Unit 15 Molecular Compounds Lewis Structures for Molecular Compounds and Ions Common Polyatomic Ions Nomenclature for Molecular Compounds

Lewis Structures for Molecular Compounds and Ions (4.6, 4.10) Nonmetals – those to the right of the stair-step line can react with each other by sharing electrons The driving force is similar to that for ionic compounds – the attainment of an octet of electrons as summarized in the Octet Rule In molecular compounds, though, the atoms do not form individual ions, they share electrons throughout their structure In this unit, we will look at drawing Lewis structures representing these molecular compounds as well as other aspects of these shared electron systems

Key Aspects of Lewis Structures (4.10) Key elements of a Lewis structure for a molecular compound include: The number of valence electrons indicated equals the total number of valence electrons in the compound or ion Electrons are shared in such a way that each atom is effectively surrounded by eight electrons (two for hydrogen, other exceptions exist). All or some of the electrons around an atom may be shared with other atoms in the structure

An Approach to Drawing Lewis Structures (4.10) Several approaches exist for drawing Lewis structures – this is just one you might consider: Draw skeletal structure for the atoms. Things that might be helpful are hydrogen can only take one bond so it has to be on the outside; carbon typically has four bonds, nitrogen has three bonds, and oxygen has two bonds; polyatomic molecules and ions typically have a central atom surrounded by other atoms. Connect each atom to its neighbor(s) with a single bond which consists of two electrons. The bond may be characterized by two electrons (:) or a single line (—) attaching the two atoms. Place electrons around the outside atoms so each has eight total electrons including all of those bonded to it as well as the electrons not bonded. (Continued on Next Slide)

An Approach to Drawing Lewis Structures Continued (4.10) Count the number of valence electrons the structure should have. This is the sum of the valence electrons on all atoms. For ions, the number of valence electrons is increased by the charge and for negative ions and reduced by the charge for positive ions. Count the number of valence electrons in your structure. If the valence electron counts match from steps 4 and 5 you are done. If there are too many valence electrons in your structure, take away a nonbonded pair and slide a nonbonded pair from an adjacent atom to make a double chemical bond. Continue this process until the number of valence electrons in the structure matches the number that should be present.

Examples of Drawing Lewis Structures (4.10)

Examples of Drawing Lewis Structures (4.10)

Examples of Drawing Lewis Structures (4.10)