Describe the structure of the periodic table

The elements in the periodic table are arranged in terms of atomic number. i.e. hydrogen is first as it has 1 proton, then helium with 2, lithium with 3 etc.Elements in the same group have the same number of outer electrons. i.e. fluorine and chlorine are both in group 7 and hence both have 7 outer electronsElements in the same period have their outer electrons in the same energy level. i.e. sodium and magnesium both have their outer electrons in the 3rd energy level