Library books are arranged by the Dewey Decimal classification scheme whereby each subject is represented by a number.
You wil find books on the same subject will have the same number. The shelfmark is preceded by the letter D and will be followed by 3 letters, which are usually the first 3 letters of the author's name.
Here are some useful shelfmarks.
D 541 Physical & theoretical chemistry
D 546 Inorganic chemistry
D 620.1129 Nanoscience
D 620.1199 Nanotechnology
D 660 Chemical engineering