SME
Grade 10 Science · Chemistry
Ionic, covalent and metallic bonding
Use this for “Draw the Lewis structure of H₂O. How many shared pairs and lone pairs does it have?”
Diagram
H₂O: 6 + 2 × 1 = 8 valence electrons.
2 shared pairs (4 electrons, the lines) and 2 lone pairs (4 electrons).
Each hydrogen has 2 electrons around it; every other atom has 8.
This is an example. Type your own values to start a new problem.
nH
Hydrogen atoms
example
nC
Carbon atoms
example
nN
Nitrogen atoms
example
nO
Oxygen atoms
example
V
Valence electrons
calculated
b
Shared pairs
worked out (not typed)
l
Lone pairs
worked out (not typed)
Formulas
V
=
nH
+
4
×
nC
+
5
×
nN
+
6
×
nO
8 = 2 + 4 × 0 + 5 × 0 + 6 × 1
b
=
(2
×
nH
+
8
×
(nC
+
nN
+
nO)
−
V
)/2
2 = (2 × 2 + 8 × (0 + 0 + 1) − 8)/2
V
=
2
×
(
b
+
l
)
8 = 2 × (2 + 2)
Assumptions
•
Each atom but hydrogen ends with 8 electrons around it; hydrogen with 2.
•
A shared pair counts for both atoms, so the electrons needed minus those on hand are two per shared pair.
•
Two or three shared pairs make a double or triple bond.
•
The drawn molecules are H₂O, NH₃, CH₄, CO₂, HCN, CH₂O, H₂, O₂ and N₂; the rule works for any molecule whose atoms all reach full shells.
Step-by-step
WE KNOW
nH = 2, nC = 0, nN = 0, nO = 1
FIND
valence electrons (V), shared pairs (b), lone pairs (l)
Step 1 · Find valence electrons (V)
Use V = nH + 4 × nC + 5 × nN + 6 × nO
Add each atom’s valence electrons: H 1, C 4, N 5, O 6.
V
=
2
+
4
×
0
+
5
×
0
+
6
×
1
V
=
2
+
6
V
=
8
Step 2 · Find shared pairs (b)
Use b = (2 × nH + 8 × (nC + nN + nO) − V)/2
Electrons needed for full shells (2 for H, 8 for the others) minus the valence electrons you have, halved, are the shared pairs.
b
=
(2
×
2
+
8
×
(0
+
0
+
1)
−
8)/2
b
=
(4
+
8
−
8)/2
b
=
4
2
b
=
2
Step 3 · Find lone pairs (l)
Use V = 2 × (b + l)
Electrons go in pairs; the pairs that are not shared are lone pairs.
l
=
V
2
−
b
l
=
8
2
−
2
l
=
2
Check
Put the numbers back in. Both sides should match.
8 = 2 + 4 × 0 + 5 × 0 + 6 × 1 ✓
2 = (2 × 2 + 8 × (0 + 0 + 1) − 8)/2 ✓
8 = 2 × (2 + 2) ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
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