A particle that contains 9 protons, 10 neutrons and 10 electrons is a

Explanation

If it were a neutral atom,the number of proton would have been equal to number of electron = 9 each

However, since the number of electrons is by 1 greater than that of proton, and since electrons having more number than proton are negatively charged, the particle must be a negative ion.

There is an explanation video available .