Water and Alcohol Molecular Size
Name: Chong S.
Date: Thursday, August 22, 2002
Is a water molecule larger than that of a alcohol
molecule, say for example ethanol?
No. Water (H2O) is one oxygen atom with two hydrogen atoms bound to it.
Ethanol (CH3CH20H) is much larger since the -OH group alone is nearly the
size of one water molecule. Ethanol is larger than water by roughly the
size of the ethyl group ( -CH2CH3 ).
The "size" of a molecule is not unique, that is, the bond lengths and bond
angles of the same bond type vary somewhat for different molecules. For H2O,
the H-O-H bond angle is about 104.5 degrees and the H--O bond distance is
about 1.0 Angstroms (100 picometers = 100pm). METHYL ALCOHOL is smaller than
ethyl alcohol. For METHYL ALCOHOL the structural parameters are: R(C--H) =
109 pm; R(C--O) = 1.43 pm; R(O--H) = 0.94. The bond angles are (H--C--H) =
108.6; (C--O--H) = 108.5. These are very nearly the same values for ethyl
alcohol with the additional angle (C--C--C) = 109.5 and the bond length is
about R(C--C) = 154 pm. You could do the geometry and calculate the overall
size of the molecule by the vector addition of the (C--H),
(C--C), (C--O), and (O--H) bonds to estimate the overall size. Even without
doing so it is clear that the "size" of the ethanol molecule is
significantly larger than the "size" of a water molecule.
No, all alcohols have molecules larger than water. Alcohols basically are
water with one of the hydrogen atoms (the smallest atom there is) replaced
by an organic group. As molecules go, water is VERY small.
Richard E. Barrans Jr., Ph.D.
Director of Academic Programs
PG Research Foundation, Darien, Illinois
All alcohol molecules are larger than water molecules. Comparatively
speaking, water has the formula: H-OH. Methyl alcohol (the smallest alcohol)
has the formula: CH3-OH. Ethanol's formula is: CH3-CH2-OH.
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Update: June 2012