Elements are organized first by their atomic number. The number on top of each element is the atomic number which identifies each element, and increases from there. It is also further organized by groups and periods. Groups are vertical columns, and elements of each group tend to share similar properties. For example, group 2A is the alkaline earth metals, these elements all tend to react with other elements similarily and share other properties as well because they tend to have the same number of valence electrons, with certain rules and exceptions particularly for the groups in the center of the periodic table Periods are the horizontal rows. The period number indicates the number of energy levels. So all the elements in period 4 have four energy levels.
