Stable, shared octets: how to map the valence electrons.
Nonmetal atoms are reactive because their outer electron shells are incomplete. To become stable, they share valence electrons, and that shared attraction holds them together. A Lewis structure is a simple two-dimensional map of that sharing, drawn with dots and lines. By the end, you can draw one for any simple molecule and check it for octet stability.
Core Claims
- Lewis Map: Represents valence electrons as dots and bonds as lines to track electron sharing in covalent compounds.
- Octet & Duet Rules: Main-group nonmetal atoms share electrons to obtain a stable set of 8 valence electrons. Hydrogen only requires 2 (duet).
- Bond Orders: Single, double, and triple bonds share 2, 4, and 6 electrons respectively between two nuclei.
H₂O Lewis Structure Map
Retrieval Checklist
- Calculate total valence electron count for a molecule.
- Assemble Lewis structures using octet/duet constraints.
- Differentiate bonding pairs from lone pairs.
The drive toward stability.
Think of two volatile gases: hydrogen (H2) and oxygen (O2). Spark them, and they react violently to form water (H2O), a stable, calm liquid. The explanation lives in their outer shell configurations. Atoms are at their lowest energy and most stable when their outermost electron shell is completely filled.
For most main-group elements, a full outer shell requires 8 valence electrons. This tendency is the octet rule. Hydrogen is the primary exception, seeking 2 valence electrons under the duet rule. Because nonmetal atoms are both electronegative, neither can strip electrons completely from the other. Instead, they share pairs of valence electrons to satisfy their octets.
To model this sharing visually, we represent the outer-shell electrons as dots around the element's chemical symbol, drawing lines to show shared electron pairs. This two-dimensional diagram is a Lewis structure (or Lewis dot diagram).
The rules of the map.
To draw any Lewis structure, follow these five steps in order. Let's trace them for carbon dioxide (CO2):
1. Find the total valence pool
Sum the valence electrons from each atom. Carbon (Group 14) has 4; oxygen (Group 16) has 6. Total: 4 + (2 × 6) = 16 electrons. If you miscount this pool, your map will be chemically invalid from the start.
2. Set the central skeleton
The least electronegative nonmetal takes the center (hydrogen is never central; carbon is always central). Place carbon in the middle with the oxygens flanking it on either side.
3. Draw single bonds
Connect each outer atom to the center with a shared pair of electrons, represented by a single line. This line is a single bond (uses 2 electrons). Our skeleton O–C–O uses 4 electrons, leaving 12 electrons in the pool.
4. Fill the outer octets
Distribute remaining electrons as pairs of dots, called lone pairs, around outer atoms until each has 8 electrons. We place 3 lone pairs (6 electrons) on each oxygen, using up all 12 remaining electrons.
5. Satisfy the central atom
Check the central atom. Carbon currently has only two single bonds (4 electrons), which violates the octet rule. Because the pool is empty, the outer oxygens share lone pairs inward. Move one lone pair from each oxygen toward carbon. Each side now has two shared pairs: a double bond.
Not counting shared electrons toward an atom's octet. A bonding pair counts for both atoms that share it.
The Lewis structure lab
Select a molecule from the list. Click a bond to cycle through no bond, single, double, and triple. Click an atom to cycle through lone pairs (up to 3). Match the target electron count, satisfy every octet and duet, then check.
Can you place every bond and lone pair so each atom has a full shell?
Water H₂O
Oxygen acts as the central atom, bonding to two hydrogen atoms. Oxygen needs 8 outer electrons to satisfy its octet, while hydrogen needs only 2 (duet).
Click bonds and atoms to start building the structure.
- Bond (click to cycle: none, single, double, triple)
- Lone pair (click an atom to add one)
- No bond yet
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
The chemical tendency of atoms to seek a full outermost shell of 8 electrons is the . Hydrogen is the exception, seeking only 2 electrons to fill its shell, which is the . Valence electrons that are not shared in a bond sit as pairs of dots, called . A single line represents a , which shares 2 electrons. Carbon dioxide uses two pairs of lines to connect carbon to oxygen, forming a , sharing electrons total in each bond. Nitrogen gas uses three shared pairs, forming a .
Why can hydrogen never act as the central atom in a Lewis structure, and why can it never form double or triple bonds?
Explain it in plain English. Writing it out creates stronger memory hooks than just thinking about it.
Hydrogen's valence shell is the very first energy level, which has a maximum capacity of only 2 electrons (the duet rule). A single covalent bond already shares 2 electrons, which completely fills hydrogen's outer shell. A central hydrogen atom or multiple bonds would force hydrogen to share 4 or more electrons. This violates its fundamental 2-electron shell capacity.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
Nitrogen trichloride (NCl3) is a reactive, yellow oil. State total valence electrons and identify the central atom. Using the octet rule, deduce how many single bonds and lone pairs surround the central nitrogen.
- Valence electrons (1 mark): You find 26: nitrogen has 5, each chlorine has 7, so 5 + (3 × 7) = 26.
- Central atom (1 mark): You name nitrogen, because it is less electronegative than chlorine.
- Nitrogen's octet (1 mark): You say nitrogen forms 3 single bonds and holds 1 lone pair, for 8 electrons.
- Chlorine atoms (1 mark): You say each chlorine forms 1 bond and holds 3 lone pairs, for 8 electrons.
Carbon monoxide poisoning is a consequence of molecular mimicry. Carbon monoxide (CO) has a triple bond and a carbon lone pair. It mimics the geometry of molecular oxygen (O₂). It binds hemoglobin 200 times more tightly than oxygen, blocking vital cellular respiration.
Adaptive Competency Practice: HS-PS1-3
Test your mastery of Lewis dot diagrams, octet and duet rules, and molecular structures one question at a time. The adaptive engine identifies individual skill gaps, provides tutoring hints on missed questions, and awards an official authenticated completion receipt.