PERIODIC TRENDS 
   
       The periodic table reflects electron configuration of the elements. 
   The table also reveals trends in the elements’ physical and chemical properties. 
         (You may want to keep your student reference sheets handy) 
   
   
   
   
   
  Atomic Radius: the distance from the center of the nucleus to the outer edge of the electron cloud. 
  Trend across a period: atomic radius generally decreases across a period from left to right. 
  Trend down a group: atomic radius increases down a group due to additional energy levels. 
  Let’s Practice! 
  For each of the following pairs, predict which atom is larger. 
    a. Mg, Sr      d. Ge, Br 
     
    b. Sr, Sn      e. Cr, W 
     
    c. Ge, Sn 
   
   
   
   
   
   
    Ionic Radius: The distance from the center of the nucleus to the outside of the electron cloud After 
    the atom has either gained or lost an electron(s). Positive ions are smaller than the parent atom; 
    negative ions are larger than the parent atom.   
    Trend across a period: Ionic radius decreases as you move from left to right across the table.  
    Negative ions are always larger than positive ions in the same period. 
    Trend down a group:  Ionic radius increases as you move from top to bottom down the table. 
    Let’s Practice! In the following questions, the charges of ions are indicated by the superscript 
    numbers and signs. 
                   
    For each of the following pairs, predict which atom or ion is larger. 
                  2                         
        a. Mg, Mg                     c. Cl , I  
               2                              3
        b. S, S                       d. Na , Al  
    Predict which of the ions, Mg2 or S2, is larger. Explain your prediction. 
     
     
     
    Ionization energy  Energy that is required to pull an electron away from an atom. 
    Atoms with high ionization energies, such as fluorine, oxygen, and chlorine, are found on the right side 
    of the periodic table and are unlikely to form positive ions by losing electrons. Instead, they usually 
    gain electrons, forming negative ions. 
     
    Atoms with low ionization energies, such as sodium, potassium, and strontium, lose electrons easily to 
    form positive ions and are on the left side of the periodic table. 
    Trend across a period: First ionization energies generally increase across a period of elements 
    primarily because the electrons to be removed are successively closer to the nucleus.  
     
    Trend down a group:  First ionization energies decrease moving down through a group of elements 
    because the sizes of the atoms increase and the electrons to be removed are farther from the nucleus.  
     
    Let’s Practice! 
    For each of the following pairs, predict which atom has the higher first ionization energy. 
        a. Mg, Na              b. Cl, I             c. S, O              d. Na, Al 
         
    For each of the following pairs, predict which atom forms a positive ion more easily. 
        a. Be, Ca              b. K, Ca             c. F, I              d. Sr, Sb 
     
     
     
    Electronegativity: the ability of an atom to attract electrons. This is important as the 
    electronegativity (measured with the Pauling Scale) can determine the type of bond formed in a 
    chemical reaction. 
    Trend across a period: electronegativity increases from left to right across a period. 
    Trend down a group: electronegativity decreases from top to bottom down a group. 
    Let’s Practice! 
    For each of the following pairs, predict which atom has the higher electronegativity. 
        a. Mg, Na                     d. Ca, Ba  
         
        b. Na, Al                     e. S, O  
         
        c. Cl, I                      f. Se, Br 
     
     
    The Octet Rule States that atoms will gain, lose, or share valence electrons to fill their outermost 
    energy level aka valence shell. For most elements, that means 8 electrons.  The exceptions are: 
    hydrogen, helium, lithium, and beryllium which need only 2 electrons to achieve noble gas stability. 
    The position of an element on the periodic table predicts the charge and oxidation number for 
    representative elements.  NOTE:  Representative elements are those found in groups 1, 2, and 13 
    through 18 – basically all of the groups not in the transition metals!  Remember! You have the valence 
    electrons for the representative elements in your periodic table families/groups foldable.  
    The relationship between valence electrons and charge of an ion is EXTREMELY IMPORTANT 
    *** We will use this the rest of the year.  
    How Do You Determine Charge? First determine the number of valence electrons from the group no. 
    1. If the element is a metal, the number of valence electrons with a positive sign is the charge. 
                                                                             2+,   2+    2+    2+
           Example:  Group 2 elements have 2 valence electrons.  Ex.  Be        Mg , Ca , Sr , …… 
    2. If the element is a nonmetal, subtract the number of valence electrons from 8, and the sign is 
    negative. 
           Example: Group 17 elements are nonmetals and have 7 valence electrons.  So, 8-7=1  so the 
                                  -         -   -    -
           charge of the ions is 1 .  Ex. F , Cl , Br , ……