Molecules are aromatic if they are

(1) Cyclic

(2) Planar

(3) Conjugated

(4) Molecules who have (4n + 2) $$\pi$$ electrons [This is called Huckel's Rule]

Aromatic compounds are highly stable.

(a)

(1) Molecule is cyclic.

(2) All atoms which are completing the cyclic ring have sp² hybridisation. So it is shape is planar. If any one atom is sp³ hybridised then molecule shape will be non planar.

(3) It is conjugated means all $$\pi$$ bonds are showing resonance with each other.

So all atoms which are forming cyclic ring are perticipating in resonance. That is called conjugated.

(b)

(1) This compounds is cyclic

(2) All atoms are sp² hybridised. So molecule shape is planar.

(3) It is conjugated as all $$\pi$$ bonds are showing resonance.

(4) It has 6$$\pi$$e^$$-$$. So obey Huckel Rule.

$$\therefore$$ Compound is aromatic.

(c)

(1) This compound is cyclic

(2) All atoms are sp² hybridised. So molecule shape is planar.

(3) It is conjugated as $$\pi$$ bond is showing resonance.

(4) It has 2$$\pi$$e^$$-$$. So obey Huckel Rule.

$$\therefore$$ Compound is aromatic.

Anti-Aromatic :-

It is highly unstable.

Anti-aromatic compounds are -

(1) Cyclic

(2) All atoms which is forming cyclic ring are planar.

(3) it is conjugated.

(4) It has 4n$$\pi$$e^$$-$$.

Here no anti-aromatic compounds presents.

Non-Aromatic :-

Those compounds which are not either aromatic or anti-aromatic.

Definition : All cyclic compounds which are comparatively stable with its open chain analogous system, compound is known as non aromatic compound.

(a)

Here one carbon is sp³ hybridised that is why it's structure is non planar. So this cyclic compound will be neither aromatic nor anti-aromatic. It will be non aromatic.

Multiple cyclic rings :-

To find nature of this ring, make maximum possible $$\pi$$ bonds in the outer ring. This is a hypothetical structure. Here outer ring should be largest conjugated ring means outer ring should have these $$\pi$$ bonds which are in resonance with each other.

And the nature of the outer ring will be same as the original molecule.

(d)

From this structure we can't decide the nature as all $$\pi$$ bonds are not present in the outer ring that is why we make right resonating structure where all $$\pi$$ bonds present in the outer ring.

Outer ring :

As outer ring is aromatic so original compound will also be aromatic.

Note :-

Even though from left resonating structure we can't decide the nature of the molecule, it does not mean it's nature is not aromatic. As different resonating structures don't have different nature. All resonating structure have same nature.