Drawing Molecules Electron by Electron
The SECS method for building any Lewis structure
A Lewis structure (or Lewis dot structure) is a two-dimensional diagram that represents a molecule's atoms, the bonds connecting them, and the valence electrons involved — both the electrons shared in bonds (usually drawn as lines) and any electrons that remain unshared as lone pairs (usually drawn as dots). These structures are the essential starting point for predicting molecular geometry (covered in the VSEPR lesson) and polarity (covered in the Polarity lesson), since both depend directly on knowing exactly how a molecule's atoms and electron pairs are arranged.
Lewis structures are built around the octet rule: most main-group atoms are most stable, and therefore most likely to form, when surrounded by eight valence electrons (matching the stable electron configuration of a noble gas) — either through a combination of bonding and lone pairs. This drive toward a stable octet is the underlying reason atoms bond together in the specific patterns and numbers that they do.
The SECS method provides a reliable, repeatable sequence for constructing a correct Lewis structure for essentially any molecule or polyatomic ion: Structure (sketch the skeletal arrangement of atoms), Electrons (count the total number of valence electrons available), Connect (place single bonds between connected atoms), Satisfy (complete octets using lone pairs and, if necessary, multiple bonds).
💡 Why the Least Electronegative Atom Goes in the Center
The first step of the SECS method — sketching the skeletal structure — relies on a specific rule of thumb: the least electronegative atom (excluding hydrogen, which never occupies a central position since it can only form one bond) is placed in the center of the structure, with the more electronegative atoms arranged around it.
This rule reflects a real structural pattern found across a huge number of common molecules and ions: central atoms are typically elements like carbon, nitrogen, phosphorus, or sulfur, capable of forming multiple bonds to surrounding atoms, while more electronegative atoms like oxygen, fluorine, and chlorine much more commonly occupy terminal (outer) positions, often completing their octet with lone pairs rather than forming additional bonds to yet more atoms. Hydrogen is a special, universal exception: because hydrogen only needs two electrons total to be stable (matching the noble gas helium, not the eight-electron octet used by larger atoms), it can only ever form a single bond, which means it can never be a central atom bonded to more than one other atom.
This rule of thumb has known exceptions (some structures genuinely require a different arrangement, particularly for asymmetric molecules where atom connectivity is given directly rather than inferred), but as a starting default for constructing an initial skeletal sketch, placing the least electronegative non-hydrogen atom centrally produces a correct structure in the overwhelming majority of introductory cases.
S-E
Structure and Electrons — the first two steps
Structure: sketch the skeletal arrangement, generally placing the least electronegative non-hydrogen atom in the center, with other atoms arranged symmetrically around it (unless the molecular formula or a stated connectivity indicates otherwise). Electrons: count the total number of valence electrons available across the entire molecule, by summing the valence electron count (equal to each element's main group number for representative elements) for every atom present. For a polyatomic ion, add one additional electron for each unit of negative charge, or subtract one electron for each unit of positive charge, since the ion's overall charge represents electrons gained or lost relative to the neutral atoms.
For CO₂: carbon contributes 4 valence electrons, and each oxygen contributes 6, for a total of 4 + 6 + 6 = 16 valence electrons to distribute across the whole structure.
C-S
Connect and Satisfy — the final two steps
Connect: draw a single bond (using 2 electrons from the total count) between the central atom and each surrounding atom, based on the skeleton sketched in step one. Satisfy: distribute the remaining electrons as lone pairs, starting with the outer (terminal) atoms, working to give each atom a complete octet (or, for hydrogen, exactly 2 electrons total). If, after distributing all lone pairs, the central atom still doesn't have a complete octet, convert one or more lone pairs on a surrounding atom into an additional bond (forming a double or triple bond) to share more electron density with the central atom, until every atom in the structure has a satisfied octet (or duet, for hydrogen).
For CO₂, after placing two single C-O bonds (using 4 of the 16 electrons) and giving each oxygen three lone pairs to complete their octets (using 12 more electrons, for 16 total), carbon only has 4 electrons around it — not a full octet — so one lone pair from each oxygen is converted into an additional bond, producing two C=O double bonds and giving carbon a complete octet.
Res/Exc
Resonance and exceptions to the octet rule
Some molecules and polyatomic ions can be drawn with more than one equally valid Lewis structure, differing only in which specific atoms carry a double bond versus a single bond and lone pair. When this happens, the true structure is understood to be a resonance hybrid — an actual, real molecule whose true electron distribution is a blend, or average, of all the valid resonance structures, rather than the molecule truly flipping back and forth between the different drawn versions. The classic example is the nitrate ion, NO₃⁻, which has three equally valid resonance structures, each with the double bond on a different one of the three oxygen atoms — reflecting the fact that, in reality, all three N-O bonds in nitrate are experimentally identical in length, intermediate between a single and a double bond.
The octet rule also has known, systematic exceptions: some molecules (like BF₃, or other compounds with boron or beryllium as the central atom) are stable with fewer than eight electrons around the central atom, called an incomplete octet. Some molecules (particularly those with a central atom from period 3 or below, like PCl₅ or SF₆) can accommodate more than eight electrons around the central atom, called an expanded octet, made possible by the availability of empty d orbitals in those larger atoms. Molecules with an odd total number of valence electrons (like NO, nitric oxide) cannot satisfy the octet rule for every atom at all, since electrons must be paired to form complete octets.
