Electron configuration describes the arrangement of electrons within an atom’s orbitals, providing a foundational understanding of its chemical behavior. This structure dictates how the atom interacts with others, as elements seek the lowest possible energy state. Gallium (Ga), element number 31, is a metal that commonly forms the \(Ga^{3+}\) cation by losing three electrons. Determining the electron configuration of this ion requires applying the rules that govern electron arrangement and removal. This process reveals the stable electronic structure Gallium adopts when bonding.
The Electron Configuration of Neutral Gallium
A neutral Gallium atom contains 31 electrons, distributed among its energy levels and sublevels according to established principles. The Aufbau principle dictates that electrons fill the lowest-energy orbitals first, while Hund’s rule governs how electrons occupy orbitals within the same subshell. Following this organized filling order leads to the full ground-state configuration for neutral Gallium.
The complete configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^1\). This notation shows that the first 30 electrons fill the first three principal energy levels (\(n=1, 2, 3\)) and the \(4s\) and \(3d\) sublevels. The final electron occupies the \(4p\) subshell, making the fourth principal quantum number (\(n=4\)) the valence shell. The inner core of 18 electrons matches the noble gas Argon, allowing the condensed notation \([Ar] 4s^2 3d^{10} 4p^1\).
Principles of Cation Formation
Forming a positive ion, or cation, involves removing one or more electrons from the neutral atom. The order of electron filling (based on energy) is not the same as the order of electron removal, which is a common point of confusion. When an atom ionizes, electrons are always removed from the orbital with the highest principal quantum number (\(n\)) first. This occurs because these electrons are furthest from the nucleus and experience the weakest attractive force.
In Gallium’s case, the highest principal quantum number is \(n=4\), containing the \(4s\) and \(4p\) orbitals. The electrons in the \(4p^1\) and \(4s^2\) subshells are the valence electrons and will be the first ones to leave. Gallium typically forms a \(+3\) ion by losing three electrons, which allows it to achieve a stable configuration. Losing these three valence electrons (\(4s^2\) and \(4p^1\)) leaves behind a completely filled set of inner orbitals. This arrangement is known as a pseudo-noble gas configuration, which increases stability.
Determining the Final Gallium Ion Configuration
Applying the cation formation principles to neutral Gallium’s configuration dictates the removal of the three valence electrons. The neutral configuration is \([Ar] 3d^{10} 4s^2 4p^1\). The single electron in the \(4p\) subshell has the highest principal quantum number, so it is removed first.
The next two electrons removed are from the \(4s\) subshell, as \(n=4\) is still the highest principal quantum number remaining. Removing these three electrons results in the \(Ga^{3+}\) ion, which now has 28 electrons. The resulting full configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10}\). The condensed notation for the Gallium ion is \([Ar] 3d^{10}\).
This final configuration shows a completely filled \(3d\) subshell, representing the highly stable pseudo-noble gas electron arrangement. The \(Ga^{3+}\) ion has an electron count matching the neutral Zinc atom (element 30). However, its configuration is characterized by the full \(3d\) shell, which provides the stability driving Gallium’s chemical behavior.