Although both CO2 and H2O are triatomic molecules, the shape of H2O molecule is bent while that of CO2 is linear.

CO₂ molecule has only two bonds which lie farthest to each other to minimise repulsion, making an angle of 180°. 

H₂O, on the other hand, has a lone pair. As lone pair- bond pair repulsion is greater than bond pairbond pair repulsion, the repulsions due to the lone pair decreases the angle between the two bonds, giving the molecule a bent shape.

In CO₂ , there are two C = O bonds. Each C = O bond is a polar bond. The net dipole moment of CO molecule is zero. This is possible only if CO₂ is a linear molecule. (O = C = O). The bond dipoles of two C = O bonds cancels the dipole moment of each other.

Whereas, H₂O molecule has a net dipole moment (1.84 D). H₂O molecule has a bent structure because here the O – H bonds are oriented at an angle of 104.5° and do not cancel the bond dipole moments of each other.