Carbon dioxide

CO₂ · carbon dioxide

Two C=O double bonds satisfy every octet with zero formal charges. The two bond dipoles are equal and opposite, so the linear molecule is nonpolar overall.

Valence electrons: 16
Charge: 0
Lone pairs: 4
Molecular geometry: linear

Lines represent covalent bonds (shared electron pairs); dots represent lone pairs (nonbonding electrons).
A single bond is one shared pair of electrons, a double bond two shared pairs, and a triple bond three shared pairs.
Each atom in a bond contributes to the shared pair and counts the shared electrons toward its own octet. The atoms do not each gain two extra electrons; they share the same pair, and both atoms count it.

Count

A must account for exactly the , so total them first: carbon is a Group 14 element, so it has 14 - 10 = 4 valence electrons; each oxygen is a Group 16 element, so it has 16 - 10 = 6 valence electrons, and with 2 of them that is 6 × 2 = 12, giving 16 valence electrons that the finished structure has to use in full, with none added and none left over.

Build

Carbon is the . With four it can form four bonds, more than any other present, so it is best suited to connect to several neighbors at once. Hydrogen cannot be central, because a single valence electron lets it form only one bond.
Connect the to every with a . Because each single bond represents one shared electron pair (2 electrons), the 2 bonds use 4 electrons, leaving 12 of the original 16 to place.
The terminal O are filled to an next: 6 among them, using 12 electrons and reducing the number still available to 0.
With the terminal octets complete, no electrons remain, so C receives no at this stage.
C is left with only 4 electrons, below an , and every has already been placed. Rather than adding electrons, 2 already on the terminal are redrawn as 2 shared pairs, turning 2 single bonds into double bonds. Because both atoms count the , C now reaches a full octet without changing the total of 16.

Verify

Now verify the electron count on every , terminal atoms included, not only the . C: 8 shared + 0 nonbonding = 8 (a full ); each O: 4 shared + 4 nonbonding = 8 (a full octet).
Compute each 's (, less , less half the ). Formal charge is a bookkeeping quantity that assumes perfectly equal sharing; it is not the atom's real charge. Across all 3 atoms the value is zero, the most stable possible distribution.

Watch out

The wrong version of CO2 is all single bonds with a starving C in the middle. If your is short, don't hunt for spare electrons: convert a neighbor's into a shared one.

Shape

The follows from the around C: 2 bonding domains and 0 total 2 domains. Mutual repulsion spreads 2 domains into linear , and that domain count sets the at sp.
Every domain carries an here, so and agree: linear, 180° angles throughout.
: each C-O bond is genuinely polar on its own, but the arrangement points those pulls symmetrically against each other. They cancel to nothing. Nonpolar overall.

Worth knowing

Two bonds, zero polar : the shape points them dead opposite so they cancel. Solid CO2 is dry ice, which skips melting entirely and jumps straight from solid to gas at -78 degrees C.