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.
Elements in group Ia are Alkali metals; group IIa the alkaline earth metals; group VIIa the halogens; group 0 the Inert gases. The position of the elements on the periodic table is based on electron structure (which gives the similar properties that have already been described). The period of the table (which horizontal row the element is in) corresponds to the principal quantum number of the elements i.e the number of shells.