Shell Model of Carbon Dioxide

Let's look at the covalent bonds within a carbon dioxide molecule.

Shell model of carbon dioxide molecule

The carbon atom (in the middle) has four electrons in its outer shell.

The two oxygen atoms each has six electrons in their outer shells.

To complete their shells (which all atoms want to do), every one of these atoms needs to have 8 electrons in its outer shell.

So the central carbon needs to gain 4 electrons, and each oxygen atom needs to gain 2.

Because of the cloud-like nature of the electron, it can be in several places at once. An electron can move round two atoms at the same time. If an atom shares one of its electrons with another atom, BOTH atoms gain an electron, so filling a hole in their outer shells. So atoms join together to share pairs of electrons.

The carbon shares two of its electrons with each oxygen, so each oxygen gains two electrons and hence gains a full outer shell. Each oxygen shares two of its electrons with the carbon, so the carbon gains four electrons, and so gets a full outer shell too.

Each pair of electrons is called a covalent bond. So the carbon atom has 4 covalent bonds, two with each oxygen atom. We call these DOUBLE BONDs.