Your instructor can tell you which method to use. The CBE style offers several methods of citing your references. This handout illustrates the Citation-Sequence method. As each source is mentioned in the text, it is numbered in sequence. Page numbers are not added .
Ozone plays an important role in photochemical smog and in the production of acid rain.
If a source is used again later, the original number is reused. On the other hand, atmospheric ozone absorbs ultraviolet radiation with wavelengths between 240 and 320 nanometers which is lethal to simple unicellular organisms and the surface cells of higher plants and animals. The complete references are listed at the end of the paper in the order they were numbered in your paper. The arrangement of references in your list depends upon how you have cited them in your text. In the Citation-Sequence system, each reference appears in the order that it is first mentioned in your paper.