What is "elective attraction"

Elective \E*lect"ive\, a. [Cf. F. ['e]lectif.]

1. Exerting the power of choice; selecting; as, an elective act.

2. Pertaining to, or consisting in, choice, or right of choosing; electoral.

   The independent use of their elective franchise.
   --Bancroft.

3. Bestowed or passing by election; as, an elective office.

   Kings of Rome were at first elective; . . . for such are the conditions of an elective kingdom.
   --Dryden.

4. Dependent on choice; that can be refused; as, an elective college course. Opposite of required or mandatory.

   Elective affinity or Elective attraction (Chem.), a tendency to unite with certain things; chemism.