Resonance structures of propenal are given below. Which of these resonating structures is more stable? Give reason for your answer.
$CH_2=CH-CH=O$ $(I)$ $\leftrightarrow$ $\stackrel{+}{CH_2}-CH=CH-\stackrel{-}{O}$ $(II)$

(A) Structure $(I)$ is more stable than structure $(II)$ due to the following reasons:
$1$. Structure $(I)$ is a neutral molecule with no charge separation,whereas structure $(II)$ involves charge separation,which requires energy and decreases stability.
$2$. Structure $(I)$ has more covalent bonds (total $9$ bonds: $7$ $\sigma$ and $2$ $\pi$) compared to structure $(II)$ (total $8$ bonds: $7$ $\sigma$ and $1$ $\pi$). Structures with more covalent bonds are generally more stable.
$3$. In structure $(I)$,all carbon and oxygen atoms have complete octets. In structure $(II)$,the terminal carbon atom has a positive charge and an incomplete octet (only $6$ electrons),making it less stable.