Consider the following… (in your notes)
Write Lewis dot diagrams for the following neutral atoms:
1) magnesium
6) calcium
2) nitrogen
7) aluminum
3) carbon
8) oxygen
4) iodine
9) sulfur
5) lithium
10) fluorine
Lewis Structures
Chemistry MYP
Ms. Khan
Lewis Structures
▪ a formula in which atomic symbols represent nuclei and innershell electrons, dashes between two atomic symbols represent
electron pairs in covalent bonds, and dots adjacent to only
one atomic symbol represent unshared electrons (lone pairs)
▪ covalent bond – a chemical bond resulting from the sharing
of an electron pair between two atoms
▪ lone pairs – a pair of electrons that is not involved in
bonding and that belongs exclusively to one atom
Steps for Drawing Lewis
Structures
Step 1 – count total valence electrons
▪ count the total number of
valence electrons indicated
by the given formula
– the number of atoms of
an element are indicated
by a subscript following
the element symbol
– if no number is written, a
“1” is understood
▪ add / subtract electrons
based on ionic charge
▪ ex: PF3
– 1 phosphorus atom
▪ 5 valence electrons
▪ 1(5e-) = 5e– 3 fluorine atoms
▪ 7 valence electrons
▪ 3(7e-) = 21e5e- + 21e- = 26eall valence electrons must
be represented in drawing
Step 2 – draw a single bonded skeleton
▪ arrange the atoms with the
least electronegative atom
in the center and all other
atoms surrounding it
▪ connect central atom to the
peripheral
atoms
with
single bonds
▪ account for electrons used
1 bond = 2e-
▪ ex: PF3
– phosphorus has a lower
electronegativity, will be
the central atom
26eF –– P –– F
– 6e|
20eF
Step 3 – fill octets
▪ remaining electrons used to
fill the octets of all atoms
– bonded pairs are shared
between both atoms
– octets are filled using lone
pairs of electrons
– octets
of
the
more
electronegative atoms are
filled first
▪ account for electrons used
1 lone pair = 2e-
▪ ex: PF3
– fluorine filled first
– each fluorine has 2e-, gets
3 lone pairs (6e-)
– phosphorus has 6e-, gets 1
..
..
..
: F –– P –– F :
¨ |
¨
:F:
¨
20e– 20e0e-
Exceptions to the Octet Rule
LESS THAN THE OCTET
▪ some atoms are satisfied
with less than 8 electrons in
the valence shell
▪ duet (2) – hydrogen
▪ quartet (4) – beryllium
▪ sextet (6) – boron
ex: CH4
1(4e-) + 4(1e-) = 8eH
8e|
– 8eH –– C –– H
0e|
H
carbon has 8eeach hydrogen has 2e-
Exceptions to the Octet Rule
MORE THAN THE OCTET
▪ some atoms can have more
than 8 electrons in the
valence shell
▪ possible for all nonmetals in
period 3 or below
▪ if electrons remain after
Step 3, added as lone
pairs to central atom
ex: SF4
1(6e-) + 4(7e-) = 34e..
..
:F:
:F:
34e\ .. /
8eS
– 24e/
\
2e:F:
:F:
¨
¨ sulfur has 10e-
Multiple Bonds
▪ when all electrons are represented in the drawing but not all
octets of all atoms are satisfied, multiple bonds can be
formed between two atoms – a double bond or a triple
bond
▪ a lone pair of electrons is reassigned as a bonded pair so
that electrons are shared by both atoms and octets can be
satisfied
Multiple Bonds
▪ all electrons represented,
sulfur does not have octet
ex: SO3
1(6e-) + 3(6e-) = 24e24e-
..
..
: O –– S –– O :
¨
|
:O:
¨
¨
6e–
18e0e-
▪ lone pair from one oxygen
will turn into a bonded pair
..
..
: O –– S –– O :
¨
||
:O:
¨