Solution:
The previous noble gas is argon (Ar) at the end of the third row. Its unchanging inner core will be present in all fourth-row atoms:

Ar:

Beyond this core, the filling diagram shows that iron will have two electrons in the 4s orbital and six elecrtons in the 3d oorbitals:

Fe:

The transition metals and inner transition metals sometimes deviate from this ideal filling scheme, by dropping one electron from the outer s orbital into the nearby d orbitals, or from the outermost d orbitals to an f orbital. This arises because half-filled shells, d and f , with one electron in each orbital, or filled d and f shells, are especially stable. If an atom is one electron short of these states, that is, d, f , d, or f , it can steal an electron from a nearby level and achieve the more stable arrangement. This is possible only because the s, d, and f orbitals have very nearly the same energy, particularly in later rows. Hence chromium (Cr) has the structure [Ar] 3d 4s instead of [Ar] 3d 4s, as would be expected from its place in the table, and gold (Au) has the structure [Xe] 4f 5d 6s, rather than [Xe] 4f 5d 6s. These minor irregularities are not nearly as important as is the understanding of the overall pattern of electron filling, and how it can be obtained from the table.