Types of cleavage of covalent bond

A covalent bond -- a shared pair of electrons holding two atoms together -- can be broken (undergo 'cleavage' or 'fission') in one of two distinct ways. In HOMOLYTIC cleavage, the bond's two shared electrons split evenly: one electron goes to each of the two previously-bonded atoms. This movement of just a single electron is drawn using a special HALF-headed curved arrow (sometimes nicknamed a 'fish-hook'), whose tail marks exactly where that electron is being taken FROM and whose head marks exactly where it is arriving TO. The direct result of homolytic cleavage is two electrically NEUTRAL fragments, each one now carrying a single UNPAIRED electron; a species carrying such a lone unpaired electron is called a FREE RADICAL (or simply a 'radical'). A free radical is inherently unstable and highly reactive, because it constantly 'seeks' another electron to pair up with, and so it typically has only a very brief, transitory existence as a short-lived reaction intermediate. Homolysis is specifically favoured under ultraviolet radiation, in the presence of suitable peroxides, or simply at high temperature, and any organic reaction that proceeds via this route is called a free-radical (or 'non-polar') reaction -- illustrated by $H_3C{-}Cl \xrightarrow{h\nu} H_3C^{\bullet} + Cl^{\bullet}$, giving a methyl free radical and a chlorine radical (chlorine atom). Structurally, a carbon-centred free radical is $sp^2$ hybridised, with a flat, trigonal-planar geometry and its single unpaired electron sitting in a $p_z$ orbital; alkyl free radicals are classified as primary (1°), secondary (2°), or tertiary (3°) exactly as for carbon atoms generally, and their OBSERVED stability order runs tert-butyl (3°) > isopropyl (2°) > ethyl (1°) > methyl -- i.e. stability increases as more alkyl groups are directly attached to the radical carbon. In HETEROLYTIC cleavage, by contrast, BOTH of the bond's shared electrons go to just ONE of the two previously-bonded atoms -- specifically, to whichever one of the two is the more electronegative. This movement of an entire electron PAIR (rather than a single electron) is drawn using an ordinary, full (double-headed) curved arrow. Heterolysis produces two oppositely CHARGED fragments: one negatively charged (an anion) and one positively charged (a cation). If it is the carbon atom itself that loses the shared electron pair (because it was bonded to something even MORE electronegative than itself), carbon is left positively charged, carrying only a sextet (six) of electrons rather than a full octet -- this species is called a CARBOCATION (an older name still occasionally seen is 'carbonium ion'), illustrated by $H_3C{-}Br \rightarrow H_3C^{\oplus} + Br^{\ominus}$ (since bromine is more electronegative than carbon). A carbocation's positively charged central carbon is $sp^2$ hybridised, trigonal planar in geometry, and carries a completely EMPTY $p_z$ orbital sitting perpendicular to the plane defined by its three ordinary sigma bonds; carbocations are likewise classified 1°/2°/3°, with the same observed stability order as free radicals -- tert-butyl (3°) > isopropyl (2°) > ethyl (1°) > methyl. If instead carbon is bonded to something MORE ELECTROPOSITIVE than itself, carbon keeps BOTH of the shared bonding electrons for itself when the bond breaks, …