Molecules having all their subshells completely filled are diamagnetic, i.e., they are not influenced greatly by a magnetic field. Paramagnetic is the opposite and it is the nature of elements possesing incompletely filled subshell(s).
The Lewis structure of NO molecule can be represented as
: N . = : O : OR : N : = . O :
It is observed that the total no. of unbonded electrons is odd. Therefore, there must be an incompletely filled subshell. Therefore, it is paramagnetic.
In NO+ , due to loss of 1 electron, the no. of unbonded electrons becomes even. Therefore, all subshells must be completely filled. Therefore, it is diamagnetic.