Which molecule would you expect to have a linear shape based on its dot and cross diagram?

The linear shape of a molecule is determined primarily by the arrangement of its atoms and the regions of electron density around the central atom. In the case of carbon dioxide (CO₂), the central carbon atom is bonded to two oxygen atoms with double bonds. The molecule has no lone pairs on the carbon atom, resulting in a total of two regions of electron density.

According to the VSEPR (Valence Shell Electron Pair Repulsion) theory, these regions of electron density will arrange themselves as far apart as possible to minimize repulsion. With only two bonds and no lone pairs to account for, the molecule adopts a linear geometry, with bond angles of 180 degrees. This spatial arrangement results in the molecule being straight, with both oxygen atoms located on opposite sides of the carbon atom.

In contrast, the other molecules mentioned possess configurations that lead to different geometries. Water has a bent shape due to the presence of two lone pairs on the oxygen, ammonia has a trigonal pyramidal shape because of three bonds and one lone pair, and methane has a tetrahedral shape with four bond pairs and no lone pairs. Therefore, the linear shape of carbon dioxide is uniquely attributed to its specific bonding and electron arrangements.