This document discusses periodic trends, including how atomic and ionic size, ionization energy, and electron affinity vary across the periodic table. It explains that atomic size increases down a group as more electron shells are added, but increases across a period from left to right as the nuclear charge felt by valence electrons increases. Ionization energy and electron affinity also increase as atomic size decreases, as it is harder to remove or add electrons to smaller atoms. The octet rule is introduced, where main group elements seek to obtain full s and p subshells of eight electrons to achieve stable electronic configurations.