Lewis Decoration Studio

Build a Lewis structure around one central atom using predefined terminal blocks, using a decoration-first adaptation of the approach of Sciaini, Ingram and Schipper. Decorate first, use formal charge to supply missing lone pairs, then redistribute pairs to satisfy the allowed shells. The electron counters provide a cross-check; there is no need to count a stock of central electrons before attaching blocks. Perspective VSEPR geometry and resonance exploration remain available.

1. Choose your centre. Select an element from the periodic-table palette: columns show groups and rows show periods. C and Si are in group 14. The centre starts with no electrons drawn. Determine its initial formal charge from its group: for the main-group centres in groups 13–18, the neutral valence-electron count is the group number minus 10. With no electrons assigned yet, that count is the positive formal charge. The centre palette omits charge labels so you can infer them; the canvas shows the calculated, circled charge and updates it after every edit. This is a drawing and bookkeeping starting point, not a claim that the molecule forms from a bare ion. H and F are terminal atoms; R– is a generic alkyl terminal group.
2. Enter molecular charge. Set the target net charge, including 0 for a neutral molecule. This does not add or remove electrons. You must adjust the drawing until e⁻ placed matches e⁻ required and the sum of formal charges equals the target.
3. Decorate with terminals. With the centre selected, click a terminal block to add it, or drag the block onto the centre. After an addition the centre stays selected, so repeated clicks add more terminals. To replace a terminal, click or tap that terminal on the canvas, then click the desired terminal block in the palette. You can also drag the new block directly onto the terminal. The selected terminal keeps its position and is replaced with the new block’s element or group, bond order and lone pairs. Central nonbonding electrons and the target molecular charge stay unchanged; electron totals and formal charges are recalculated. The replacement remains selected. Select the centre again to resume adding terminals. Undo restores the previous block. The terminal positions follow the current VSEPR group arrangement. If a central electron group has already been added, you can select its direction before attaching; the attachment does not consume that group. H and R– sit together because both use one attachment bond; the other terminals are arranged by group.

   Each terminal block shows its bonding pairs and lone pairs. A dashed connection with a dot at each end represents a complete bonding pair (2e⁻), already included in the block. Count both dots when checking the terminal’s shell. The neutral terminal blocks therefore already represent complete octets (a duet for H) and zero formal charge; the zero is not printed. For formal charge, assign one electron from each bonding pair to the terminal, together with all its nonbonding electrons. O⁻ is the explicitly charged exception: it has a complete octet and formal charge −1. Attaching a single-, double- or triple-bonded block adds its one, two or three existing bonding pairs and its displayed lone pairs to the drawing. No electrons are taken from the centre. The centre’s formal charge falls by one per added bond, while the terminal retains its displayed formal charge. O⁻ stays −1; neutral terminals stay neutral. The animation shows each newly added pair moving into its bond; it is bookkeeping, not a reaction mechanism.

   OH and R– are fixed terminal groups. R– represents an alkyl group whose internal bonds and electrons are omitted; when R is present, the counters cover only the displayed framework. A completed R block is neutral and single-bonded. Its internal electrons are not editable; exploratory pair moves that violate this pattern are marked Not allowed.

4. Check formal charge, then refine. After attaching the intended terminals, compare the formal-charge sum with the molecular-charge target. Add missing lone pairs to the centre: each pair lowers its formal charge by two. Individual electrons are also available. If the centre exceeds its allowed shell, move bonding pairs onto terminals; these transfers conserve electrons and the charge sum. Use Structure check & guidance to find the next correction. When available, Show the next move displays a suggested electron-pair arrow; Apply this move carries out that move. You can also move the pair yourself. The panel provides feedback throughout construction and does not automatically change your drawing.

   The canvas reports Not allowed when a check fails, Not the best, it can be improved for a valid but less-preferred arrangement, or Preferred Lewis structure when the drawing passes the checks and has a preferred formal-charge pattern among the structures the studio considers.

   The checks cover electron count, formal-charge sum, supported shells and the six-group limit. The preference check first minimizes Σ|FC|. When this total is tied, it favours the arrangement with the smaller largest individual formal-charge magnitude, before favouring negative charge on more electronegative atoms. For example, terminal charges of −1 and −1 are preferred over 0 and −2 when the centre and total charge are unchanged. A complete-octet drawing can therefore be acceptable without being the best. Supported period ≥3 centres automatically allow expanded octets; a complete-octet arrangement is not preferred when expansion improves this ranking.

Select atoms and edit electrons

Click or tap an atom to select it. A selected terminal is the destination for replacement terminal blocks; a selected centre is the destination for new attachments. Electron blocks always edit the selected atom. Click Electron (1e⁻) or Lone pair · 2e⁻ to add electrons to the selected atom, or drag the block directly onto the intended atom. In Atom controls, use + electron, − electron, + lone pair or − lone pair. These edits change the drawing’s charge, while the molecular-charge target stays fixed.

Formal charge is calculated as $\mathrm{FC}=V-N-\frac{B}{2}$, where V is the neutral atom’s valence-electron count, N is its nonbonding electrons and B is its bonding electrons. Assign all nonbonding electrons and half of each bond’s electrons to the atom. Equivalently, subtract the nonbonding electrons and one electron per bonding pair from V.

