CH-01 Rev. 0 Page 12 THE PERIODIC TABLE DOE-HDBK-1015/1-93 Fundamentals of Chemistry THE PERIODIC TABLE All known elements fall into a pattern when placed in a periodic table, and the position in this pattern is decided by the element's atomic number.  This chapter will discuss the significance of this fact. EO 1.4 DEFINE  the following subdivisions of the periodic table: a. Periods of the periodic table b. Groups of the periodic table c. Classes of the periodic table EO 1.5 Given   a   periodic   table,   IDENTIFY   the following subdivisions: a. Periods of the periodic table b. Groups of the periodic table c. Classes of the periodic table EO 1.6 LIST the characteristics that elements in the same group on the periodic table share. EO 1.7 DEFINE the term valence. Periodic Table Over many years of chemical investigation, scientists have discovered a remarkable feature of the elements.  If the elements are arranged in the order of their atomic numbers, the chemical properties of the elements are repeated somewhat regularly.  To a lesser extent, the physical properties are also repeated periodically.  This periodic repetition can be seen in Table 3.  Compare the properties of lithium (Li), sodium (Na), and potassium (K), and also those of beryllium (Be), magnesium (Mg), and calcium (Ca).  In the list of elements shown in Table 3 the properties are repeated every eighth element. TABLE 3 Description of the Properties of the First Twenty Elements Element Symbol Atomic Atomic Description of Properties Number Weight Hydrogen H 1 1.008 Colorless gas, reacts readily with oxygen to form H O; 2 forms HCl with chlorine. Helium He 2 4.003 Colorless gas, very non-reactive chemically.