From Chemguide:

The most likely example of geometric isomerism you will meet at an introductory level is but-2-ene. In one case, the CH3 groups are on opposite sides of the double bond, and in the other case they are on the same side.

Would it seem strange to a native speaker of English if I used the there:

The most likely example of geometric isomerism you will meet at an introductory level is but-2-ene. In one case, the CH3 groups are on the opposite sides of the double bond, and in the other case they are on the same side.

.. or would it still be okay?

According to this Ngram, the use of "on the opposite sides of" has been on a decline relative to "on opposite sides of":

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An example of "on the opposite sides":

Even at present, by comparing the differences of the inhabitants of the sea on the opposite sides of a continent, and the nature of the various inhabitants of that continent in relation to their apparent means of immigration, some light can be thrown on ancient geography. (Darwin, The Origin of Species, Chapter 14)

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    I'd use 'the'... but not sure whether it's a non-native way! – Maulik V Mar 9 '16 at 6:59
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    If you remove the "s" in "sides" you will see that with "the" is more common. And in the dictionary, it agrees with the results. If "side" used in the plural form, without the article is more common. – Ghaith Alrestom Mar 9 '16 at 7:22
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    It depends on the following noun phrase. Conservatives and Liberals are usually on opposite sides of an issue. On such matters, we tend to be on opposite ends of the spectrum. There, we would not be likely to use "the". But here we would: Draw a line to connect the opposite sides of the parallelogram. The article is used, I think, when there is felt a need to distinguish opposite from non-opposite. And it is avoided when the noun phrase is so general that "the" would refer too specifically. "The opposite sides of an issue" is a little incongruous. – Tᴚoɯɐuo Mar 9 '16 at 13:08
  • @TRomano - so "double bond" is a vague enough thing not to admit "the" in "(the) opposite sides of the double bond"? Or is it somewhere in between "an issue" and "the parallelogram"? – CowperKettle Mar 9 '16 at 13:15
  • There, I believe there is felt no need to distinguish opposite from non-opposite. What would the non-opposite sides of the double-bond be? – Tᴚoɯɐuo Mar 9 '16 at 13:17

The two lovers lived on opposite sides of the U.S.

Here what we have is vague. It is not clear where they live, we only know that they live far apart from each other, and this is the focus in this sentence, the distance, and not the location.

The two lovers lived on the opposite sides of the U.S.

In this sentence the focus changes. Now we're interested in where they live (even though it is not yet clear!), making it a more definite and clear construction.

In your case you need to verify what you want to focus (I'm no good at chemistry so I cannot help you with this pal), but if you want to emphasize in the distance of the CH3 groups you omit the "the". If you want to emphasize the location of the CH3 groups, you write "the".

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  • "In your case you need to verify what do you want to focus" - we don't use the inverted word order here ("what do you want"). So, "you need to verify what you want to focus". (The Penthouse Principle) Or "You need to be sure of what you want to focus". – CowperKettle Mar 15 '16 at 6:44

You use the definite article before an adjective to use the adjective in a qualifying sense. ie to specify which item(s) out of a larger set you are talking about.

Without the definite article, the adjective is used in a descriptive sense.

If you are talking about all of the items, you can use the adjective in either a qualifying or a descriptive sense- ie with our without the the.

The double bond has two sides: if you are talking about both of them then "opposite sides" or "the opposite sides" are valid. If you want to talk about just one side, you must add the definite article "the opposite side".

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  • I've added a quotation from Darwin. He clearly talks about both sides of a continent, and uses the. – CowperKettle Mar 9 '16 at 7:42
  • Understood. I did not say that it's wrong to use "the", I said that it is not necessary. – JavaLatte Mar 9 '16 at 8:07
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    ... .. but I wonder whether he would have said "on the opposite sides of the continent"... or "on opposite sides of the continent". A continent implies more continents, more sides, so he is talking about two among many... – JavaLatte Mar 9 '16 at 8:22
  • Ah, so to you as a native speaker the definite article does not seem out of place in my example sentence. Thanks - that was my key question. (0: And thanks for the point about the article before "continent", I'll look into it. – CowperKettle Mar 9 '16 at 8:38

Gramatically, both "opposite sides" and "the opposite sides" are correct, but may have a slightly different meaning depending on context.

Taking the chemistry context into account, it is appropriate in this case to use "the opposite sides", since with a double bond there are only two sides on which the ch3 groups can be placed.

Contrast this with Two opposite sides of a trapezoid are parallel - in this case there are two pairs of opposite sides, so using "the" leaves ambiguity in what pair is reffered to (and in fact only one pair is parallel!). Consequently, adding "the" to this sentence would be incorrect (though gramatically it has the same structure).

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Obviously, and the question understands this, both structures are identical in meaning (and are both correct). Whether one says "on opposite sides", or "on the opposite sides", the listener will understand the meaning. However, "on the opposite sides" is more verbose, and has gone out of favor in common usage.

This is not a matter of correctness, it is a matter of ease of understandability. As a rule of thumb, one should apply the usage with the fewest and simplest words, for the broadest understandability among the audience. Accordingly, "the opposite sides" has fallen into disuse, in favor of "opposite sides".

There is one caveat here. The original question used a chemistry specific usage as an example. It is common to see field-specific usage that differs from normal English usage. Most careers use a career-specific language differentiation technique to isolate "insiders" from "outsiders". (E.G. Medical professionals use "apoptosis" when writing papers, rather than the more commonly understandable "cell death".) This use of "the" could be such a usage.

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