Net Dipole

Molkit can draw a molecule’s net dipole as an arrow over the structure. Following the textbook convention, the arrow points toward the negative end and carries a small plus mark at the positive end. The direction is estimated from an idealized 3D shape of the molecule, as a teaching aid; the canvas prints no number.

To see it, draw water, select the whole molecule with the Select tool (double-click one of its bonds), and tick Show net dipole (μ) in the Object Properties panel. The arrow points from the hydrogens toward the oxygen.

Turning it on

Select a single, complete molecule. The Object Properties panel then shows a MOLECULE section with a Show net dipole (μ) checkbox. Tick it to draw the arrow.

The checkbox only appears for a whole covalent molecule. A partial selection, or a selection spanning more than one molecule, hides it, because a net dipole is a property of the molecule as a whole.

The first time you turn it on, Molkit loads its 3D chemistry library to build the molecule’s shape, so the arrow can take a moment to appear.

Reading the arrow

The arrow runs from the positive end of the molecule to the negative end, with the arrowhead at the negative end and a plus crossbar near the tail.

For a molecule whose bond dipoles cancel, such as carbon dioxide or carbon tetrachloride, there is no direction to draw, and a small μ ≈ 0 marker appears instead of an arrow.

If the dipole points straight out of or into the page and the flat drawing gives no sideways direction either, a circle appears: a dot inside means the dipole points toward you, a cross means it points away.

The arrow’s length scales with the molecule’s bond length and does not change with the size of the dipole, so two arrows cannot be compared for strength.

The arrow sits in the molecule’s own layer, so it travels with the structure when you move it.

How Molkit works out the direction

Molkit builds an idealized 3D shape for the molecule, turned to match your drawing, and estimates the dipole from it in one of two ways:

Partial charges. Molkit assigns each atom a small partial charge from electronegativity (the Gasteiger method) and adds up charge times position. The canvas arrow uses this estimate whenever every element in the molecule is one the method covers (H, C, N, O, F, Cl, Br, I, S and P), which includes most molecules in an intro course.
Bond dipoles. Otherwise Molkit adds up one arrow per polar bond, each pointing toward the more electronegative atom and scaled by the electronegativity difference. This is the rule of thumb taught in intro courses. Lone pairs are not included.

If you have drawn the molecule looking straight down its dipole axis (for example ammonia drawn with nitrogen in the middle and three hydrogens at 90 degrees), the 3D estimate has almost nothing in the page plane. Molkit then takes the direction from the bonds as you drew them, so the arrow still points toward the lone-pair side.

Freely rotating O-H, N-H and S-H

In a 2D drawing, the hydrogen of an -OH, -NH2 or -SH group can sit on either side of its bond, and in the real molecule it rotates freely. In the bond-dipole estimate, Molkit therefore averages that X-H bond over a full rotation, which leaves only its component along the C-X bond. That estimate no longer depends on where the hydrogen happens to be drawn. The partial-charge estimate is not averaged this way.

This averaging applies only when the O, N or S has exactly one other heavy-atom neighbour and that neighbour is carbon. Hydrogen peroxide, hydrazine and hydroxylamine are not averaged, because rotation about an O-O or N-N bond is not free and their dipole depends on the actual shape.

The 3D viewer does not average: it shows one specific arrangement of the atoms, so it uses that arrangement exactly.

Carbon monoxide and nitric oxide

The electronegativity rule puts the negative end of CO on oxygen and of NO on oxygen. The measured dipoles point the other way: the negative end is on carbon in CO and on nitrogen in NO. For these two diatomic molecules Molkit draws the arrow in the measured direction and marks it as an exception to the rule of thumb. The exception applies only to a molecule made of exactly those two atoms.

Ions get no arrow

A species with a net charge has no single well-defined dipole, because the value depends on where you put the origin. Molkit draws no arrow for an ion. Whether a molecule is an ion is worked out from the formal charges implied by its electrons, and the charges you drew are ignored for this test, so both of these count as ions:

an ion with charges on its atoms, such as NH4+ drawn with + on nitrogen;
an ion drawn in brackets with all atoms neutral and the charge outside, such as [OH]-, [CN]-, [H3O]+ or [NO3]-.

A neutral molecule drawn with only part of a charge-separated pair (for example nitrogen dioxide with only the O- shown) still sums to zero formal charge, so it keeps its arrow.

A molecule that contains a generic placeholder such as R or X has no defined 3D shape, so it gets no arrow.

Adjusting it

With the dipole shown, the same MOLECULE section has a Color swatch (with transparency and a reset button), Head size and Thickness sliders for the arrowhead and stem, and a Position reset button. Drag the arrow with the Select tool to move it over the molecule; the reset button returns it to the center.

In the 3D viewer and exports

The 3D Structure Viewer has its own Show net dipole toggle that draws the arrow over the 3D model, with the same CO and NO exceptions and no arrow for ions.

When you export to SVG with chemistry metadata, both estimates are written into the file’s embedded data. If the molecule matches an entry in Molkit’s table of 79 measured gas-phase dipole moments (from the NIST CCCBDB database), the measured value in debye, with its source, is written into the file as well, so a reader can compare the estimate with experiment. A value of exactly zero for a symmetric molecule such as SF6 is marked as a symmetry zero, not a measurement. See SVG metadata.