Water and Alcohol Molecular Size ```Name: Chong S. Status: student Age: 17 Location: N/A Country: N/A Date: Thursday, August 22, 2002 ``` Question: Is a water molecule larger than that of a alcohol molecule, say for example ethanol? Replies: 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 ). Darin Wagner 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. Vince Calder 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 Chong S, 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. Regards, ProfHoff 455 Click here to return to the Chemistry Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs