How many resonance forms can be written for the nitrate ion,$(NO_3^-)$?

$CH_2 = O \leftrightarrow \overset{\oplus}{C}H_2 - \overset{\ominus}{O} \leftrightarrow \overset{\ominus}{C}H_2 - \overset{\oplus}{O}$
Which of these structures is practically not a valid canonical structure for formaldehyde?

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

Which of the following are not resonance structures of each other?

What is the difference between the resonance structures of $CH_3CH_2COO^{-}$ and $CH_3COOH$?

Explain the structure of $CO_{2}$ molecule.