To understand the mid-summer nocturnal mesospheric K layer over the Arecibo Observatory (18.35¿N, 66.75¿W) a one dimensional model has been developed. Focusing on nights with no sporadic layer activity, we identify possible molecular reservoirs and sinks in the K layer using ab initio calculations to estimate the thermochemistry. Reactions KN2+ + O, KN2+ + CO2, and KHCO3 + H play primary roles in the chemistry of ionic and neutral potassium in the mesospheric cycle. The predominant potassium species is KHCO3 as a result of the influence of odd oxygen/hydrogen chemistry. The study of the metal layer contributes to a better understanding of chemical and dynamic changes that affect atmospheric composition.