What is the characteristic of the phospholipid bilayer that allows uncharged small molecules can pass through

such as oxygen and carbon dioxide.
Bio homework.
Help please.

Those molecules along with other small uncharged molecules can pass directly through the bilayer, as they are small enough to fit through the gaps between the phospholipid molecules, and are uncharged, so won't interact with the hydrophilic polar head of the phospholipid. They can go through the bilayer down a diffusion gradient. The characteristics of the bilayer are that it's got a hydrophilic head part and a hydrophobic tail, so they can prevent other charged molecules which may be harmful from entering the cell.