Individual atoms of nonmetals are unstable and reactive because their outer electron shells are incomplete. To become stable, they share valence electrons, forming a network of attraction that holds them together. To model and predict this electron sharing, we use a simple, two-dimensional map of dots and lines. By the end of this page, you'll be able to draw these maps for any simple molecule — and check them for octet 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 chemical tendency is the octet rule (hydrogen is the main exception, seeking only 2 valence electrons to fill its first and only shell, a tendency called the duet rule). Since nonmetal atoms are both highly 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).
To draw any Lewis structure, follow these five steps in order. Let's trace them for carbon dioxide (CO2):
Select a molecule from the list. Click on the bonds (dashes) to cycle through Single, Double, Triple, or No bonds. Click on the atoms (letters) to cycle through lone pairs (up to 3 pairs). Satisfy the target electron count and all octet/duet rules, then verify.
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).
Formaldehyde (CH2O) is a toxic chemical used to preserve specimens. Carbon is the central atom. How many total valence electrons must be drawn in its Lewis structure?
Total valence = 12 electrons. Carbon (Group 14) has 4. Oxygen (Group 16) has 6. Each hydrogen has 1. Math: 4 + 6 + (2 × 1) = 12. If your finished Lewis structure uses 10 or 14 electrons, it is chemically impossible for formaldehyde.
Methanol (CH3OH) has the skeleton shown below. The carbon has 4 single bonds, and oxygen is bonded to carbon and hydrogen. How many lone pairs must sit on the oxygen atom to satisfy the octet rule?
H — C(H)₂ — O — H
Oxygen needs 2 lone pairs. In the skeleton, oxygen shares two single bonds (one to carbon, one to hydrogen). This gives oxygen 4 shared electrons around it. To satisfy the octet rule (8 electrons total), oxygen needs 4 more electrons, which must sit as 2 unshared lone pairs. Carbon has 4 single bonds (8 electrons, full octet), and all hydrogens have 1 single bond (2 electrons, full duet).
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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. Forming double or triple bonds, or making hydrogen a central atom (which requires bonding to at least two other atoms, i.e., sharing 4 or more electrons), would force hydrogen to hold more than 2 electrons, violating its shell limit.
Give yourself a point for each idea you actually wrote down. The flag (⚑) marks the step that separates a complete answer from a partial one.
Nitrogen trichloride (NCl3) is a reactive, yellow oil. State its total number of valence electrons, predict its molecular arrangement (which atom sits in the center), and explain — using the octet rule — how many single bonds and lone pairs must sit on the central nitrogen atom in its correct Lewis structure.
Self-score: 4 = all four · 3 = missing chlorine details · 2 = valence count + nitrogen bonds, no EN/octet explanation · ≤1 = valence count only.
Carbon monoxide poisoning is a consequence of molecular mimicry. Carbon monoxide (CO) has a triple bond and a lone pair on carbon, structurally mimicking the size and shape of an oxygen molecule (O₂) so perfectly that it binds to hemoglobin 200 times more tightly and blocks oxygen transport in our blood.