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UC San Diego has trial access to the ACS Guide to Scholarly Communication through the end of 2020.
Why Cite Your Sources
It's important to cite the sources in your writing (also posters, presentations, etc).
It can help you avoid plagiarizing, by distinguishing your own ideas from other works that you are quoting or paraphrasing.
You are demonstrating your knowledge of prior research in the field, where your work fits within the current body of research, and that you are building on that existing research rather than simply repeating them.
It gives readers a map to find other sources related to your paper, for the same reason that databases like Web of Science include the cited sources for every article you find there.
ACS Citation Style
It's difficult to completely copy the citation formatting as seen on the published ACS journal articles:
ACS has technical staff who format references for the accepted articles before they're published. They no longer require submitting authors to use any specific format for their citations, as long as they include the article titles. Until recently, most references in ACS journals omitted the article titles.
The HTML and PDF versions are formatted differently for readability. HTML articles use parenthetical numbering like (1) or (1, 2) for the in-text citations, while the PDFs use superscript numbering like1 or1,2 for the in-text citations. The reference lists are also formatted differently.
For a student paper, the simplest approach would be to use the HTML version, with parenthetical numbering of citations within the text, followed by a numbered references list without indenting. But you should also consult with your instructor if needed.