In chemistry, the double bond rule states that elements with a principal quantum number (n) greater than 2 for their valence electrons (period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds).
Double bonds for these heavier elements, when they exist, are often weak due to poor orbital overlap between the n>2 orbitals of the two atoms.
Although such compounds are not intrinsically unstable, they instead tend to dimerize or even polymerize.
[2] An example is the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of O2.
Likewise, bromobenzenes are generally inert, whereas benzylic bromides are reactive.