Species are often defined as a group of individuals with similar characteristics, where they can interbreed to produce fertile offsprings, typically by sexual reproduction. To differentiate between types of living organisms, species are used as the principal natural unit in biology.

Magnitude of any given physical quantity may vary over a wide range, therefore, different units of same physical quantity are required.
For example:
1. Mass ranges from 10-30 kg (for an electron) to 1053 kg (for the known universe). We need different units to measure them like miligram, gram, kilogram etc.
2. The length of a pen can be easily measured in cm, the height of a tree can be measured in metres, the distance between two cities can be measured in kilometres and distance between two heavenly bodies can be measured in light year.

Prime factorising 98

{tex}sqrt(98) = sqrt(2*7*7){/tex}

{tex}= sqrt(7^2 *2){/tex}

=7√2

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