Crescent School
Grade 11 Chemistry
Drawing Electron Dot (Lewis) Structures
Guidelines:
 for H, 2 electrons = ☺
(why?____________________________________)
 for all other elements, ☺ = 8 electrons, with some exceptions which
will be explained later
 the least electronegative element will be in the centre. See p 71 in 11
Chemistry for an electronegativity chart; there is also one on our data
sheet.
 H cannot be the central atom
(why?________________________________)
 There will be no ring structures for any of the following. Where
necessary, the skeletal structure will be supplied. Later this year you
will see some molecules that are ring-shaped.
1.
Simple Ionic Compounds
 Ionic bonds involve electron __________________________ .
 Ionic compounds do not exist as discrete. molecules. The ions exist in
a 3-D crystal lattice. The formula of an ionic compound represents the
lowest terms ratio of cation : anion. Hence the term formula unit.
Examples
NaCl
SrCl2
Al2O3
2.
Covalent (molecular) Compounds (single, double, triple, coordinate
covalent)
 Covalent bonds involve electron _____________________ .
 Covalent compounds exist as discrete molecules.
a)
Covalent compounds that contain only single bonds. A single
bond involves the sharing of one pair (2) electrons.
Examples
CH4
b)
HF
Covalent compounds that contain a double bond. A double
bond involves the sharing of two pairs (4) of electrons.
Examples
O2
CO2
c)
Covalent compounds that contain a triple bond. A triple bond
involves the sharing of three pairs (6) of electrons.
Examples
CO
N2
 Notice that electrons—bonding and non-bonding—work in pairs
whenever possible.
 Always show lone pairs on the central atom. You’ll learn why later.
d)
C
oordinate covalent bonds occur when one atom supplies both of
the electrons required for, say, a single bond.
Examples
SO2
e)
N2O
When drawing Lewis structures for ions, don’t forget to add
electron(s) for an anion or take away electron(s) for a cation.
Examples
K+
Cl–
NH4+
NO3–
Now complete the exercise with the Fruit Loops. Be sure to write down the
Lewis structures in your notes after they have been checked by the teacher.
Practice Problems
Draw the Lewis structure for each of the following. Reread the beginning of
this handout now.
HBr
HCl
NH3
PH3
H2O
H2S
H2O2 (linear molecule with Hs at the ends)
CN– (Don’t forget to add an extra electron for the negative charge.)
SO42–
SO3
C2H2 (linear molecule with Hs at the ends)
CO32–
—fin—