We have carried out an extensive quantum chemical exploration of gas-phase alkali metal cation affinities (AMCAs) of archetypal neutral bases across the periodic system using relativistic density functional theory. One objective of this work is to provide an intrinsically consistent set of values of the 298 K AMCAs of all neutral maingroup-element hydrides XHn of groups 15-18 along the periods 1-6. Our main purpose is to understand these trends in terms of the underlying bonding mechanism using Kohn-Sham molecular orbital theory together with a canonical energy decomposition analysis (EDA). We compare the trends in XHn AMCAs with the trends in XHn proton affinities (PAs). We also examine the differences between the trends in AMCAs of the neutral XHn bases with those in the corresponding anionic XHn-1 - bases. Furthermore, we analyze how the cation affinity of our neutral Lewis bases changes along the group-1 cations H+, Li+, Na+, K+, Rb+, and Cs+.