Question

what particle did Chadwick discovered in 1932 that was hard to detect because it was no electrical charge

Answer 1

Nuetrons.

An electron has a negative charge.
A proton has a positive charge.
Therefore if you can detect the presence of electrical charge, you could observe these two.

Not so with nuetrons. The absence of electrical charge in the neuron make it difficult to detect, but Chadwick used a radiation tube to detect the existence of the neuron