Chemistry · Lesson

Electronegativity, bond polarity and molecular dipoles

Distinguish unequal electron sharing in a bond from the total dipole of a molecule. Use direction and geometry to explain cancellation, while keeping schematic arrows separate from measured dipole moments.

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By the end, you can

Explain electronegativity and partial-charge notation within a bond model.
Combine equal bond arrows in linear and bent arrangements.
State what a schematic vector sum does not measure.

State the arrow convention

A common chemistry bond-polarity arrow has a crossed tail at the more positive end and points toward the more negative end. The physical electric dipole vector uses the opposite direction, from negative toward positive. This chapter’s diagram uses the chemistry convention throughout. Read the legend before adding arrows from different sources.

An arrow’s direction and its magnitude have different roles. The diagram assigns every shown bond arrow one arbitrary unit to illustrate vector addition. Those chosen equal lengths do not compare measured C–O and O–H bond moments and are not values in Debye. Partial-charge signs and drawn atom sizes are schematic too.

Geometry controls how directions combine

Linear, symmetric CO₂ has two equivalent C–O bonds. In the simple chemistry-arrow construction, one equal arrow points left toward oxygen and the other right. Their sum is zero. A molecule can therefore have polar bonds while having no permanent molecular dipole in its isolated symmetric description.

Water is bent. Two equal O–H chemistry arrows point toward oxygen at an angle. Their horizontal components cancel, but their components along the angle bisector add. The illustration uses an approximate 104.5° geometry and gives a nonzero vector sum. VSEPR explains a bent arrangement qualitatively; it does not calculate that numerical angle exactly.

Equal polar bonds can add differently

Chemistry convention: the crossed tail marks the more positive end; the arrowhead points toward the more negative end.

The physical electric dipole vector uses the opposite direction, from negative toward positive. The arrows shown here use the chemistry convention.

Each bond arrow is assigned one arbitrary unit in this comparison. Equality within each molecule is the point; these chosen lengths do not compare measured C–O and O–H bond moments. The resultant is the sum in this simple construction.

The water angle is approximate. VSEPR helps explain a bent shape; it does not calculate this angle exactly. Nuclei, connections, sizes and spacing are schematic; no lone pairs or electron-density distribution are drawn.

The δ+ and δ− labels denote partial-charge signs. They do not give full ionic charges or measured atomic charge values.

CO₂: the equal, opposite bond arrows cancel. H₂O: the equal bond arrows meet at an angle, so the horizontal components cancel while their components along the angle bisector add. This explains cancellation versus a nonzero sum qualitatively; the full molecular dipole depends on the entire charge distribution.

Underlying illustrative vector data
Arrow in the model	x component	y component	Magnitude
CO₂: toward left O	-1	0	1
CO₂: toward right O	1	0	1
CO₂: sum	0	0	0
H₂O: from right H toward O	-0.791	0.612	1
H₂O: from left H toward O	0.791	0.612	1
H₂O: sum	0	1.224	1.224

For this coordinate choice, +x is right and +y is upward along the water angle bisector. Components are rounded here; the exact sine and cosine construction is retained in the model data. All vector values shown here use arbitrary units, never Debye.

The linear carbon dioxide model has two equal chemistry bond arrows pointing in opposite directions toward oxygen. Their sum is zero. Bent water has two equal arrows toward oxygen; their horizontal components cancel and their upward components add. The arrows are schematic, not measured dipole moments.

The molecular dipole concerns the whole distribution

A simple bond-arrow sum helps explain symmetry and qualitative cancellation. The full molecular dipole depends on the entire charge distribution, including all nuclei and electrons. It is not necessarily obtained by adding two arbitrarily chosen independent bond moments. A real magnitude needs suitable evidence or a defined electronic calculation.

Cancellation requires compatible magnitudes as well as directions. Two bonds pointing opposite ways need not cancel if their electronic contexts differ. A molecule with no permanent dipole can still be polarizable and interact with surrounding fields. Later intermolecular and spectroscopy lessons connect these distinctions to observations.

Worked examples

Add equal, opposite chemistry arrows

In the schematic linear CO₂ model, the two arrows are (−1, 0) and (1, 0) in arbitrary units. What is their sum?

Add horizontal components: −1 + 1 = 0.
Add vertical components: 0 + 0 = 0.
Retain equal magnitude and opposite direction as the cancellation assumptions.

The sum is (0, 0). This expresses cancellation in the stated symmetric model, not a measurement of individual bond moments.

Try it yourself

Can a molecule with polar bonds have zero permanent molecular dipole?
Show the answer and reasoning
Yes, under an appropriate symmetric arrangement.
Equal opposing contributions can cancel, as in the stated linear CO₂ example.
Does δ− necessarily mean a whole −1 ionic charge?
Show the answer and reasoning
No.
It denotes a partial-charge sign in the stated model, without assigning that full integer charge.
Where does the crossed chemistry arrow point in this diagram?
Show the answer and reasoning
Toward the more negative end.
The crossed tail marks the more positive end. The physical electric dipole-vector convention points oppositely.
Does the water diagram’s arbitrary-unit sum establish its measured dipole in Debye?
Show the answer and reasoning
No.
The construction illustrates geometry and cancellation. A real molecular dipole depends on the whole charge distribution.

What to carry forward

Separate bond polarity from the molecular dipole.
Read the arrow and partial-charge conventions.
Cancellation depends on both geometry and magnitudes.
Keep illustrative arrow sums distinct from measured molecular dipoles.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.