Characterizing how students construct causal mechanistic explanations for chemical phenomena can provide us with important insights into the ways that students develop understanding of chemistry concepts. Here, we present two qualitative studies of undergraduate general chemistry students' reasoning about the causes of London dispersion forces in nonpolar species such as helium atoms. In the first study, we used semi-structured interviews to examine students' verbal explanations for how and why electrical interactions arise between helium atoms. In the second, we used an online short-answer version of the interview task to examine the prevalence of the drawing and explanation types we observed in student interviews. We present a characterization of students' explanations and drawings in terms of increasing sophistication and demonstrate how it may be used as a model for assessing students' ability to engage in scientific practices such as explanation.