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.
from wikipedia the free encyclopedia the periodic table of the chemical elements is a tabular display of the known chemical elements the elements are arranged by electron structure so that many chemical properties vary regularly across the tableeach element is listed by its atomic number and chemical symbol group is a vertical column in the periodic table of the elements there are 18 groups in the standard periodic table elements in a group have similar configurations of their valence shell electrons which gives them similar properties