Hii praveen...

Value of m for dz2 is 0 because  of following

m stands for magnetic qunatum number.

Magnetic quantum number explains orientation of 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

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