Transition Elements & Electron Configuration Worksheet printable pdf download

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Transition Elements & Electron Configuration: Understanding the Exceptions

The Aufbau Principle, Pauli Exclusion Principle and Hund’s Rule give us a set of rules to determine the order in which

electrons occupy the energy sub-levels and orbitals within each sub-level. These rules are consistent and clear cut

when it comes to determining the electron configurations for the representative elements (all the elements in groups

1, 2 and 13 through 18). These rules still hold true for the transition elements and inner transition elements, but

applying them is not as clear cut as it is with the representative elements. The exceptions for the inner transition

elements are very complex to understand and beyond the scope of an introductory college chemistry course.

However, we can figure out the exceptions for the transition elements with two additional pieces of information.

The ns and (n-1)d sub-levels are very close to each other in energy. In other words, there is not much

difference in energy between the 4s sub-level and the 3d sub-level. The same holds true for the 5s and 4d

sub-levels, the 6s and 5d sub-levels, and the 7s and 6d sub-levels.

Half-filled and completely filled sub-levels are more stable than sub-levels that are not half-filled or

completely filled.

This extra stability for half-filled sub-levels coupled with small energy difference between the ns and (n-1)d sub-levels

results in electrons sometimes occupying the (n-1)d sub-level before the ns sub-level is completely filled. Specifically,

this occurs in transition metals when leaving the ns sub-level half-filled results in a half-filled or completely filled (n-

1)d sublevel.

Let’s look at the two exceptions that occur in the fourth period (aka row) of the periodic table. We would expect

chromium (Cr) to have an electron configuration of [Ar]4s

, but its actual electron configuration is [Ar]4s

The expected electron configuration for chromium would leave the 3d sub-level less than half-filled.

↑↓

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The actual electron configuration for chromium results in half-filled 4s and 3d sub-levels, which is more stable than

the expected configuration.

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The other exception in the fourth period occurs with copper (Cu). We would expect copper (Cu) to have an electron

configuration of [Ar]4s

, but its actual electron configuration is [Ar]4s

The expected electron configuration for copper would leave the 3d sub-level more than half-filled, but not

completely filled.

↑↓

↑

The actual electron configuration for chromium results in a half-filled 4s sub-level and a completely filled 3d sub-

level, which is more stable than the expected configuration.

↑

↑↓

Transition Elements & Electron Configuration Worksheet