) 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.

uranium-238 i believe is your answer

in scientific notation, the value of the  speed of light in a vacuum

  will be written as 2.995 * 10^8. when this value is represented in the form of

  a * 10^b, then, a = 2.995 and b = 8.

the value of the speed of light in a vacuum given above can also be rounded up to approximately 300000000. in this case, the scientific notation will be written as 3 *10^8.