Operational Skills Using Lewis symbols to represent ionic bond formation. Writing electron configurations of ions. Using periodic trends to obtain relative ionic radii. Using electronegativities to obtain relative bond polarity. Writing Lewis formulas. Writing resonance structures. Using formal charges to determine the best Lewis formula. Relating bond order and bond length. 2

Ionic and Covalent Bonding

Electronegativities of the Elements

Electronegativities of the Elements How are these used? Subtract the electronegativities (Nonmetal - metal) or (anion - cation) Electronegativity of 1.7 or greater is an ionic bond (table S) Eg. Na + Cl NaCl electroneg- (3.2-0.9=2.3) 0.9 3.2 ionic

Ionic Bonds Noble gas configurations and ions with noble gas configurations are particularly stable. When an atom loses an electron it becomes a cation (positive). Na (Na+) - 1e- When an atom gains an electron it becomes an anion (negative). Cl (Cl+) + 1e- 2

Electron Configurations of Ions As metals lose electrons to form cations and establish a “noble gas” configuration, the electrons are lost from the valence shell first. Mg Ne + 1e- ] 2

Ionic Radii The ionic radius is a measure of the size of the spherical region around the nucleus of an ion within which the electrons are most likely to be found. Ionic radii increase down any column because of the addition of electron shells. In general, across any period the cations decrease in radius. 2

Ion Radius in the Lithium Iodide (Lil) Crystal

Comparison of Atomic and Ionic Radii

Lewis Structures 1 Draw “skeleton” structure The most electronegative atoms will be the terminal atoms The least electronegative will be the central atom H is always terminal 2. Calculate the number of electrons in the valence shell of the molecule or ion. The number of valence electrons for each atom is the group number (i.e. N = 5, O = 6, H = 1). Adjust for charge of ions (for +ve charge, remove electrons; for -ve charge, add electrons) 3. Distribute the electrons such that each atom has a share of 8 i) Give to the terminal atoms first (most electronegative) ii) Give 8 to the central atom iii) If the central atom has less than 8, use double (or triple) bonds iv) If there are excess electrons at this point, put them on the central atom v) H atoms never from double bonds