Atom map visibility must be turned on in the View settings dialog to display the atom map numbers.
Unlike atom indices, map numbers remain constant during editing the molecule.
Set Map Numbers to Individual Atoms
You can add map numbers to every single atom manually through the Atom properties dialog.
Open the Basic tab in the dialog, and activate the Map field by clicking the Padlock icon next to the input field.
Enter the number, then click OK.
You can change the number in this menu anytime. Shortcuts are also available for atom map number 1-9, simply hold Alt and press the desired number while the cursor is over an atom.
Deleting Map Numbers from Individual Atoms
To remove atom map numbers from atoms, you can delete the Map value in Atom properties dialog by closing the Padlock next to the input field.
Assigning the Same Number to Corresponding Atoms
In some cases, you need to map atoms with the same number as what another atom has.
Select the smart Reaction tool from the Tools toolbar, then press the left mouse button on the atom you would like to map, and drag to the other atom, which receives the same map number. As soon as the second atom gets golden-brown feedback, release the mouse button.
The same happens when you drag from an atom that has no map number to another one which has one. Both of the atoms will get the same map number as what the latter one had.
When there is no map number on any of the atoms, both of the atoms get the smallest number which is not present on the canvas.
Overwriting the Atom Map Number
Dragging from an atom which already has an atom map number to another atom with an atom map number, the latter atom will get the same number as what the former atom has.
Deleting the Atom Map Number
Dragging from an atom with an atom map number to another atom with the same number results that both of the map numbers will be deleted from the canvas.
Click the Auto Map button. In this case, every atom in the reaction will get an atom map number automatically. By clicking the Auto Map button, every atom in the reaction gets an atom map number automatically.
Please note that the Auto-map function only works for structures that are in the same reaction. (In other words: there should be only one reaction (one arrow) on the canvas where every molecule would be a reactant or a product of it.)