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.
Two other predicted elements were later discovered, lending further credit to Mendeleev's table. Modern day periodic tables are expanded beyond Mendeleev's initial 63 elements. Most of the current periodic tables include 108 or 109 elements. It is also important to notice how the modern periodic table is arranged. Although we have retained the format of rows and columns, which reflects a natural order, the rows of today's tables show elements in the order of Mendeleev's columns.