Structures_Fakhreddine - Lewis Dot Structures for Molecules...

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Lewis Dot Structures for Molecules The goal is to sketch a 2-D visual representation of a chemical. Multiple methods exist. Guidelines: Begin from the chemical formula and use elemental symbols. Determine the central atom o Often, the least electronegative element o Or, the element with the fewest atoms Arrange the remaining atoms around the central atom, as symmetrically as possible. Many elements follow the octet rule . Each dot represents a valence electron. A minimum of two dots are needed to attach two atoms in a bond. (These are called bonding electrons.) Multiple bonds are possible: four shared electrons form a double bond, six a triple bond. (Use S=N-A to test.) Lone pairs of electrons are possible. (These are called nonbonding or lone electrons. They are not involved in bonds, but are part of the molecule.) Electrons prefer to be paired. Unpaired electrons are reserved for radicals (aka “free radicals”) . Octet Rule: Most of the representative elements achieve stability by obtaining a valence of 8 electrons by sharing, giving, or acquiring electrons. For molecules, we focus on the sharing of electrons. When an atom achieves a valence of 8, we say that it has a “filled octet”. Example: Draw the Lewis structure of HF. Example: What is the simplest compound to form from the combination of nitrogen and fluorine? All atoms in the molecule follow the octet rule. Example: What is the simplest compound to form from the combination of carbon and fluorine? All atoms in the molecule follow the octet rule. Exceptions to the octet rule: H Be B Row 3 and below Radicals
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Example: Draw the Lewis dot structure for CH 2 Cl 2 . S=N-A A test to check if multiple bonds exist in the molecule. S = number of shared electrons (number of electrons involved in bonds) N = number of bonding electrons needed in structure A = number of available electrons Notes: For each H, the N = 2. For Be, A=4. For B or Al, A=6. For other elements, N = 8 per atom. The A must be adjusted for ions. Add to A for negative ions; subtract for positive ions. To determine number of bonds, use S/2. The equation S=N-A will give unbelievable results for molecules using the expanded octet, because N is based on the octet rule. For this reason, S=N-A can be employed as a check for expanded octet. (Example later) Example: CO 2 U = A – S Calculate the number of unshared (nonbonding) electrons in the molecule.
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