Previous research has shown that students' existing conceptions are critical to subsequent learning because there is interaction between the new knowledge that the students encounter and their existing knowledge from previous lessons. Taber (1999a) found A-level students in the UK had difficulty in understanding the principles determining the magnitude of ionization energy because of their pre-existing octet rule framework. In a related study, Tan et al. (2005) explored the conceptions of ionization energy of A-level students in Singapore, and found that these students also applied the octet rule framework to decide if an isolated sodium ion would recombine with an electron to reform the sodium atom, and whether another electron could be removed from the sodium +1 (Na[superscript +]) ion. In addition, the study found that there appeared to be an offshoot of the octet rule framework, he "stable fully-filled and half-filled sub-shell" thinking, which the students in Singapore used to explain the trend of ionization energies across Period 3. (Contains 5 tables.)