Hydrogen Cyanide (HCN) or commonly known as prussic acid is most well know as a poison which is a very small, fast-acting, deadly chemical which can exist both as a gas and crystaline salt and can be lethal in high enough concentration. It is said to have a 'bitter almond' smell, however, this is not a reliable indicator as it is not always detectable and some people cannot always detect it. It is found abundantly in nature as well as in many man-made materials, e.g in the seeds of apples and peaches
Cyanide is known to form very strong bonds with metals and is one of the quickest acting poisons. It is toxic because when cyanide atoms are present in the human body they quickly bind to iron atoms inside cells, this prevents an enzyme cytochrome C oxidase which is inside the mitochondria of the cell. This enzyme catalyses the final stage of glucose oxidation. On consumption, it takes minutes for the victim to become unconscious and slowly their heart gives up. Interestingly chemical industries produce enough hydrogen cyanide every month to kill every living person on earth. Most of it is used to produce a variety of organic compounds. A 160-pound adult has a 90% chance of death after consuming only 0.4g of potassium cyanide. A teaspoon contains 60 times this amount.
http://www.atsdr.cdc.gov/toxprofiles/tp8-c4.pdf
https://pubchem.ncbi.nlm.nih.gov/compound/hydrogen_cyanide#section=Top
https://en.wikipedia.org/wiki/Hydrogen_cyanide