Move pairs, erase and undo

Drag a terminal lone pair into its bond or onto the centre to increase the bond order. You can also drag a central lone-pair balloon into a bond or onto its terminal. Drag a bonding pair onto either the centre or its terminal atom to place a lone pair on that atom. Moving the pair onto the terminal reduces the central shell; moving it onto the centre leaves the central shell electron count unchanged and reduces the terminal shell by two electrons. The new central lone pair counts as a group. More than six groups is marked Not allowed, but the pair move is still permitted. Hover highlights the selectable pair or bond, and the drop target highlights when it can receive the selected pair. A highlighted target indicates an available move, not chemical validity. These transfers conserve the total number of electrons and the sum of formal charges. All moves involving an existing pair are enabled, including moves from a preferred structure to a structure that is Not allowed. Shell, bonding-pattern and six-group rules determine the feedback after the move. Invalid drawings remain editable: reverse the move, use another move or choose Undo. Only preferred structures are coloured; invalid drawings are excluded from resonance contributors. You cannot move a pair that is not present. Drop a lone pair along the bond or onto the atom at its other end; the highlighted target confirms an available move. Alternatively, select the bond or terminal and choose Terminal lone pair → bond, Bond → terminal lone pair, Central lone pair → bond or Bond → central lone pair. Moving the last bonding pair leaves a disconnected terminal, labelled no bond and marked Not allowed. Keep it in place and restore a bond by moving a lone pair into the gap or onto the other atom, or use Remove this terminal to remove the group entirely.

Hold Shift and point at a terminal atom, electron or lone pair to highlight it in red; Shift-click erases that item. Removing a terminal removes its nonbonding electrons, internal OH bond if present, and all its attachment bonding pairs. It does not add electrons to the centre. Central lone pairs already present remain. The centre’s formal charge rises by one for each removed attachment bond. Undo restores the entire previous drawing. The centre itself cannot be erased this way. On a phone, select the terminal and tap Remove this terminal, or use the electron controls. Keyboard users can focus a terminal and press Shift+Enter or Shift+Space. Undo restores the previous edit.

Read and adjust the perspective view

Ideal geometry describes all central electron groups; Molecular geometry describes the atom positions. Each attached terminal and each central lone pair occupies one group; a multiple bond still counts as one group. A completed structure supports up to six groups, with octahedral ideal geometry. Exploratory pair moves can exceed that limit; those drawings are marked Not allowed and are not assigned an ideal geometry. Central lone pairs appear as balloons with two dots. Unpaired central electrons occupy provisional construction directions; the studio checks completed closed-shell structures.

Drag empty canvas space to rotate the view. Use − and + to zoom, Fit to reset the view, and Refine VSEPR to fit the geometry with the smallest reorientation. H follows the duet rule; period-2 centres follow the octet rule, with a neutral six-electron B exception. OH and R– do not become additional centres. A supported Lewis drawing does not by itself establish chemical stability.

Explore resonance and averages

Once the drawing is valid and balanced, open Resonance. Select a contributor card to see it in the large workspace, or use the previous/next arrows. Play arrow steps repeats the electron-pair sequence; press Pause arrow steps to stop. Use this contributor copies the selected arrangement into the editable drawing. You can also drag a pair or use the pair-transfer buttons directly on an individual contributor: a successful move opens the edited structure in Build. Undo restores the previous editable drawing. Average views cannot be edited as individual electron arrangements; select Show contributors first.

Include reasonable nonequivalent forms adds supported, less-preferred contributors. By default, every displayed contributor is limited to formal charges of −1, 0 or +1 on every atom. The additional Extend formal charge limit to ±2 on all atoms option becomes available when nonequivalent forms are enabled. Select it to also include supported contributors with formal charge −2 or +2 on the central atom, terminal atoms, or both. Every atom must have a formal charge between −2 and +2. Switching it off restores the original limits. This display option does not change the preferred-structure ranking or permit invalid shells. Show averages displays equal-weight bond orders and formal charges for the selected equivalent family as reduced fractions; nonequivalent families are not mixed. Dashed bond lines indicate the fractional part of a bond order. Return to Show contributors to inspect individual lone pairs. The arrows illustrate electron bookkeeping, not molecules switching between drawings; averaged formal charges are not measured atomic charges.

Appearance, export and saving

Colour atoms colours only preferred structures, with equivalent terminal groups sharing a colour. Switch it off for monochrome. Choose light or dark backgrounds independently for Controls and Canvas; toggle Sound and Celebrate as desired. Celebration marks a preferred structure when enabled.

Export SVG and Export PDF create vector drawings using the current orientation, canvas background and resonance view. Use Save project and Open project below to keep an editable copy. Your current drawing is also saved automatically in this browser, separately from Lewis Centre Studio. An imported drawing retains the electrons it already contains; only a newly chosen centre starts without electrons.

Sciaini, Ingram & Schipper · From Atom Decoration to Pattern Recognition (2025)