APA stands for American Psychological Association. APA is often called name-and-date style. APA Format is widely used in psychology, business, education, engineering and the social sciences.The latest APA manual is called: Publication Manual of the American Psychological Association, sixth edition.
The elements are arranged in order of their increasing atomic number. Also the columns or groups are arranged by the number of valence electrons that a element has. For example, all the elements in column 1 have 1 valence electron meaning it has 1 electron in its outer shell. Elements in the same column tend to have similar if not identical properties.
Periods are rows across the periodic table.    The old table of elements was arranged by atomic weights. The new  one is arranged by shells and subshells (s, p, d, f...). For  example, you start with the first shell that is an s-subshell (1s).