Question

Explain why the ionization energy for aluminum, contrary to the general trend, is less than the ionization energy for magnesium.

Short answer

Aluminum has a lower ionization energy than magnesium due to its less stable, partially filled 3p subshell.

Step-by-step solution

Understanding Ionization Energy

Ionization energy is the amount of energy required to remove an electron from an atom in its gaseous state. Generally, as you move across a period in the periodic table from left to right, the ionization energy increases due to increased nuclear charge, which holds electrons more tightly.

Exploring Electron Configuration

To understand the anomaly between aluminum and magnesium, let's examine their electron configurations. Magnesium (Mg) has an electron configuration of \(1s^2 2s^2 2p^6 3s^2\), and aluminum (Al) has \(1s^2 2s^2 2p^6 3s^2 3p^1\). Magnesium has a filled 3s orbital, while aluminum has an additional 3p electron.

Analyzing Subshell Stability

Electrons in a filled or half-filled subshell are generally more stable. Magnesium's filled 3s subshell provides stability, making it harder to remove an electron. In contrast, aluminum's 3p subshell is partially filled (3p^1), making it easier to remove this less stable electron.

Concluding the Anomaly

Because the 3p electron in aluminum is easier to remove compared to the electrons in magnesium's filled 3s subshell, aluminum has a lower ionization energy than magnesium, contrary to the general trend.

Key concepts

Electron Configuration

Electron configuration is the arrangement of electrons in an atom's orbitals. This distribution is crucial in understanding an element's chemical behavior, including its ionization energy. For magnesium, the electron configuration is \(1s^2 2s^2 2p^6 3s^2\). The electrons fill up the lowest available energy levels first, filling the 3s subshell entirely.
The electron configuration for aluminum differs slightly with an extra electron: \(1s^2 2s^2 2p^6 3s^2 3p^1\). This configuration illustrates the presence of a single electron in the higher energy 3p subshell.
The differences in electron arrangements explain a lot about the general stability and energy requirements of removing an electron. When an electron is more stable, the ionization energy tends to be higher. Observing these configurations helps predict anomalies like the one seen between aluminum and magnesium.

Periodic Table

The periodic table is an incredible tool of organization for elements, arranged based on increasing atomic number. It showcases repeating patterns in properties, known as periodic trends. One such trend is the ionization energy trend. Usually, as you move across a period (from left to right), ionization energy increases because electrons are more strongly attracted to the nucleus due to an increased nuclear charge.
Magnesium and aluminum are both in the third period of the periodic table. Despite the general trend, aluminum's ionization energy is less than magnesium's. This happens because of the different placements of their outer electrons and the presence of subshells, which break the expected trend.
Although the periodic table hints at trends, individual electron configurations and other factors can sometimes deviate from these patterns. Understanding these deviations is crucial, especially when electrons populate new subshells, as in aluminum's case with its 3p electron.

Subshell Stability

Subshell stability plays a significant role in determining an atom's ionization energy. Subshells, denoted as s, p, d, and f, can be completely filled, half-filled, or partially filled. Fully filled or half-filled subshells are often more stable, providing a lower energy state for the electrons.
Magnesium's 3s subshell is fully filled \(3s^2\), which means greater stability, akin to having a fulfilled electron seating arrangement. This high stability makes magnesium's electrons harder to remove, resulting in higher ionization energy.
Aluminum, with its partially filled 3p subshell \(3p^1\), showcases less stability because it does not enjoy the stability that comes with a fully or half-filled subshell. As a result, it's easier for aluminum to lose its 3p electron. This reduction in subshell stability leads to lower ionization energy compared to magnesium, contrary to what the periodic trend suggests.