Aluminium Atom: Bohr Shells, Slater Z-eff & Ionisation (2D)…

Bohr Shells and Electron Configuration

Aluminium, with atomic number 13, has a ground state electron configuration of 1s² 2s² 2p⁶ 3s² 3p¹. This configuration is visualized in the Bohr model as shells surrounding the nucleus, each representing an energy level where electrons reside.

The Aufbau principle dictates that electrons fill orbitals starting from the lowest available energy level to the highest, ensuring stability and minimizing repulsion between electrons.

Slater Shielding Effect

The Slater shielding effect refers to the reduction in effective nuclear charge experienced by outer electrons due to inner-shell electrons. This is crucial for understanding why successive ionisation energies increase as more electrons are removed.

In aluminium, the first electron removal from 3p¹ results in a significant change in Z_eff (effective nuclear charge) compared to subsequent removals, reflecting the different shielding effects of inner shells.

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Shell Radii and Ionisation Energies

The radii of Bohr shells for aluminium can be calculated using the formula r_n = a0·n²/Z_eff, where a0 is the Bohr radius (approximately 52.9 pm), n is the principal quantum number, and Z_eff represents the effective nuclear charge experienced by electrons in that shell.

The successive ionisation energies of aluminium are 577.5 kJ/mol, 1816.7 kJ/mol, 2744.8 kJ/mol, and 11577 kJ/mol for Al to Al3+. These values illustrate the increasing energy required to remove electrons due to reduced shielding.

Stability of Al3+ Ion

The Al3+ ion is stable because it has a full outer shell (1s² 2s² 2p⁶), resembling the noble gas neon. This configuration minimizes electron-electron repulsion and provides stability, making Al3+ the most common ion form of aluminium.

Understanding these principles helps in predicting chemical behavior and reactivity of elements like aluminium.

Frequently asked questions

What is the significance of Slater shielding?

Slater shielding reduces the effective nuclear charge experienced by outer electrons, which explains why ionisation energies increase with each successive electron removal from an atom.

Why does Al3+ have a stable configuration similar to neon?

Al3+ has a full valence shell (1s² 2s² 2p⁶), resembling the noble gas neon, which is highly stable due to minimal electron-electron repulsion.

How does the Bohr model explain aluminium's ionisation process?

The Bohr model visualises aluminium’s electron configuration and shell radii, showing how electrons are removed in sequence, each time with a slightly higher energy due to reduced shielding by inner electrons.

What determines the stability of an ion like Al3+?

An ion is stable when its electron configuration resembles that of a noble gas, as in the case of Al3+, which has a full outer shell and thus minimal repulsion between electrons.

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