Here's why:

* Principal Quantum Number (n): This number describes the energy level of an electron. It can be any positive integer (1, 2, 3, etc.). Higher values of n correspond to higher energy levels.

* Angular Momentum Quantum Number (l): This number describes the shape of an electron's orbital. It can range from 0 to n-1.

* l = 0 corresponds to an s orbital (spherical shape)

* l = 1 corresponds to a p orbital (dumbbell shape)

* l = 2 corresponds to a d orbital (more complex shape)

* l = 3 corresponds to an f orbital (even more complex shape)

In the case of the 3d sublevel:

* The "3" indicates that the electrons in this sublevel are in the third energy level (n = 3).

* The "d" indicates that the sublevel has l = 2.

So, for any d sublevel, the value of n is always one greater than the value of l.