A Lewis structure allocates valence electrons among covalent bonds and lone pairs. It is a bookkeeping model for connectivity and valence electron distribution, not a literal picture of a molecule's three-dimensional shape.

For many main-group molecules, a useful first model is the octet rule: atoms tend to appear in structures with eight valence-shell electrons around them. Hydrogen is an important exception because its first shell is filled by two electrons.

To construct a simple Lewis structure:

1. Count all available valence electrons, adding electrons for negative charge and subtracting them for positive charge.
2. Choose a plausible connectivity and join bonded atoms with single bonds. Each bond uses two electrons.
3. Complete the valence shells of terminal atoms.
4. Place remaining electrons on the central atom.
5. If the central atom still lacks an octet, convert suitable neighboring lone pairs into multiple bonds.

For $CO_2$, the total is $4+2(6)=16$ valence electrons. Starting from $O-C-O$, completing both oxygen octets leaves carbon electron-deficient. Converting one lone pair from each oxygen into a bond gives

O=C=O

with two lone pairs on each oxygen and an octet around every atom.

The octet model is highly useful but not universal. Electron-deficient, odd-electron and expanded-valence species require explicit treatment beyond this basic construction method.