Encyclopedias are good starting places to find in-depth introductions to science topics. These articles may include historical information, economic aspects, uses or applications, and recent research highlights, with a bibliography or list of references to the primary literature. We have science encyclopedias, chemistry encyclopedias, and more specialized reference works that cover energy, atmospheric sciences, nanotechnology, inorganic chemistry and more.
With each of these titles, you can browse the articles by title or topic, or search the full text of the entire work.
THIS IS NOT A COMPREHENSIVE LIST. There may be more specialized encyclopedias in our collection (including this longer list in the Chemistry Research Guide), as well as books that cover your topic broadly enough to provide you with background information.
Wikipedia is a good starting point to find out "what is [something]?" But citing Wikipedia is often discouraged by instructors, and there are some good reasons. Unlike the articles in our science encyclopedias, it may be difficult to identify who's editing a Wikipedia article--and subsequently their authority or expertise on that topic. Bias is also something to be aware of for topics with very "pro-" and "anti-" stances. Here's an example of such gatekeeping.
However, the more developed Wikipedia articles often have good bibliographies and lists of additional resources, including books and scholarly articles. These may be worth a look.