Explain the various transitions occurring in the UV-vis electronic spectroscopy withproper illustration and examples.

Ultraviolet-visible (UV-vis) spectroscopy involves the spectroscopy of photons and is usually used in chemical analysis to determine the absorption of certain wavelengths of light by specific substances.

There are four types of electronic transitions that can be studied with this method:

1. π - π* Transition: This transition occurs in molecules which have pi bonds (π bonds), and it's the most common type of transition in UV-Vis spectroscopy. An example of this would be ethene (C2H4). In this transition, an electron moves from a pi (π) orbital to a pi* (π*) antibonding orbital.

2. n - π* Transition: This transition occurs in molecules which have lone pair of electrons. An example of this would be formaldehyde (CH2O). In this transition, an electron moves from a non-bonding orbital (n) to a pi* (π*) antibonding orbital.

3. σ - σ* Transition: This transition occurs in molecules which have sigma bonds (σ bonds). An example of this would be methane (CH4). In this transition, an electron moves from a sigma (σ) orbital to a sigma* (σ*) antibonding orbital.

4. n - σ* Transition: This transition is less common but occurs in molecules which have non-bonding electrons and sigma bonds. An example of this would be water (H2O). In this transition, an electron moves from a non-bonding orbital (n) to a sigma* (σ*) antibonding orbital.

In each of these transitions, the electron absorbs energy (in the form of light), which causes it to move to a higher energy level. The wavelength of light absorbed during this process is what is measured in UV-Vis spectroscopy. Different substances will absorb different wavelengths of light, which allows for their identification and analysis.