The internet is prevalent in society today, and user-friendly web-based productivity tools are readily available for developing diagnostic instruments. This study sought to determine the affordances of a web-based diagnostic instrument on ionisation energy (wIEDI) based on the pen-and-paper version, the Ionisation Energy Diagnostic Instrument (IEDI) that was previously developed and reported on. The Google Forms platform was used to develop the wIEDI and it allowed a degree of personalisation such that specific second-tier options are offered in response to the student's choice of answer in the first tier. Students could choose one or more reasons in the second tier or supply their own reasons, and they were asked to indicate their confidence in their choice of answer–reason combinations. The wIEDI was administered to 274 A-level students (257 Grade 11 and 17 Grade 12), and answer–reason combinations indicating alternative conceptions were highlighted only if 5% or more students expressed confidence in them in the third-tier confidence measure. The results showed that all the possible alternative conceptions of ionisation energy reported in the previous study were also identified in the present study. Additional alternative conceptions were indicated as new reasons had to be developed for many items in the wIEDI to ensure that there were sufficient reasons for each first-tier response, and students were allowed to choose more than one reason for their answer. The wIEDI better facilitated responses reflecting the consistency of the use of specific ideas in student thinking and provided direct evidence of students’ possible manifold conceptions and thinking within each question as well as across a range of questions. It also allowed easy collation of the comments students typed in response to the ‘Others’ and ‘I do not know the answer’ options. Thus, the study makes a case for researchers and teachers using such technology in the diagnostic assessment of students.