In science writing, journal titles are frequently abbreviated in citations. If the style you are using abbreviates journal titles, you need to use an official abbreviation.
To find official abbreviations of journal titles, look in a major database which covers that area of science, like GeoRef or MathSciNet. Additionally, some of the professional society web sites have lists or PDFs of standard journal abbreviations (e.g. Mathematical Reviews' Abbreviations of Names of Serials or PubMed's Journals Reference in the NCBI Databases or the American Chemical Society's CAPlus Core Journal Coverage List). The library also has some older print guides like Periodical Title Abbreviations which may help identify appropriate abbreviations.
If you are using citation management software (EndNote, Mendeley, etc.), the software may abbreviate titles for you in the correct format for a particular style.