Lewis Structure Review
Lewis Structure Review
How To Write Lewis Structures
1. Determine the total number of valence electrons from all of the atoms in the molecule or ion. • Add one electron for each unit of negative charge. • Subtract one electron for each positive charge. 2. Write the correct skeletal structure. • For molecules of the formula ABn, place the least electronegative element in the center, and the more electronegative elements in the terminal positions. • H is always a terminal atom, and NEVER a central atom, since it can only form one bond. • Draw single bonds between pair of bonded atoms.
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Drawing Lewis Structures
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How To Write Lewis Structures
3. Distribute the remaining valence electrons, giving octets to as many atoms as possible. • Place the remaining electrons as lone pairs on the terminal atoms first. • Once all of the terminal atoms have 8 electrons (or 2 for H), place any remaining electrons on the central atom. 4. If any atoms lack an octet, form double or triple bonds as necessary. • This is done by moving lone pairs from the terminal atoms in between the terminal atom and the central atom. • The formal charge can be used as a guideline for placing double bonds.
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Formal Charges
Formal charge = valence e- – (½ bonding e-) – (lone pair e-) • Formal charge is the difference between the number of valence electrons on the free atom and the number of valence electrons assigned to the atom in the molecule. – The sum of the formal charges must equal the charge on the species. – Smaller formal charges are better (more stable) than larger ones. – Like charges on adjacent atoms are not desirable. – When a formal charge cannot be avoided, negative formal charges should reside on more electronegative atoms.
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Examples: Drawing Lewis Dot Structures
1. Draw Lewis dot structures for the following molecules and ions. Show all lone pair electrons, and all formal charges that are not zero. a. CH4 b. NH3 c. H2O d. NH4+ e. HCN f. CO2
How To Write Lewis Structures
1. Determine the total number of valence electrons from all of the atoms in the molecule or ion. • Add one electron for each unit of negative charge. • Subtract one electron for each positive charge. 2. Write the correct skeletal structure. • For molecules of the formula ABn, place the least electronegative element in the center, and the more electronegative elements in the terminal positions. • H is always a terminal atom, and NEVER a central atom, since it can only form one bond. • Draw single bonds between pair of bonded atoms.
1
Drawing Lewis Structures
•••———•••
2
How To Write Lewis Structures
3. Distribute the remaining valence electrons, giving octets to as many atoms as possible. • Place the remaining electrons as lone pairs on the terminal atoms first. • Once all of the terminal atoms have 8 electrons (or 2 for H), place any remaining electrons on the central atom. 4. If any atoms lack an octet, form double or triple bonds as necessary. • This is done by moving lone pairs from the terminal atoms in between the terminal atom and the central atom. • The formal charge can be used as a guideline for placing double bonds.
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Formal Charges
Formal charge = valence e- – (½ bonding e-) – (lone pair e-) • Formal charge is the difference between the number of valence electrons on the free atom and the number of valence electrons assigned to the atom in the molecule. – The sum of the formal charges must equal the charge on the species. – Smaller formal charges are better (more stable) than larger ones. – Like charges on adjacent atoms are not desirable. – When a formal charge cannot be avoided, negative formal charges should reside on more electronegative atoms.
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Examples: Drawing Lewis Dot Structures
1. Draw Lewis dot structures for the following molecules and ions. Show all lone pair electrons, and all formal charges that are not zero. a. CH4 b. NH3 c. H2O d. NH4+ e. HCN f. CO2