Chapter 8- Drawing Lewis Structures for Covalent Compounds Rules for writing lewis structures: NASL method All atoms need to follow the octet rule; except hydrogen, helium, and boron. Steps for the NASL method 1. Arrange atoms so that the least electronegative atom is in the center 2. Calculate as the sum of electrons needed for all atomsby the octet rule. N (Needed) Remember the exceptions: H=2, Be=4, B=6 3. Calculate A (Available) as the sum of all valence electrons. For negatively charged ions, add the number of electrons equal to the charge of the anion to A. For positively charged ions, subtract thenumber of electrons equal to the charge of the cation. 4. Calculate as the difference between S (Shared) N A – 5. Divide S by 2 to obtain the number of bonds to be extended from the central atom 6. Calculate L (Lone-pair electrons or simply as the difference between A S. “dots”) – Place the L dots into the skeleton as to fill the octet of every element, except “A-group” hydrogen. Remember that hydrogen has only one bond and NO dots 7. Check that the number of electrons used is equal to the electrons available Charge on individual atoms = Valence of atom Number of Bonds extending from atom– dots – on atom. All atoms in CF4 are neutral. Carbon = 4 4 0 = 0 (neutral) – – Fluorine = 7 1 6 = 0 (neutral) – – On a separate piece of paper, write the Lewis structures for the following: H2O d. CO3-2 g. N2 j. N2O SO2 e. h. SOCl2 k. O2 CO2 f. NH4+ i. CS2 l. BeCl2
Chapter 8- Drawing Lewis Structures for Covalent Compounds Rules for writing lewis structures: NASL method All atoms need to follow the octet rule; except hydrogen, helium, and boron. Steps for the NASL method 1. Arrange atoms so that the least electronegative atom is in the center 2. Calculate N (Needed) as the sum of electrons needed for all atomsby the octet rule. Remember the exceptions: H=2, Be=4, B=6 3. Calculate A (Available) as the sum of all valence electrons. For negatively charged ions, add the number of electrons equal to the charge of the anion to A. For positively charged ions, subtract thenumber of electrons equal to the charge of the cation. 4. Calculate S (Shared) as the difference between N – A 5. Divide S by 2 to obtain the number of bonds to be extended from the central atom 6. Calculate L (Lone-pair electrons or simply “dots”) as the difference between A – S. Place the L dots into the skeleton as to fill the octet of every “A-group” element, except hydrogen. Remember that hydrogen has only one bond and NO dots 7. Check that the number of electrons used is equal to the electrons available Charge on individual atoms = Valence of atom – Number of Bonds extending from atom– dots on atom. All atoms in CF4 are neutral. Carbon = 4 – 4 – 0 = 0 (neutral) Fluorine = 7 – 1 – 6 = 0 (neutral) On a separate piece of paper, write the Lewis structures for the following: H2O d. CO3-2 g. N2 j. N2O SO2 e. h. SOCl2 k. O2 CO2 f. NH4+ i. CS2 l. BeCl2
nasl_method.docx