Efforts to understand changes in teacher curricula following the adoption of reform-based standards, such as the "Next Generation Science Standards" (NGSS) remain incomplete and prior scholarship has identified several topics in the standards (e.g., nuclear chemistry and kinetics) that remain infrequently addressed in teachers' introductory chemistry classes. This study provides an initial insight into how teachers decide what to teach, how they teach it, and why it might be valuable to include in their curriculum. To accomplish this, two teachers' units on nuclear chemistry and kinetics were explored as part of a case study methodology. The research questions sought answers to help understand why some topics found in the standards remain marginalized in many teachers' curricula while other topics receive extensive attention and coverage. Similarly, the study attempted to understand how teachers' curricular knowledge and orientations to the teaching and learning of science influence their curricular decision-making process around the topics of nuclear chemistry and kinetics. Findings suggest that subject-matter knowledge as well as curricular knowledge plays a significant role in shaping how teachers understand a particular topic and what type of knowledge students should be developing. Both participants independently sought learning opportunities (e.g., professional development) to augment their subject-matter knowledge and curricular knowledge around a unit on nuclear chemistry but did not do so for a unit on chemical kinetics. Similarly, individual teachers' orientations to the teaching and learning of science were generally consistent across the topics studied but differed greatly between the two participants. Both teachers also reported a desire to bring chemistry as it relates to the "real world" into their classes, though their understandings of what "real world" means differed significantly as did their subject-matter knowledge about each topic. For the goals underlying standards such as NGSS to be realized, further work must be done to understand barriers to implementation and for targeted professional development to be designed and offered to support those needs. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]