An atom’s electron configuration describes the arrangement of its electrons within the various energy levels and subshells surrounding the nucleus. This organization dictates how an element interacts with others, influencing its chemical behavior and physical properties. Molybdenum (Mo), a transition metal, has an atomic number of 42, meaning a neutral atom contains 42 electrons. Determining the precise placement of these electrons is necessary to understand the element’s unique chemistry.
The Foundation of Electron Configuration
The theoretical placement of electrons in any atom follows established rules that minimize the atom’s overall energy. Atomic structure is defined by principal energy levels (\(n\)), which contain subshells designated by \(s\), \(p\), \(d\), and \(f\). These subshells hold a maximum of 2, 6, 10, and 14 electrons, respectively.
The Aufbau principle dictates the order for filling subshells, requiring electrons to occupy the lowest available energy levels first. This establishes a standard filling sequence (e.g., \(1s\), \(2s\), \(2p\), \(3s\), \(3p\)). Hund’s rule applies when filling orbitals of equal energy, requiring electrons to fill each orbital singly before pairing.
The Pauli Exclusion Principle ensures that a single orbital holds a maximum of two electrons with opposite spins. By applying these three rules, scientists can predict the electron configuration for the majority of elements. This systematic approach provides a framework for understanding the electronic structure that governs chemical bonding and reactivity.
Determining Molybdenum’s Configuration
To determine the electron configuration for Molybdenum (42 electrons), we use the noble gas notation, where \([\text{Kr}]\) accounts for the first 36 electrons. The remaining six electrons must be placed into the subsequent subshells according to the expected energy order.
The standard order predicts that the \(5s\) subshell fills before the \(4d\). Following conventional rules, the first two remaining electrons would occupy \(5s^2\), and the final four would enter the \(4d\) subshell. This leads to the theoretically expected configuration of \([\text{Kr}] 5s^2 4d^4\).
However, the actual, experimentally observed ground-state configuration for Molybdenum deviates from this prediction. The true electron configuration is \([\text{Kr}] 5s^1 4d^5\), showing one electron in the \(5s\) subshell and five electrons in the \(4d\) subshell. This arrangement accounts for all 42 electrons and minimizes the atom’s total energy.
The Stability Anomaly
The deviation from the predicted configuration results from energetic factors that favor enhanced stability. Subshells that are exactly half-filled (\(d^5\)) or completely filled (\(d^{10}\)) possess a lower energy state than partially filled subshells. This stability arises from electron exchange energy and spherical symmetry.
The expected configuration, \(5s^2 4d^4\), is one electron away from achieving the highly stable half-filled \(4d^5\) state. Since the energy difference between the \(5s\) and \(4d\) orbitals is small, an electron is promoted from \(5s\) to \(4d\). This promotion results in the configuration \(5s^1 4d^5\).
The \(5s^1 4d^5\) configuration is more stable because achieving the half-filled \(d\) subshell outweighs the energetic cost of having a half-filled \(s\) subshell. This rearrangement provides a net gain in stability for the atom. Molybdenum, along with Chromium, exhibits this anomalous behavior to achieve a lower overall energy state.