Abstract Existing instructional materials for chemistry offer a huge range of different external representations that can be used by chemistry teachers to support students’ understanding of chemical concepts like the concept structure of matter. In science, different kinds of representations are usually combined forming multiple external representations. Examples are combinations of texts, pictures, figures, diagrams, graphs, tables, schemes etc. However, these multiple external representations often have problematic features and/or do not meet students’ subject-related learning needs. For example, many external representations do not take different representational levels into account and/or mix information on the macroscopic level with those from the submicroscopic level. Such representations have the potential to favor students’ misconceptions who often struggle with separating different representational levels. Therefore, it is important to highlight crucial characteristics of external representations that potentially facilitate students’ learning of chemical concepts at lower secondary schools (age group 10–14). When chemistry teachers consider and reflect crucial characteristics of representations and adapt existing external representations or develop new ones, these new representations can become powerful cognitive tools helping to make instruction in chemistry more effective and coherent. This article answers the question What makes representations good representations in science education? by describing features of effective learning with decisive characteristics of multiple external representations and highlighting these characteristics by means of concrete examples from chemistry learning. Finally, an online tool will be outlined that can help teachers to improve multiple external representations for use in chemistry classes.