-CH2-

Methylene \Meth"yl*ene\, n. [F. m['e]thyl[`e]ne, from Gr. ? wine
   + ? wood; -- a word coined to correspond to the name wood
   spirit.] (Chem.)
   A divalent hydrocarbon radical, {-CH2-}, not known in the
   free state, but regarded as an essential residue and
   component of certain derivatives of methane; as, methylene
   bromide, {CH2Br2}; -- formerly called also {methene}.
   [1913 Webster]

   {Methylene blue} (Chem.), an artificial dyestuff consisting
      of a complex sulphur derivative of diphenyl amine; --
      called also {pure blue}.
      [1913 Webster]
    

molecular formula \mo*lec"u*lar form"u*la\, n. (Chem.)
   An expression representing the composition of elements in a
   chemical substance, commonly consisting of a series of
   letters and numbers comprising the atomic symbols of each
   element present in a compound followed by the number of atoms
   of that element present in one molecule of the substance.
   Thus the molecular formula for common alcohol (ethyl alcohol)
   is {C2H6O}, meaning that each molecule contains two carbon
   atoms, six hydrogen atoms, and one oxygen atom. The molecular
   formula may be written to provide some indication of the
   actual structure of the molecule, in which case structural
   units may be written separately. Thus, ethyl alcohol can also
   be written as {CH3.CH2.OH} or {CH3-CH2-OH}, in which the
   period or dash between functional groups indicates a single
   bond between the principle atoms of each group. This formula
   shows that in ethyl alcohol, the carbon of a methyl group
   ({CH3-}) is attached to the carbon of a methylene group
   ({-CH2-}), which is attached to the oxygen of a hydroxyl
   group ({-OH}). A {structural formula} is a graphical
   depiction of the relative positions of atoms in a molecule,
   and may be very complicated.
   [PJC]