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periodic trends electronegativity learning objectives define electronegativity describe how the electronegativity changes within a period describe how the electronegativity changes within a group analyze the importance of electronegativity in determining ...

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                    Periodic Trends: Electronegativity 
                    Learning Objectives 
                         •    Define electronegativity. 
                         •    Describe how the electronegativity changes within a period. 
                         •    Describe how the electronegativity changes within a group. 
                         •    Analyze the importance of electronegativity in determining bond polarity 
                    Periodic trends are specific patterns that are present in the periodic table that illustrate different 
                    aspects of a certain element, including its size and its electronic properties. Major periodic trends 
                    include: electronegativity, ionization energy, electron affinity, atomic radius, melting point, and 
                    metallic character. Periodic trends, arising from the arrangement of the periodic table, provide 
                    chemists with an invaluable tool to quickly predict an element's properties. These trends exist 
                    because of the similar atomic structure of the elements within their respective group families or 
                    periods, and because of the periodic nature of the elements. We have already studied metallic 
                    character,  atomic  radius,  and  ionization  energy.  We  are  now going to have  a closer look  at 
                    electronegativity. 
                    Electronegativity Trends 
                    Electronegativity can be understood as a chemical property describing an atom's ability to attract 
                    and  bind  with  electrons.  Because  electronegativity  is  a  qualitative  property,  there  is  no 
                    standardized  method  for  calculating  electronegativity.  However,  the  most  common  scale  for 
                    quantifying electronegativity is the Pauling scale (Table A2), named after the chemist Linus 
                    Pauling. The numbers assigned by the Pauling scale are dimensionless due to the qualitative nature 
                    of electronegativity. Electronegativity values for each element can be found on certain periodic 
                    tables. An example is provided below. 
                                                                                                                                                     
                                                                                                                                                     
                    Electronegativity measures an atom's tendency to attract and form bonds with electrons. This 
                    property exists due to the electronic configuration of atoms. Most atoms follow the octet rule 
                    (having the valence, or outer, shell comprise of 8 electrons). Because elements on the left side of 
                    the periodic table have less than a half-full valence shell, the energy required to gain electrons is 
                    significantly higher compared with the energy required to lose electrons. As a result, the elements 
                    on the left side of the periodic table generally lose electrons when forming bonds. Conversely, 
                    elements on the right side of the periodic table are more energy-efficient in gaining electrons to 
                    create  a  complete  valence  shell  of  8  electrons.  The  nature  of  electronegativity  is  effectively 
                    described thus: the more inclined an atom is to gain electrons, the more likely that atom will pull 
                    electrons toward itself. 
                         •    From left to right across a period of elements, electronegativity increases.  If the 
                              valence shell of an atom is less than half full, it requires less energy to lose an electron than 
                              to gain one. Conversely, if the valence shell is more than half full, it is easier to pull an 
                              electron into the valence shell than to donate one. 
                         •    From top to bottom down a group, electronegativity decreases. This is because atomic 
                              number increases down a group, and thus there is an increased distance between the valence 
                              electrons and nucleus, or a greater atomic radius. 
                         •    Important exceptions of the above rules include the noble gases, lanthanides, and 
                              actinides. The noble gases possess a complete valence shell and do not usually attract 
                              electrons. The lanthanides and actinides possess more complicated chemistry that does not 
                              generally follow any trends. Therefore, noble gases, lanthanides, and actinides do not have 
                              electronegativity values. 
                         •    As for the transition metals, although they have electronegativity values, there is little 
                              variance among them across the period and up and down a group. This is because their 
                              metallic properties affect their ability to attract electrons as easily as the other elements. 
                    Patterns of electronegativity in the Periodic Table 
                    The distance of the electrons from the nucleus remains relatively constant in a periodic table row, 
                    but not in a periodic table column. The force between two charges is given by Coulomb’s law. 
                                                                              F=kQ1Q2 
                                                                                   r2 
                    In this expression, Q represents a charge, k represents a constant and r is the distance between the 
                                                       2                              2                              2
                    charges. When r = 2, then r = 4. When r = 3, then r  = 9. When r = 4, then r  = 16. It is readily seen 
                    from these numbers that, as the distance between the charges increases, the force decreases very 
                    rapidly. This is called a quadratic change. 
                    The result of this change is that electronegativity increases from bottom to top in a column in the 
                    periodic table even though there are more protons in the elements at the bottom of the column. 
                    Elements at the top of a column have greater electronegativities than elements at the bottom of a 
                    given column. 
                    The overall trend for electronegativity in the periodic table is diagonal from the lower left corner 
                    to the upper right corner. Since the electronegativity of some of the important elements cannot be 
                    determined by these trends (they lie in the wrong diagonal), we have to memorize the following 
                    order of electronegativity for some of these common elements. 
                                                      F > O > Cl > N > Br > I > S > C > H > metals 
                    The most electronegative element is fluorine. If you remember that fact, everything becomes easy, 
                    because electronegativity must always increase towards fluorine in the Periodic Table. 
                    According to these two general trends, the most electronegative element is fluorine, with 3.98 
                    Pauling units.  The least is cesium, at 0.79.  Francium is rated as lower, but as it is a radioactive 
                    element, its reactivity is not typically a consideration.  
                    SUMMARY 
                    Trends 
                                                                                                                                                      
                         •    Electronegativity refers to the ability of a nucleus to attract electrons or to retain electrons 
                              during chemical bonding.  
                         •    The electronegativity of the elements within a period generally increases from left to 
                              right. This is because the nuclear charge is increasing faster than the electron shielding, 
                              so the attraction that the atoms have for the valence electrons increases. 
                         •    The electronegativity of the elements within a group generally decreases from top to 
                              bottom. This is because as you go from top to bottom down a group, the atoms of each 
                              element have an increasing number of energy levels. The electrons in a bond are thus 
                              farther away from the nucleus and are held less tightly. 
                         •    Atoms with low ionization energies have low electronegativities because their nuclei do 
                              not have a strong attraction for electrons. Atoms with high ionization energies have high 
                              electronegativities because the nucleus has a strong attraction for electrons.  
                         •    Although the noble gases possess very high ionization energies, He, Ne and Ar do not 
                              have listed electronegativity values, as they do not bond with other elements. Kr and Xe 
                              do form  
                               
       QUESTIONS: 
       1) What is electronegativity? 
        
        
        
       2) Considering also the periodic trends in atomic radius and ionization energy, explain why 
       fluorine has the highest electronegativity. 
        
        
        
       3) Why are there no values of EN for He, Ne and Ar? 
        
        
        
       4) a. What is the trend in EN across a period (row) from left to right? 
       b. What causes this trend? 
        
        
        
       5) a. What is the trend in EN down a group (column) from top to bottom? 
       b. What causes this trend? 
        
        
        
       6) In each pair, select the element which has the higher electronegativity: 
        
          a.  N and As 
          b.  Mg and Sr 
          c.  Na and S 
          d.  K and Br 
        
        
       7) Which group would generally have the lowest electronegativity? 
        a.  Transition Metals (Groups 3-12) 
        b.  Alkali Metals (Group 1) 
        c.  Noble Gases (Group 18) 
        d.  Alkaline Earth Metals (Group 2) 
        e.  Halogens (Group 17) 
       Justify your response. 
        
        
        
       9) Low electronegativty is considered a property of 
        a.  Metals  
        b.  Nonmetals 
       Justify your response 
        
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