Electronegativity and reactivity of molecules

In general molecules are more stable if there is large difference between electronegativities of their atoms but less stable and more energetically reacting if they are made of atoms with similar electronegativites (EN) excluding those elements with 4 free electrons like carbon that can form strong cubical crystal structure.
For example most metals react with oxygen when they could come in contact with that. Also pure sodium and other alkali metals start burning in water but they lose reactivity after combining with chlorine or other element from opposite side of periodic table. While nitrogen doesn't react easily with organic materials it can react in contact with lithium or magnesium. EN difference of 1,5 or more is usually enough to start energetic (often flaming) chemical reaction without any need to add heat but with smaller differences heat or catalyst are needed like between carbon and oxygen.

Energy releasing metabolism and burning usually create molecules with larger difference of EN between its atoms than before. For example fats and sugars often have mainly carbon and hydrogen connected to each other which have 0,35 difference in EN but after turning them into CO2 (0,89) and water (1,2 difference) this difference grows and resulting molecules need much energy (heat or energetic electric current) to break those up.

Precious metals in middle columns are relatively nonreactive or resistant to most acids, oxygen and other reactive substances compared to other metals. Among the most stable metals are metals with EN at least 2,2 (or more) like gold and those with less EN can rust like silver but others between like Mo usually need heat to start combining with oxygen without visibly rusting in room temperature air.

Many infrared sensors use unstable molecules that create electron flow with even less energy than visible light provides. These are often cooled to 60-100 degrees above absolute zero to avoid detectors from blinding itself by creating electron currents due to room temperature and cooling is commonly needed to see room temperature heat. At the same time these same sensors without cooling are usually only useful for detecting temperatures that are hundreds of degrees above room temperature. As example PbSe has elements with EN differences of 0,2 and can detect infrared with 5-6 micrometer wavelength. Heat from human body is about 10 micrometer infrared radiation.
One of the most sensitive infrared sensor materials is mixture of Hg, Te and Cd which can detect almost 2-3 times less energetic infrared radiation than PbSe and it has about 2 times smaller difference in EN. Due to sensitivity HgTeCd has to be about 77 degrees above absolute zero to detect weaker infrared wavelength (up to ~12 micrometer wavelength for this mixture).