In MarvinSketch, the charge of an atom is initially set to be neutral.

As bonds are added or removed, MarvinSketch adjusts the number of implicit hydrogens to let the charge remain neutral.

You may change the charge of any atom using the Atom popup menu. The number of implicit hydrogens will be adjusted, if possible, to accommodate the new charge.

MarvinSketch will then perform a valence check and highlight the atom if an error is found.

Optionally, it is possible to display the charge symbols in circles. To set this option, go to the Display tab of the Preferences dialog located in the Edit menu. Here, you can also change the font type/size of the circled charge symbols.