SF₆ (sulfur hexafluoride) has sulfur bonded to six fluorine atoms, requiring 12 electrons around the central sulfur atom — a clear expanded octet, made possible because sulfur is in period 3 and has accessible d orbitals unavailable to period 2 elements.
🔬 Applied Scenario — Building Lewis Structures for Common Molecules
Working through a range of real molecules and ions shows how the SECS method, resonance, and octet exceptions all fit together in practice.
A
Water, H₂O — a straightforward application. Oxygen (6 valence electrons) is central (hydrogen can never be central), with two hydrogen atoms attached by single bonds (using 4 electrons total). The remaining 4 electrons form two lone pairs on oxygen, giving oxygen a complete octet and each hydrogen its required 2 electrons — no multiple bonds needed.
B
The nitrate ion, NO₃⁻ — a resonance example. After following the SECS method, nitrogen ends up needing one double bond to one of the three oxygen atoms to complete its octet, but there is no chemical reason to prefer any one specific oxygen for that double bond over the other two — this ambiguity is resolved by recognizing all three possible structures as valid resonance forms, with the true molecule being an equal blend of all three.
C
Boron trifluoride, BF₃ — an incomplete octet exception. Following the SECS method for BF₃ (boron contributes 3 valence electrons, each fluorine contributes 7, for 24 total) produces a stable structure with boron surrounded by only 6 electrons (three single bonds, no lone pairs on boron) rather than a full octet — this is a recognized, stable exception, not an error in the method, and it's exactly why BF₃ readily acts as a Lewis acid, as covered in the Acid-Base Theory lesson.
D
Formal charge as a tool for choosing between multiple valid structures. When more than one Lewis structure seems chemically plausible for the same molecule, calculating the formal charge on each atom (formal charge = valence electrons − nonbonding electrons − half of bonding electrons) helps identify the most realistic structure — the preferred structure generally minimizes formal charges overall, and places any necessary negative formal charge on the more electronegative atom.
⚠️ Most Common Lewis Structures Mistakes
Resonance structures are NOT the molecule switching back and forth between different forms — this is one of the most common misconceptions in this topic. Students sometimes describe resonance as if the actual molecule oscillates or alternates between the different drawn structures over time. In reality, the true molecule has a single, fixed, blended (hybrid) structure at all times — the multiple drawn resonance structures are just different ways of representing that one true, averaged electron distribution, none of which is individually 100% accurate on its own.
Multiple bonds count differently than students often expect when checking an atom's total electron count. A double bond contributes 4 electrons total to the atoms it connects (not 2), and a triple bond contributes 6. Students sometimes undercount electrons around an atom involved in a double or triple bond, incorrectly concluding the octet isn't satisfied when it actually is.
An incomplete or expanded octet is not a mistake in the Lewis structure — it's a legitimate, recognized exception for certain elements. Students sometimes keep trying to force a complete octet onto boron or beryllium (which are stable with fewer than 8 electrons) or refuse to place more than 8 electrons around large central atoms like sulfur or phosphorus (which can legitimately exceed the octet using available d orbitals) — both approaches produce an incorrect structure by fighting against a genuine, well-documented exception rather than a rule violation.
✓ Quick Self-Test
1. What do the four letters in the SECS method stand for, and what happens at each step?
2. What is the octet rule, and what is the exception for hydrogen?
3. What general rule of thumb determines which atom is placed in the center of a Lewis structure's skeleton?
4. What is a resonance structure, and what does it mean for the "true" structure of a molecule like nitrate (NO₃⁻)?
5. Name and briefly describe the three main types of exceptions to the octet rule.
Answers:
1. SECS stands for Structure (sketch the skeletal arrangement of atoms, typically with the least electronegative non-hydrogen atom central), Electrons (count the total valence electrons available across the whole molecule or ion), Connect (draw single bonds between the central atom and surrounding atoms), and Satisfy (distribute remaining electrons as lone pairs, converting lone pairs into additional bonds if needed to complete every atom's octet).
2. The octet rule states that most main-group atoms are most stable, and most likely to form, when surrounded by 8 valence electrons (through a combination of bonding and lone pairs), matching a stable noble gas electron configuration. Hydrogen is an exception — it only needs 2 electrons total to be stable, matching helium's configuration, not 8.
3. The least electronegative atom (excluding hydrogen, which can never be central since it only forms one bond) is generally placed in the center of the structure, with more electronegative atoms arranged around it.
4. A resonance structure is one of two or more equally valid Lewis structures that can be drawn for the same molecule or ion, differing only in which atoms carry double bonds versus single bonds and lone pairs. For nitrate (NO₃⁻), three valid resonance structures exist, and the true structure of the molecule is a resonance hybrid — a real, single, blended average of all three valid structures — not a molecule that flips or alternates between the different drawn versions.
5. The three main exceptions are: incomplete octets (some central atoms, like boron or beryllium, are stable with fewer than 8 electrons around them), expanded octets (central atoms from period 3 or below, like sulfur or phosphorus, can accommodate more than 8 electrons using available d orbitals), and odd-electron molecules (molecules with an odd total number of valence electrons, like NO, cannot have every atom satisfy the octet rule since electrons must be paired to form complete octets).