Footnotes are not just busy work. It's very important that you give credit where credit is due by indicating where you found specific pieces of information. If you don't cite your sources, or if you copy or paraphrase text without attribution, you are implying that you are the creator of that information, and obviously that's not the case most of the time. Plagiarism is one of the most serious breaches of academic ethics, and it can ruin your career.
What you can (and should) cite
It's fine to cite these kinds of literature in your assignments, papers, and lab reports:
Journal articles and reviews
Preprints (e.g. arXiv or ChemRxiv)
Encyclopedias and textbooks
Books and book chapters
Reputable factual databases (such as NIST Chemistry WebBook, DIPPR, etc.)
Reputable government or institutional web sites (such as EPA, NIST, IUPAC, NIH, etc.)
You should generally NOT cite things like this:
Wikipedia articles (or other crowd-sourced sites)
Miscellaneous personal or unofficial web pages, blogs, wikis, etc.
Why not? Because they are not "published" or peer-reviewed, and may not be accurate, reproducible, or safe, and they may not persist over time. There is also a professional bias against such sources. If you are giving a talk or presenting a poster at a conference, and a person asks you where you got a certain piece of information, "I got it from Wikipedia" is definitely the wrong answer!