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 byforming 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 atomicnumber. 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 contributenegligible 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 differentfrom 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 theisotope ratio 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 issimply 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 itsgroup or period them you could make predictions about its behaviour. Most periodic tables are color coded so that you can see at a glance which elements share common properties witheach other. Sometimes these clusters of elements are called element groups. Chemists also refer to the columns of the periodic table called element groups. Elements in the same columnor 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 tendto 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 theperiodic 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 ofelements 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 chartof element information is because the arrangement of elements according to periodic properties helps predict properties of unfamiliar or undiscovered elements. You can use the locationof 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 commonproperties 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 thatare just touching. Ionic radius is half the distance between two atomic ions that are barely touching