Exceptions To Simple Lewis Structures

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A force that holds atoms together and makes them function as a unit Atoms bond to achieve a noble gas electron configuration (8 electrons in the outer level - octet rule)
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Determine the total number of valence electrons 2. Form the bonds between the atoms 3. Distribute the remaining electrons as lone pairs (may need to incorporate double or triple bonds) 4. For ions, add 1 e- for each negative charge and subtract 1 e- for each positive charge; draw brackets around the structure with the charge outside

Exceptions to Simple Lewis Structures
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Dative Bond - bonds formed when both electrons in a bond come from the same atom (a lone pair of electrons); once formed, they are no different than other covalent bonds
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NH4+ CO H3O+

Be and B are atoms with small radii; does not allow 4 pairs to be present (significant repulsion of the pairs)

BeCl2 BF3

Bond Characteristics

Bond strength - measure of energy needed to break the bond; increases as more electrons are shared (triple > double > single); caused by greater attraction between nucleus and electrons Bond length - distance between two bonded nuclei; decreases as more electrons are shared due to the strength (triple < double < single)

Bond C-C C=C C=C

Length (nm) 0.154 0.134 0.120

Strength (kJ mol-1) 347 612 838

Polarity - charge created as a result of uneven sharing of electrons; if the ∆ in electronegativities is greater than 0.5, one end of the bond has a larger electron density, resulting in a partial - charge (the other end is a partial + charge) - dipoles if atoms involved in a colvalent bond have similar electronegativities (∆ less than 0.5), they share the electrons somewhat evenly; electrons do not have tendency to be around one nuclear center than another; the molecule is said to be non-polar

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