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It is a chemical term used to denote the hybridization of an atom. It is an initialism of "s orbital and p orbital" to get "sp".
"sp" starts with a consonant letter so usually, we use the article "a" in front of it, but when pronounced it sounds like "ES-PEE" giving the vowel sound "E" and hence the article "an" can be used. Which article is most appropriate for sp?
My textbook uses both "a" and "an" interchangeably:

(c) Benzylic halides
These are the compounds in which the halogen atom is bonded to an sp³-hybridised carbon atom next to an aromatic ring

(a) Vinylic halides
These are the compounds in which the halogen atom is bonded to an sp²-hybridised carbon atom of a carbon-carbon double bond (C = C). (edit: the printed textbook has an "a" instead of the "an" given above)

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  • Your book is consistent. It always uses "an".
    – James K
    Mar 4, 2023 at 8:14
  • Hmm yes, that must be a printing error in my book then, the online version is right. "an" it is then. Mar 4, 2023 at 8:24

2 Answers 2

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It should be "an sp³ atom". The use of "an" is completely dependent on the sound of the next word as spoken.

The two examples are correct, they both use "an". Using "a" would be a mistake, but a minor one that a native speaker might easily make (in writing but not in speech).

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The form "an" exists to aid pronunciation, as having two vowels sounds together is awkward. So it whether to use it goes by pronunciation, not spelling. For instance, whether "an herb" or "a herb" is correct depends on one's dialect. It's "an hour", even though "hour" starts with a consonant letter, and "a union", even though "union" starts with a vowel letter.

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