Learning Outcomes:
i. Explain the concept of the Aufbau principle and its application in determining electron configurations.
ii. Identify the exceptions to the Aufbau principle observed in chromium and copper.
iii. Analyze the factors that contribute to the deviations in the electron configurations of chromium and copper.
iv. Apply the concept of orbital stability to explain the unusual electron configurations of these elements.
Introduction
The Aufbau principle, a fundamental concept in chemistry, dictates the filling of electrons in orbitals based on their energy levels. However, certain elements exhibit exceptions to this principle, particularly among the d-block transition metals. In this lesson, we will focus on the anomalies in the electron configurations of chromium and copper, exploring the reasons behind these deviations from the expected Aufbau pattern.
i. Aufbau Principle and Its Application
The Aufbau principle, also known as the Aufbau ordering rule, states that electrons occupy orbitals in order of increasing energy. The energy levels of orbitals are represented by principal quantum numbers (n), starting from n=1 for the innermost shell. Within each principal energy level, orbitals are further divided into subshells based on their angular momentum quantum number (l). The subshells are designated as s, p, d, f, and so on, with increasing values of l.
According to the Aufbau principle, electrons are filled into orbitals starting from the lowest energy level (n=1) and progressing to higher energy levels, filling each subshell completely before moving on to the next with higher energy. This rule applies to most elements, providing a general guideline for determining their electron configurations.
ii. Exceptions in Chromium and Copper
Chromium and copper, two d-block transition metals, exhibit notable exceptions to the Aufbau principle. Their actual electron configurations differ from those predicted by the Aufbau rule.
Chromium: The expected electron configuration for chromium (atomic number 24) is 1s2 2s2 2p6 3s2 3p6 3d4 4s2. However, the actual configuration is 1s2 2s2 2p6 3s2 3p6 3d5 4s1.
Copper: The expected electron configuration for copper (atomic number 29) is 1s2 2s2 2p6 3s2 3p6 3d9 4s2. However, the actual configuration is 1s2 2s2 2p6 3s2 3p6 3d10 4s1.
These deviations from the Aufbau principle can be explained by considering the stability of electron configurations.
iii. Orbital Stability and Deviations
The stability of an electron configuration depends on the energy of the orbitals involved. Half-filled and fully filled d orbitals are generally more stable than partially filled d orbitals. This is due to the pairing of electrons and the minimization of electron-electron repulsion.
In the case of chromium, the half-filled 3d5 configuration is more stable than the expected 3d4 4s2 configuration. This is because the pairing of electrons in the 3d orbital lowers the overall energy of the configuration.
Similarly, in copper, the fully filled 3d10 configuration is more stable than the expected 3d9 4s2 configuration. The complete filling of the 3d orbital stabilizes the electron configuration.
The exceptions observed in the electron configurations of chromium and copper highlight the limitations of the Aufbau principle. While the Aufbau rule provides a general framework for predicting electron configurations, it is not always applicable to transition metals due to the stability of half-filled and fully filled d orbitals. Understanding these deviations is crucial for accurately representing the electron configurations of transition elements and predicting their chemical behavior.
