydrogen gas is liberated in the process.
2Al + 2 NaOH + 6 H₂O →2Na⁺ [Al(OH)₄]^- + 3H₂

(i) Electron deficient compounds. Hydrides of group 13 (i.e. BH3, AlH3 etc.) have lesser electrons to form normal covalent bonds and hence are called electron deficient hydrides. To make up this deficiency, these hydrides generally exist in polymeric forms such as B2H6 , B4H10, (AlH3)n   etc. They act as Lewis acids i.e. electron acceptors.
(ii) Electron-precise compounds. Electron precise compounds have the required number of electrons to write their conventional Lewis structures. All elements of group 14 form such compounds (i.e. CH4 , Si H4)), which are tetrahedral in geometry. They do not act as Lewis acids or Lewis bases.
(iii) Electron rich compounds. Electron rich hydrides have excess electrons which are present as lone pairs. Elements of group 15, 16, 17 form such compounds (NH4, PH3, H2O, HF, HCl etc.). They all behave as Lewis bases i.e. electron donors.

UV rays are very harmful to living things. It can cause diseases like skin cancer and can also alter the climate drastically. The ozone layer protects us from these harmful rays and is essential for life on earth.

The ozone layer is not uniform throughout the earth; it is found in a thick layer at some places and a thin layer at others. If this layer becomes too thin, it cannot stop the UV rays from entering the earth and we say that a hole is formed in the ozone layer. Antarctica has the biggest ozone hole followed by the Arctic region and the Tibetan plateau. In these places, the layer was naturally thin, but over the years due to air pollution, these holes have grown bigger and thinner. They are a serious cause of worry across the world.

Causes of Ozone Layer Depletion

The primary reason behind the reduction of the ozone layer is the use of Hydro- Chloro-fluoro-carbons (HCFCs) and chloro-fluoro-carbons ( CFCs).  They are compounds of chlorine, fluorine, and carbon such as CF3Cl, CHCl2F etc. These are used as refrigerants in refrigerators, air conditioners, and in cooling plants. These molecules can destroy O3 molecules and therefore depletion of the O3 layer.

Nitrogen oxides such as nitrous oxide are also very reactive to O3 and are also responsible for holes in the ozone layer. These molecules are released by burning fossil fuels by cars and especially airplanes which fly near the ozone layer.

Since 1975, the hole has increased in size due to depletion in the ozone layer. Reductions of up to 70% have been found in some areas.

Prevention of Ozone Layer Depletion

All is not lost though. The depletion of the O3 layer has almost stopped today and there are signs that it can grow back. This is because countries around the world have agreed to stop the production and use of CFCs and HCFCs.

They have also introduced the application of bio-control agents for controlling the plant pests. In January 1989, the Montreal protocol was signed to limit the use of CFCs and HCFCs. 197 countries have ratified this protocol which has reduced CFC production by 98% today. It remains the most successful environmental treaty to this date.

Today there are better CFC free refrigerants available that do not pollute the atmosphere. Almost all the air-conditioners and refrigerators you buy today do not contain these harmful pollutants.

Vapour pressure is the pressure exerted by vapours when they are in equilibrium with the liquid phase at a given temperature. It depends on the nature of the liquid and temperature.
Vapour pressure of a liquid increases with increase in temperature due to increase in kinetic energy of molecules.

Real gases deviate from ideal behaviour because their particles (atoms for inert gases or molecules) occupy some finite space and do exert interactive forces among them. Completely ideal behaviour is hypothetical because of the reasons above. At low pressure and high temperature, real gases behave approximately as ideal gases. In ideal behaviour, gas particles don't occupy space and do not have any interaction, as assumed in the kinetic theory of gases. But in reality this is not the case: we get errors by applying the ideal gas law. That's why van der Waals corrected it by introducing suitable constants.

Wurtz reaction: Alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms present in the halide. This reaction is known as Wurtz reaction.

RX + 2Na + XR’ → R—R’ + 2NaX

Electronegativity is defined as the tendency of an element to attract the shared pair of electrons towards itself in a covalent bond. There is no specific unit for electronegativity. Electron gain enthalpy is the energy released when one mole of electron are added to gaseous atoms of an element. It can be negative or positive depending upon whether the electron is added or removed. An element has a constant value of the electron gain enthalpy that can be measured experimentally.