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.
Today, the periodic table organizes the elements in horizontal rows, or periods, by order of increasing atomic number, which equals the number of protons in the atomic nucleus of each element. The elements are also organized in vertical columns, or groups, based on similar physical characteristics and chemical behavior. This arrangement developed side by side with atomic theory over about 200 years, and it continues to evolve as new elements are discovered.