The integer number above the element symbol is its atomic number. The atomic number is the number of protons in every atom of that element. The number of electrons can change by forming ions or the number of neutrons can change by forming isotopes. But the proton number defines the element. The modern periodic table orders the element by increasing atomic number. The number below the element symbol is called the atomic mass or the atomic weight. It is the sum of the mass of protons and neutrons in an atom as the electrons contribute negligible mass. But you may notice it is not the value you had get if you assumed the atom had an equal number of protons and neutrons. The atomic weight values may be different from one periodic table to another because it is a calculated number based on the weighted average of the natural isotopes of an element. If a new supply of an element is discovered the isotope ration may be different from what scientists previously believed. In that case the number may change. If you have a sample of a pure isotope of an element the atomic mass is simply the sum of the number of protons and neutrons of that isotope. The periodic table gets its name because it arranges the elements according to recurring or periodic properties. The groups and periods of the table organize elements according to these trends. Even if you did not know anything about an element but you knew about one of the other elements in its group or period them you could make predictions about its behaviour. Most periodic tables are colour coded so that you can see at a glance which elements share common with each other. Sometimes these clusters of elements are called elements groups. Chemists also refer to the columns of the periodic table called element group. Elements in the same column or group have the same electron shell structure and the same number of valence electrons. Since these are the electrons that participate in chemical reactions the elements in a group tend to react similarly. The Roman numerals listed across the top of the periodic table indicate the usual number of valence electrons for an atom of an element listed below it. The rows of the periodic table are called periods. Atoms of elements in the same period have the same highest unexcited electron energy level. As you move down the periodic table the number of elements in each group increases because there are more electron energy sublevels per level. The reason chemists and other scientists use the periodic table rather than some other chart of element information is because the arrangement of elements according to periodic properties helps predict properties of unfamiliar or undiscovered elements. You can use the location of an element on the periodic table to predict the types of chemical reactions it will participate in and whether it will form chemical bonds with other elements. In addition to the common properties of elements in groups and periods the chart also organizes elements according to trends in ionic or atomic radius. Atomic radius is half the distance between two atoms that are just touching. Ionic radius is half the distance between two atomic ions that are barely touching.