Electron Configuration

Electron configuration is a standard notation that describes the distribution of electrons that are present in the shells, subshells, and orbitals of an atom. The electron configuration is written according to the Aufbau principle.

Aufbau Principle

The Aufbau principle states that the orbitals are filled with electrons from the lowest energy levels to higher energy levels. That means the lower energy levels are filled first. According to the Aufbau principle, the pattern that fills the orbitals can be represented in Figure 01.

But there are some instances that some elements deviate from the Aufbau principle.

Cr and Cu deviate from the Aufbau principle. Because the d orbitals stabilize when they are completely or a half with electrons.

• Chromium obtains 4s¹ 3d⁵ configuration to stabilize with a half-filled orbital.
• Copper obtains 4s¹ 3d¹⁰ configuration to stabilize with completely filled orbitals.

Electron configuration of cations

Atoms form positively charged ions (cations) by removing electrons. When atoms remove 1, 2, or 3 electrons respectively, they will form +1, +2, or +3 ions respectively. Thus, the electrons are removed starting from the highest energy orbital in the highest energy level to the lowest.

As an example, Sodium (Na) removes one electron and forms a +1 charged ion. This electron is removed from the 3s orbital which is the highest energy orbital that Sodium has.

• Na - 1s² 2s² 2p⁶ 3s¹
• Na⁺ - 1s² 2s² 2p⁶

But there are some atoms where the d orbitals are filled after the s orbital of the next energy level is filled. As an example, in copper atoms, the 3d orbital is filled after the 4s orbital. In such cases, electrons are removed from the 4s orbital first.

• Cu - 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰
• Cu²⁺ - 1s² 2s² 2p⁶ 3s² 3p⁶ 4s⁰ 3d⁹

Electron configuration of anions

Atoms form negatively charged ions (anions) by gaining electrons. When atoms remove 1, 2, or 3 electrons respectively, they will form -1, -2, or -3 ions respectively. Electrons are filled to the last subshell of the valance shell.

• O - 1s² 2s² 2p⁴
• O^2- - 1s² 2s² 2p⁶