8. The Machinery Behind The      Periodic Table   Previous PageNext Page
     Buildup of Atoms and the Periodic Table

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:


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


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.

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