Nickel (Ni) is a metallic element classified as a transition metal, often used in coins, alloys, and rechargeable batteries. Its atomic number dictates the number of electrons it contains. For a neutral nickel atom, the total electron count is 28.
The Total Electron Count in Neutral Nickel
The number of electrons in any neutral atom is determined by its atomic number (Z). The atomic number of Nickel (Ni) is 28, which means its nucleus contains 28 protons.
In an electrically neutral atom, the total positive charge must be perfectly balanced by an equal negative charge. Electrons are the negatively charged subatomic particles. Therefore, a neutral nickel atom must possess exactly 28 electrons to counteract the 28 positive charges from the protons.
Understanding Electron Arrangement
While the total number of electrons is 28, their arrangement around the nucleus is complex and highly specific. These electrons exist in distinct energy levels (shells) and sub-levels (orbitals). The electron configuration describes the precise distribution of these 28 electrons in a neutral nickel atom.
The full configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^8\), though it is often written in shorthand as \([Ar] 4s^2 3d^8\). The \([Ar]\) notation represents the 18 electrons of the preceding noble gas, Argon. The remaining ten electrons are distributed in the \(4s\) and \(3d\) orbitals.
The \(4s\) orbital is filled before the \(3d\) orbital, following the Aufbau principle. This rule states that electrons will occupy the lowest energy orbitals first, and for neutral atoms, the \(4s\) orbital is slightly lower in energy than the \(3d\) orbital. The \(4s\) orbital is complete with two electrons, and the \(3d\) subshell contains eight electrons, which results in the \(3d^8 4s^2\) arrangement.
The \(d\) subshell has five distinct \(d\) orbitals, which can hold a maximum of ten electrons. Hund’s rule mandates that electrons will first occupy separate \(d\) orbitals before pairing up. The eight electrons in the \(3d\) subshell fill four of the five orbitals completely, leaving two electrons unpaired.
When Nickel Forms Ions
Nickel readily forms positively charged ions (cations) by losing electrons. The most common ions formed are nickel(II) (\(\text{Ni}^{2+}\)) and nickel(III) (\(\text{Ni}^{3+}\)). These ions have fewer electrons than the neutral atom.
The \(\text{Ni}^{2+}\) ion forms by losing two electrons from the neutral atom. Since the neutral atom starts with 28 electrons, the \(\text{Ni}^{2+}\) ion has 26 electrons. Electrons are lost from the \(4s\) orbital first, even though the \(3d\) orbital was filled later.
The \(4s\) orbital is spatially farther from the nucleus than the \(3d\) orbital. Once the \(3d\) orbital begins to fill, its electrons effectively shield the \(4s\) electrons from the nucleus’s attractive pull, making them the easiest to remove. The electron configuration for \(\text{Ni}^{2+}\) becomes \([Ar] 3d^8\), with the \(4s\) orbital empty.
The less common \(\text{Ni}^{3+}\) ion is formed by removing a third electron in addition to the two \(4s\) electrons. This third electron is removed from the \(3d\) subshell. This ion therefore possesses 25 electrons, and its electron configuration is \([Ar] 3d^7\).