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Bonding Recitation - |CHEM1310|Fall2009 The Formation of...

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Georgia Institute of Technology | CHEM 1310 | Fall 2009 The Formation of the Chemical Bond Recitation Information and Worksheet The two most important types of bonds for this course are ionic and covalent bonds. We have previously detailed the mechanism in which positively charged cations and negatively charged anions form from the loss (ionization) or gain (electron affinity) of electrons. Energy is the most important consideration with regard to formation of chemical bonds. When observing metals, electrons are generally lost because it takes less energy for electrons to be transferred to other atoms when forming chemical bonds. The less energy required the more spontaneous the chemical process. Because metals transfer ions to other atoms (generally nonmetals which have high electron affinities), they form cations with positive charges and the counter species in the bond has a negative charge. Background Coulomb’s Law is the basis for the formation of ionic bonds. Ionic bonds are held together by electrostatic interactions resulting from the two unlike charges being close to one another. The formula for Coulomb’s Law is shown in equation (1). In general, the greater the charges the stronger the ionic bond. Such a generalization can be readily made by observing that the force (V) in the expression is directly proportional to the magnitude of the respective charges. Ions e Between th Distance r Constant Charge Q Bond) Chemical the of (Strength Force (1) 4 2 1 ! ! ! ! " # $# V r Q Q V Covalent bonds form as a result of electron-electron sharing. The Leonard-Jones curve shown below can be used to describe the formation of covalent bonds. Energy Separation of Nuclei Attraction Bonding Repulsion
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Georgia Institute of Technology | CHEM 1310 | Fall 2009 When covalents bond form, they form because of electron sharing; however, the electrons are not necessarily equally shared. This arises because some atoms have a greater attraction (or pull) for electrons than others. This attraction is called electronegativity and Linus Pauling developed a quantitative scale for characterizing electronegativity. With the electronegativity scale, Francium (Fr) has the lowest value (the least attraction) with a value of 0.7 while fluorine (F) has the greatest value of 4.0. Electronegativity increases up a family and across a row on the periodic table. The Pauling scale is shown in Figure 1 below. Figure 1: The Pauling Scale The electronegativity values are commonly used to describe the extent to which a covalent bond forms. Because the electrons are not equally shared in most covalent systems, they are said to have a certain degree of ionic character. The only systems that have a true covalent bond (with equal sharing) are the diatomic molecules: hydrogen, oxygen, nitrogen, fluorine, chlorine, bromine, and iodine. Other systems have covalent bonds, but they have an unequal sharing of electrons. Covalent bonds with an unequal sharing of electrons are said to have polar bonds. We will discuss more about polarity when describing dipole moments observed with select molecules. Before discussing dipole moments, it is first
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