The button attaches additional valences (shown as hydrogen atoms) to the center clicked
with the mouse. The new valences will be placed as far as possible from the other atoms attached to the center.
Similarly, the button removes
atoms and open valences. When used, the function will eliminate the
selected atom and all single valences (hydrogens) attached to it. It
may also be used to remove a dangling bond by clicking on the bond
stick.
Deleting a Dangling Bond (Open Valence)
Clicking on it will remove the open valence.