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Valence
"Valence" is an option under the Main Edit menu that allows you to either add or subtract a valence hydrogen to or from a selected atom. MSC Pro 5.1 normally maintains the proper valence state on all atoms automatically. However, you can override the default by using this option. Valence would normally be used to alter the default state for an atom that can exist at different states such as N or S. For example, to create a phosphate group in MSC, you would use Valence to add two additional H to the P atom to convert it from its default valence state of 3 to the valence of 5 required for the phosphate group. If you are working with ring carbons, you would probably want to turn on Show Atoms to display the result of any changes to confirm that you have what you want.
Valence may be selected in a variety of ways, including selection from either the Edit menu or the Popup menu, which can be accessed by right-clicking on the screen, or by pressing the short-cut keys (F4 to add H; F5 to subtract). The mouse cursor will change to either H+ or H-, depending on whether H will be added or subtracted from an atom that is clicked on. Move the pointer over the atom of interest and click the left mouse button to either increment or decrement the valence hydrogens by one. If there are no remaining H available on that atom, clicking to subtract a H will have no effect. This process can be repeated on the same or other atoms repeatedly. To turn off Valence either select the option again to toggle it off, or press the <Esc> key. In either case, the pointer will then revert to the standard single arrow version. Note that you can switch directly between H+ and H- by pressing the short-cut keys, either F4 or F5, as indicated.