1. Valence electron deficiency requires more bonds to achieve stable electron configurations such as noble gas configurations. Large central atoms tend to reduce repulsions between bonding and nonbonding electron pairs . 2. Hydrogen may form only one single bond. 3. A dash represents two bonding electrons. 4. "Expanded valence" beyond eight electrons is possible for central atoms that have empty d orbitals (3rd period and higher). LEWIS ELECTRON STRUCTURES for Molecules and Polyatomic Ions Rule 1 Decide which atoms are bonded together and write atomic skeletons. C Central atoms are often the most deficient in valence electrons and / or are the largest atoms. 1 C Hydrogen atoms (and often halogen atoms) cannot be central. 2 Rule 2 Count all the valence electrons. C Neutral molecules: Sum the valence electrons of each atom within the molecule. C Polyatomic ions: Sum the valence electrons of all atoms and then subtract (for + ions) or add (for - ions) the correct number of electrons to account for the overall charge. Rule
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