October 1, 2016 Chapter 2: Structure and Bonding 2.1 Ionic and Covalent Bonding Valence Electrons and Symbols: only valence electrons participate in bonding. When drawing chemical structures, it is only necessary to consider the valence electrons, which are normally indicated by dots around an atom. Non bonding electrons are considered fully owned by the atom, dots, and the bonded electrons shared by the atoms are represented by a line. Ionic Bonding: when the difference in electronegativity between 2 bonded atoms is greater than 1.9, it is assumed that there is a complete transfer of an electron from the atom of lower electronegativity to the atom with higher electronegativity, and the bond is classified as ionic. This usually occurs between metal and a nonmetal. Crystal Lattice: The attractive forces in crystal lattices are exceptionally strong and account for the very high melting points of ionic compounds. The attractive forces between the anions and cations result in the formation of crystal lattices in the solid state. These attractive forces are known as ionic bonds. The magnitude of the attractive force between anions and cations is determined by Coulombs law: 2 Force= k Q1(Q2) r Where k= is a proportionality constant known as the Coulombs law constant, Q1 and Q2 refer to the value of the charges on the ions, and r is the difference between the ions. Covalent Bonding: involve the sharing of electrons between 2 nonmetals. The sharing is deemed to be unequal if the difference in electronegativity between 2 atoms is less than about 1.9, but greater than 0.5. This is called a polar covalent bond, the uneven distribution of charge due to the electronegativity difference between the 2 atoms gives rise to a dipole movement. Nonpolar covalent bonds involve atoms with electronegativity difference of less that 0.5, the sharing of electrons is approximately equal, therefore there are no permanent dipole movements. Lewis Structures: are representations of a molecule that show covalent bonds, paired and unpaired electrons, and any charges that may be present on the atoms. 1. Count the total number of valence electrons. 2. Draw a skeleton structure, joining atoms by a single bond. 3. Count how many single bonds are present, and calculate how many valence electrons are in these bonds. 4. The remaining valance electrons must be distributed in the structure. Try to provide all the atoms, except H, with a full octet. a. If there are any electrons left, they are placed on the central atom. The elements in the 3 row and higher often have more than 8 valence electrons. b. If there are not enough electrons to provide an octet for all the atoms, distribute the electrons to the most electronegative atoms first.