So plants and mud have a smell just like your ice-cream so when the water droplet touches the mud and makes it wet 

When lightning strikes,diatomic molecules of oxygen and nitrogen (containing 2atoms) are split, and rearrange to create nitric oxide (NO)and ozone, or O3. Ozonemolecules are carried down by droplets of rain to contribute to the scent. 

It is also helped by 

Actinomycetes, a type of bacteria found in soil, secrete a compound called geosmin , which is released from soil into the air by raindrops. Geosmin in the air can be detected by the human nose at less than 5 parts per trillion

And also 

During dry weather, plants produce compounds that accumulate in between rocks and in soil. When it rains, these compounds are released into the air to add to the earthy smell of petrichor 

It also used the help of 

Stearic acid and palmitic acid they are common plant oils. They are fatty acids with a long hydrocarbon chains with a carboxyl group on one end and a methyl group on the other.

Palmitic Acid (C16H32O2) Stearic Acid (C18H36O2)

In 2015, scientists from the Massachusetts Institute of Technology (MIT) used high-speed cameras to record how the scent moves into the air. The tests involved approximately 600 experiments on 28 different surfaces, including engineered materials and soil samples. When a raindrop lands on a porous surface, air from the pores forms small bubbles, which float to the surface and release aerosols. Such aerosols carry the scent, as well as bacteria and viruses from the soil.

Raindrops that move at a slower rate tend to produce more aerosols; this serves as an explanation for why the petrichor is more common after light rains. Actinomycetes is the bacterium responsible for producing spores in soil.

So in conclusion we got to learn a lot about the smell of rain or petrichor so I hope you enjoy the lesson and I will meet you again later