How can I determine if and when a molecule has or needs a resonance structure?

Hint:. Lewis dot structures also known as Lewis electron-dot formulas. It uses dots arranged around the chemical symbol for an element to represent the valence electron configuration of the atoms in the element.

Complete step by step answer:
- When we draw two or more than two Lewis dot structures for a molecule, which differs only in the locations of the electrons then we draw the resonance structure for those molecules. The actual structure is the resonance hybrid of all of the structures.
- Resonance structures are basically two or more Lewis structures which describes the delocalization of electrons in a molecule in the polyatomic ion. Not all the resonance structures are equivalent. For this we have to determine which one is the best and which one is describing the actual bonding.

Additional information:
- Negative resonance- It is shown by the substituents or groups which withdraw the electrons by the delocalization mechanism from the rest of the molecules and is denoted by –R.
- Positive resonance- It is shown by the substituents and groups which release electrons to the rest of the molecules by delocalization and it is denoted by +R.
- The higher the electron density on the compound and lower the electronegativity, higher will be the ability for +R groups.

Note: In general, when we have to solve the question from organic chemistry be careful with the inductive and resonance effect as in many of the cases resonance effect is stronger and in many cases inductive effect is stronger as compared to resonance effect.