Why compounds like 1,3-cyclobutadiene, 1,3,5,7- cycloctatetrane and ions like cyclopropenyl anion, cyclopentadienyl cation are not aromatic?

Explanation

Though the above species appear to be aromati, they are not aromatic. They are cyclic compounds almost all carbons are `sp^(2)` hybridised and also they exhibit reasonance.
In these compounds are unstable and are called antiaromatic compounds.