Atoms and Elements
Molkit has no separate atom tool. Atoms are placed and changed with the Bond tool (B): click empty canvas to place a lone atom of the active element, click an existing atom to change its element, and drag from an atom to grow the structure. What changes from one atom to the next is the active element, chosen from the picker column or a single key.
For example, to turn ethane into ethanol, press O, then drag from one carbon of ethane outward: the new atom is an oxygen. To turn that oxygen into nitrogen (ethylamine), press N and click the oxygen.
The element picker
The picker column on the right edge of the canvas sets the active element. It holds the common elements C, N, O, H, S, P, F, Cl, Br, I, plus R (generic substituent) and X (generic leaving group). R and X are placed as generic placeholder atoms, not as carbon; the More button next to them lists further generic groups. The grid button after I opens a full periodic table popup; click any cell to make that element active. Picking any element switches you to the Bond tool if another tool was active.
Below a separator, the same column carries functional group buttons (OH, NH2, CH3, COOH, NO2, CN) and abbreviation buttons (Me, Et, Ph, OMe, tBu, Ac), each with a “More” browser. The Abc/Str toggle controls whether a group is placed as a label or as a drawn structure. See Abbreviations.
Element shortcuts
Single keys set the active element: C, N, O, S, P, F, I. Like the picker buttons, they also switch to the Bond tool. Two are conditional: P opens the Properties panel instead when atoms or bonds are selected, and F flips the bow of a selected curved arrow first. There are no shortcuts for H, Cl, or Br (H and B are taken by other tools).
Changing an existing atom
There are three ways to change an atom that is already drawn.
With the Bond tool active, click the atom to set it to the active element. Clicking a carbon while carbon is active shows or hides its “C” label instead. To show every carbon label in the document at once, check Structure > Display > Show Carbon Labels.
With the Select or Bond tool active, you can also hover an atom and type an element symbol (1 or 2 letters, e.g. Cl, Br). The label previews as you type. The letters R, X, A, M, L, Q, Y, Z, D, T are accepted too, optionally with trailing digits (R1, X2). They make the atom a generic placeholder, the same as the R and X buttons do. Y is the exception: it is a real element (yttrium), so typing it gives yttrium; use the More list in the picker for a Y placeholder. See Generic Labels. Enter commits immediately, Esc cancels and restores the previous label; otherwise the change commits half a second after you stop typing, or when the pointer leaves the atom. While you hover an atom, letter keys other than G go to hover-type instead of the tool and element shortcuts. This hover-type is off when View > Single-Key Shortcuts is unchecked.
For a custom label such as CO₂Et, open the label editor, an inline rich-text editor. Click the atom with the Text tool, or double-click it: with the Bond tool a single double-click opens the editor, and with the Select tool double-click the atom once it is selected (a double-click on an unselected atom selects its molecule; see Double-click shortcuts). Enter saves, Esc reverts, and Shift+Enter inserts a line break for multi-line labels. Bold, italic, subscript, and superscript formatting are available while editing.
Label styling beyond the text itself (size, weight, color, halo) lives in the Object Properties Panel.
Implicit hydrogens
Molkit can display implicit hydrogens on labeled heteroatoms (for example OH, NH2), with the count derived from standard valence adjusted for bond order and charge. This display is off by default; turn it on with Structure > Display > Show Implicit Hydrogens. The H text is placed on the side opposite the atom’s bonds: left or right is preferred, with above or below used only when the bonds run strongly vertical. Isolated atoms default to H on the right. Override the automatic placement per atom via the right-click “Aux Position” submenu (Auto, Left, Right, Above, Below) or the Properties panel. Implicit-H custom labels such as CH3 are mirrored to H3C when bonds approach from the right, so the bonded element stays adjacent to the bond.