Understanding the Electron Configuration of Technetium: [Kr] 4d^6 5s^1

Technetium (Tc) is a chemical element with an atomic number of 43, and its electron configuration is represented as [Kr] 4d^6 5s^1. This particular configuration arises from several principles and electronic rules governing the filling of atomic orbitals.

Atomic Number and Electron Count

Technetium (Tc) has an atomic number of 43, meaning it possesses 43 electrons. Following the principle that the total number of electrons must equal the atomic number, we can deduce that we need to account for all 43 electrons.

Noble Gas Core

The notation [Kr] signifies that the electron configuration of technetium starts from the nearest noble gas preceding it, which is krypton (Kr), with the electron configuration [Kr] 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6. This noble gas core accounts for the first 36 electrons of technetium.

Filling Order and the Aufbau Principle

According to the Aufbau principle, electrons fill the lowest energy orbitals first. The order of filling generally follows the sequence: 1s, 2s, 2p, 3s, 3p, 4s, 3d, and then 4p before moving to the 5s and 4d orbitals. Therefore, after the krypton core, the remaining 7 electrons fill the 4d and 5s orbitals.

Filling the Orbitals

The 5s orbital, which can hold 2 electrons, fills first. Hence, 2 electrons fill the 5s orbital: 5s^2 This leaves 5 electrons to place into the 4d orbital. 4d^5 However, in the case of transition metals, there is an energy stabilization that occurs when the 5s electron is reduced to 1 to allow for a half-filled 4d subshell, which is more stable. Therefore, the 5s orbital only contains 1 electron instead of 2, resulting in the configuration: 4d^6 5s^1

This explains why the electron configuration of technetium is written as [Kr] 4d^6 5s^1.

Stability and Half-Filled Subshells

The electron configuration [Kr] 4d^6 5s^1 is more stable because it allows for a half-filled 4d subshell (which can hold 10 electrons, but here only 6 are present), which is energetically favorable. This configuration provides additional stability to the atom.

In summary, the electron configuration [Kr] 4d^6 5s^1 is the most stable configuration for technetium due to the Aufbau principle and the additional stability provided by having a half-filled 4d subshell.

For a more detailed and comprehensive understanding of electron configurations, refer to authoritative sources such as the WebElements Periodic Table.