) Why is the Lewis structure for CO2 usually written as O = C = O rather than O ≡ C - O ?

The Lewis structure for CO2 is O = C = O rather than  O ≡ C - O because of the covalent nature of the bonds that makes CO2 a polar molecule and to become stable it is important to fill valence shell of both atoms equally.

CO2 can not be written as O ≡ C - O because then there will be one non-paired electron in the oxygen atom that can disturb the stability of CO2.

particle   p is an electron and particle q is a neutron