Lewis Structures

drawing Lewis structures -

valence electrons - electrons involved in chemical bonding
dots placed on 4 sides of chemical symbol
number of valence electrons in representative elements same as group number
add all valence electrons
- same # as group # for representative elements
- add electron for each negative charge in anions
- subtract electron for each positive charge in cations
- only keep track of total number of electrons (no need to associate electrons to certain atoms)
write symbols for atoms
- central atom usually the least electronegative
- single bond (2 shared electrons) represented by dash between atomic symbols
complete octets of atoms
- arrange dots so all atoms have 8 electrons
- hydrogen only needs 2 in outer shell
place extra electrons around central atom
- even if it adds up to more than 8
use multiple bonds if not enough electrons
- use unshared electron pairs to make double/triple bonds

formal charge - charge of an atom if all atoms had the same electronegativity

tells how electrons are distributed in a molecule
# of valence electrons - # of electrons assigned to the atom
- # of assigned electrons = all nonbonding electrons + 1/2 of bonding electrons
sum of formal charges = overall charge of molecule

resonance structures - placement of electrons changes, but atom placement doesn’t

exceptions to octet rule -

odd # of electrons - not possible for octet to form around all atoms
- not possible for complete pairing of electrons
less than an octet - weird exception for boron/beryllium
- usually due to halogen’s reluctance to form double bonds
more than an octet
- extra electrons placed around central atom
- expanded electron shells only possible for elements in period 3 and beyond
- d subshell can be used for bonding
- central atom usually bonded to smallest/most electronegative atoms

Draw Lewis Structures for PCl3, PO33-, XeFl4, BCl3

Subject:

Chemistry [1]

Subject X2:

Chemistry [1]