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Comprehensive Guide to Drawing Lewis Structures

This guide offers a detailed procedure for drawing Lewis structures, emphasizing the counting of valence electrons (VSE) based on the periodic table and molecular charge adjustments. It outlines key steps, including constructing molecular frameworks with single bonds, ensuring correct octet configurations, and addressing exceptions for specific elements like hydrogen and boron. Learn to manage lone pairs and create double or triple bonds as necessary. The guide also includes practical exercises for various compounds, reinforcing the concept of expanded octets and correct electron pair counts in molecular drawings.

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Comprehensive Guide to Drawing Lewis Structures

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  1. Lewis Structures and Molecular Geometry

  2. Drawing Lewis Structures A recommended procedure : • Count the total number of valence electrons (VSE) for the atoms (based on the column of the atom in the periodic table) and charge. • (add one electron for each negative charge; subtract one electron for each positive charge). • HCN = 1+ 4+ 5 -0 = 10 VSE = 5 VSE pairs • C2H6O = 2(4) + 6(1) + 6(1) -0 = 20 VSE = 10 VSE pairs • CO32-= 4 + 3(6) + 2 = 24 VSE = 12 VSE pairs

  3. Drawing Lewis Structures • Draw the framework with single bonds. Some knowledge of the way the atoms are connected may be required. H C N • Complete octets around all the atoms. H C N Exceptions: H only has one pair of electrons, Be only has two and B has only three pairs.

  4. Drawing Lewis Structures • Count the number of pairs of electrons depicted. If this number is less than the total, add electrons to the central atom until the total number of electrons is correct. H C N • There are 7 pairs in the drawing and there should only be five. Unshared pairs are non-bonding electrons

  5. Drawing Lewis Structures • If there are extra pairs of electron in the drawing use lone-pairs (non- bonding pairs) to make double or triple bonds and reduce the total number of electron pairs. H C N  H C N  H C N  H C N • Two unshared pairs = one bonded pair

  6. Drawing Lewis Structures Draw the Lewis structure for PCl5 • VSE = P (5) and Cl (7x5) = 40 e-, or 20 pairs of electrons. • Place P in the center because it is a singleton, and the chlorines around it. • Give each Cl an octet and count the pairs. • P has more than four pairs called an expanded octet.

  7. Phosphorus Pentachloride

  8. Review: • Lewis structures are a 2 dimensional picture of the arrangement of atoms in a molecule. • The total pairs of electrons must be correct in your final drawing. • Each atom must have four pairs (an octet) of electrons, except H, Be and B. • Some atoms may have more than four pairs, like the second row p orbitals.

  9. Lewis Structures Practice Draw Lewis Structure for the following compounds. • CO2 7. NO3- • H2O 8. SO2 • CH4 9. NH3 • BeCl2 10. NH4+ • PO43- • PCl6-

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