How the value of m is zero for dz²?
Value of m for dz2 is 0 because of the following:-
m stands for magnetic quantum number.
Magnetic quantum number explains the orientation of the orbit in the space.
Values of m for other subshells -
For d(xy), m = -2
For d(yz), m = -1
For d(z2), m = 0
For d(zx), m = +1
For d(x2-y2), m = +2