Lewis Dot Structures • Lewis structures show presence of all valence electrons in a compound. • If ionic, metallic or covalently bonded compounds may all be represented. • This unit concentrates on covalently bonded molecular compounds but ionic compound representation is required. • Ionic transfer of v e- • Metallic free v e- Another ionic compound One e- from Ca atom may be given to each fluorine atom to satisfy octet rule Lewis Structure for CaF2 Your Turn • Sodium Carbonate • Magnesium Chloride • Covalent compounds and polyatomic ions can be described by • Molecular formula: kind/types numbers of atoms but give no information of bonds connectivity • Structural formula: depicts arrangement of atoms in space But gives no information regarding arrangement of valence electrons Lewis Structures Does … • Shows valence e- arrangement • Lewis structures written for molecules that obey Octet Rule! • (exceptions: deficiency, extended and odd#) Lewis Dot Diagram Rule 1: • • • • Count valence e- of each atom Add totals for all atoms Make adjustment for charge if any (-1 charge add e+2 charge 2 less e-) Rule 2: • Arrange atoms in the diagram as follows: • Choose least EN atom as central atom- usually atom closest to left side of PT or largest • Arrange other atoms around central atom (CA) • Hydrogen can only form one bond- never CA • Carbon has 4 single bonds except in CO and CN (Has 3 bonds and unshared v e- pair) Rule 2 continued… Arrange other atoms around central atom (CA) Hydrogen can only have one bond, why? Carbon has 4 single bonds except CO and CN-1 Your Turn… Calculate val e- and organize CA and outside atoms in the following: Carbon dioxide Ammonia Carbon tetrachloride Methane Carbon ion Ammonium ion Rule 3 • Place vale e- around CA as follow: • Connect outer atoms to CA with single covalent bond- each bond takes 2 e• Calculate # of remaining val e• Fill octets of outer atoms • Place any remaining e- on CA Rule 4 • Check CA if Octet Rule satisfied • IF NOT- make multiple bonds double or triple • (this done by moving unshared e- pair on outer atom to CA • ONLY FEW ELEMENTS CAPABLE OF FORMING MULTIPLE BONDS: C N O P S • AND RARE-CHLORINE Your Turn.. Return to these diagrams Add val eMake Lewis Structures for each Carbon dioxide Ammonia Carbon tetrachloride Methane Carbon ion Ammonium ion EXCEPTIONS TO OCTET RULE?