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Lessons · Bonding & structure

Polar bonds, nonpolar molecule: why CO₂ is linear

Electron ledger machine-checked — valence total, octets, formal charges, all re-derived by ChemKernelGeometry rule-sourced (openstax-chemistry-2e)3 modeling assumptions (disclosed)

Carbon dioxide has two carbon–oxygen bonds, and oxygen pulls electrons hard, so each bond is polar. It seems obvious the molecule should be polar too. It isn't — CO₂ is perfectly nonpolar, and it comes down to shape. Water, built from the same 'one central atom, two outer atoms' recipe, is bent and polar; CO₂ is straight and nonpolar. We'll build CO₂ one accounting step at a time and find exactly where the two stories split — and why polar bonds do not add up to a polar molecule.

Molecule
CO2\mathrm{CO_{2}}
16 valence electrons
Shape (VSEPR) sourced
linear
2 domains · 180°
Polarity model
nonpolar
from the shape + bond dipoles
  1. 1

    Count the valence electrons

    machine-checked

    C 1×4 = 4 + O 2×6 = 12 → 16 valence electrons

    Count the valence electrons — the outer-shell electrons that do the bonding. Carbon brings 4; each oxygen brings 6, for 12; the total is 16 valence electrons, or eight pairs, to place. (No charge to add or subtract — CO₂ is a neutral molecule.) The engine derives this total from each atom's group in the periodic table and checks every later step against it.

  2. 2

    Build the Lewis structure

    machine-checked

    8 bonding (4 shared pairs) + 8 nonbonding (4 lone pairs) = 16 ✓

    AtomLone pairsBonds (Σ order)Formal charge
    CC1040
    OO1220
    OO2220

    Put carbon in the middle and try single bonds first: C–O–?—no. With one shared pair to each oxygen, carbon would have only 4 electrons around it, far short of an octet. The fix is double bonds: each C=O shares two pairs. Now carbon has two double bonds = four bonding pairs = 8 electrons, a full octet; each oxygen has one double bond (2 pairs) plus two lone pairs, also 8. Every formal charge is 0, and they sum to 0. The ledger closes exactly: 8 bonding electrons (4 shared pairs) + 8 nonbonding (4 lone pairs) = the 16 we counted. ChemKernel refuses any structure whose electrons don't conserve or whose atoms miss their octets — so the double bonds aren't a guess, they're forced.

  3. 3

    Predict the shape (VSEPR)

    rule-sourced

    2 electron domains (2 bonding + 0 lone) → electron geometry linear, molecular shape linear.

    ideal angle 180°

    Now the shape — and the split from water. VSEPR counts electron domains on the central atom, where a double bond still counts as one domain (it's one region of electrons, just a denser one). Carbon has two domains (two C=O bonds) and — crucially — no lone pairs. Two domains get as far apart as possible by pointing in exactly opposite directions: linear, 180°. This is where CO₂ and H₂O diverge. Both have a central atom bonded to two others, but water's oxygen carries two lone pairs (4 domains → bent); carbon carries none (2 domains → linear). The lone pairs — present in water, absent here — decide the shape.

  4. 4

    Decide the polarity

    model-assumed
    • C=O ×2ΔEN 0.89 · polar covalent

    Net: nonpolar — The molecule is linear, so the two equal, opposite C=O bond dipoles cancel exactly — polar bonds, but no net molecular dipole.

    Each C=O bond really is polar: oxygen is more electronegative than carbon (ΔEN 0.89), so it pulls the shared electrons and each bond becomes an arrow pointing toward its oxygen. But the two arrows point in exactly opposite directions along the straight line, and they're equal, so they cancel: the net dipole is zero. CO₂ is nonpolar — polar bonds, no molecular dipole. That is the whole lesson: polarity is decided by the bond dipoles and the geometry that arranges them, not by the bonds alone. Water, bent, cannot cancel its dipoles and so is polar; CO₂, straight and symmetric, cancels its own and is not. (This is why CO₂ dissolves poorly in water, and why it's a gas at room temperature while water — polar, hydrogen-bonded — is a liquid.)

VerificationEvery claim below was proven at build time — not asserted.
  • ✓ Electrons conserved: 8 bonding + 8 nonbonding = 16 valence [electron ledger]
  • ✓ Every atom completes its shell — octet (a duet for H) [octet check]
  • ✓ Formal charges sum to the molecular charge (0) [formal-charge sum]
  • ✓ Electron-domain count keys the sourced VSEPR geometry [VSEPR table]
✗ Common misconception: “CO₂ has two polar C=O bonds and oxygen is strongly electron-pulling, so the molecule must be polar.”

Polar bonds don't add up to a polar molecule — the geometry has the final say. Each bond here is polar (ΔEN 0.89), but the central atom has no lone pairs, so the molecule is linear (2 domains). Its equal bond dipoles point in exactly opposite directions and cancel — the net dipole is zero, so the molecule is nonpolar. A bent molecule (like water) can't cancel its dipoles and is polar; a symmetric one cancels its own and isn't.

Modeling assumptions — author-asserted, disclosed not discharged
  • model The Lewis localized-pair model: the valence electrons sit in shared bonding pairs (a double bond is two shared pairs between the same two atoms) or in lone pairs on a single atom. A model of where electrons are — predictive, not a photograph.
  • model VSEPR: electron domains repel and spread as far apart as possible, setting the geometry — and a double bond counts as a single domain (one region of electron density). Two domains → linear at 180°.
  • model Molecular polarity is the vector sum of the bond dipoles over the geometry. In a linear, symmetric molecule two equal bond dipoles point exactly opposite and cancel to zero, so the molecule is nonpolar even though each bond is polar.

Concepts in this lesson

Linked into the Chemical Atlas where an entry exists; the rest fill in as the Atlas grows.

Practice this

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