A bond dipole is a vector from the δ+ atom to the more electronegative δ− atom. A molecule's net dipole is the vector sum of those bond dipoles, so shape decides polarity — symmetric molecules cancel to nonpolar, bent or asymmetric ones stay polar.
1. Electronegativity makes a bond polar
When bonded atoms differ in electronegativity, the greedier atom pulls the shared pair closer and turns δ−, leaving its partner δ+; the bigger the gap, the more polar the bond.
2. A bond dipole is a vector from δ+ to δ−
The dipole arrow points from δ+ to δ− (crossed end on the positive side), and its length grows with the charge separation — a vector, so dipoles add head-to-tail.
3. The molecular dipole is the vector sum of bond dipoles
Add every bond dipole (plus lone-pair effects) as vectors: water's bent shape and ammonia's pyramid keep their arrows from cancelling, leaving a net dipole.
4. Symmetry makes bond dipoles cancel
Symmetrically arranged polar bonds sum to zero: linear CO₂ and tetrahedral CCl₄ and CF₄ have polar bonds but no net dipole.
Break the symmetry and cancellation fails — dichloromethane and chloroform keep an unbalanced net dipole and are polar.
5. Polarity controls boiling point and solubility
Dipole–dipole attraction (and H-bonding) raises boiling points and drives "like dissolves like" — polar solvents dissolve polar solutes, nonpolar dissolve oils.
6. The δ+ carbon is where organic reactions begin
A polar bond leaves the δ+ carbon electrophilic, so nucleophiles attack there — the carbonyl carbon and the alkyl-halide carbon are the classic targets.
7. Summary
Electronegativity gap makes a bond dipole (δ+→δ−) · net dipole = vector sum, so shape decides · symmetric cancels to nonpolar, asymmetric stays polar · polarity sets boiling point and solubility · the δ+ carbon is where nucleophiles attack.
Quiz yourself
Tap a question to reveal the answer — free, no login.
Shape. CO₂ is linear, so its two C=O bond dipoles point in exactly opposite directions and cancel to zero. Water is bent (~104.5°), so its two O–H dipoles do not cancel — they add to a net dipole, making water polar.
The arrow points from the δ+ (less electronegative) atom toward the δ− (more electronegative) atom. Its length represents the dipole's magnitude, which increases with a larger charge separation and a longer bond.
CCl₄ is tetrahedrally symmetric: four identical C–Cl dipoles pull outward evenly and sum to zero. In CH₂Cl₂ the two C–Cl dipoles are no longer balanced by opposing C–Cl bonds (the other two positions hold weakly polar C–H bonds), so a net dipole survives.
The polar bond leaves that carbon electron-poor and electrophilic, so electron-rich nucleophiles attack there. This is the starting point for nucleophilic addition to C=O carbonyls and for substitution/elimination at the C bonded to a halogen.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.