Drawing radicals can be achieved in two ways in Marvin JS:
You can select radical electrons from a predefined list in the Atom properties dialog, which is available via the Atom or the Selection context menu.
Set radicals manually with the Increase radical button on the Tools toolbar. With successive clicking on the atom, you can increase the number of the radical electrons until four. For the fifth clicking, all of the unpaired electrons will be removed.
When automatic lone pair calculation is active, the correct lone pair count will be assigned to the atoms on the canvas.
When the automatic lone pair calculation is turned off, you can set lone pairs to atoms manually with the Increase lone pair button from the Tools toolbar. (In this case, the other atoms will have 0 lone pairs.)
Please, note that lone pairs are only visible when the 'Show lone pairs' option is checked in the View settings dialog.