SME
Grade 10 Science · Chemistry
Molecular shape, polarity and intermolecular forces
Use this for “Water has 2 bonded atoms and 2 lone pairs on oxygen. What is its shape and bond angle?”
Diagram
2 bonded atoms and 2 lone pairs: 4 electron domains spread out as a tetrahedral arrangement.
The shape is bent, bond angle 104.5° (for example H₂O); lone pairs push the bonds a little closer.
The bond dipoles (crossed arrows, toward the more electronegative atom) add up: the molecule is polar.
Electron domains: 4
This is an example. Choose your own values to start a new problem.
b
Bonded atoms on the central atom
example
l
Lone pairs on the central atom
example
d
Electron domains
calculated
θ
Bond angle
worked out (not typed) · °
Formulas
d
=
b
+
l
4 = 2 + 2
θ
=
bond
angle
with
b
bonded
atoms
and
l
lone
pairs
104.5 = bond angle with 2 bonded atoms and 2 lone pairs
Assumptions
•
Electron domains spread as far apart as they can; a double bond counts as one domain.
•
Lone pairs push harder than bonds, so angles shrink: the angles are measured ones, such as water’s 104.5° and ammonia’s 107°.
•
A molecule is polar when its bond dipoles don’t cancel.
•
This page covers central atoms with 2 to 4 electron domains.
Step-by-step
WE KNOW
b = 2, l = 2
FIND
electron domains (d), bond angle (θ)
Step 1 · Find electron domains (d)
Use d = b + l
Every bonded atom and every lone pair is one electron domain.
d
=
2
+
2
d
=
4
Step 2 · Find bond angle (θ)
Use θ = bond angle with b bonded atoms and l lone pairs
The domains spread as far apart as they can; lone pairs take more room and squeeze the bonds together.
θ
=
bond
angle
with
2
bonded
atoms
and
2
lone
pairs
4 domains: tetrahedral; 2 lone pairs, so the shape is bent, like H₂O
θ
=
104.5°
Check
Put the numbers back in. Both sides should match.
4 = 2 + 2 ✓
104.5 = bond angle with 2 bonded atoms and 2 lone pairs ✓
Topic check
See what you know: 5 questions on this topic
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