Answer
Heptalene: anti-aromatic
Azulene: aromatic
Pentalene: anti-aromatic
Work Step by Step
For compounds to be aromatic they have to pass three tests:
1) Planar - if not planar then they are non-aromatic
2) Hybridization - should be sp/ $sp^2$
3) Huckle's rule- for aromatic 4n+2= # pi electrons (should be a whole number when solved)
and for Anti-aromatic 4n = # pi electrons (should be a whole number when solved)
Heptalene is anti-aromatic because it has 12 pi electrons which give a whole number when 4n is solved for n.
Azulene is aromatic becasue it passes all the tests. Has 10 pi electrons which give a whole number when 4n+2 is solved for n.
Pentalene is anti-aromatic because it has 8 pi electrons which give a whole number when 4n is solved for n.
