What is the primary difference between the Aufbau Principle and Hund's Rule?

The Aufbau Principle determines the order in which different sub-shells are filled based on energy levels. Hund's Rule describes how electrons are distributed within a single sub-shell of equal-energy orbitals to minimize repulsion.

How does the electronic configuration of a transition metal cation differ from its neutral atom?

In transition metal cations, electrons are removed from the outermost $s$ orbital (e.g., $4s$) before the $d$ orbitals (e.g., $3d$). This is because once the $3d$ orbitals begin to fill, the $4s$ electrons are effectively further from the nucleus and easier to remove.

What is the distinction between a 'shell' and an 'orbital'?

A shell is a major energy level defined by the principal quantum number $n$. An orbital is a specific sub-region within a shell (like $2p_x$) that can hold a maximum of two electrons with opposite spins.

What error is made if a student writes the configuration of Copper as $[Ar] 4s^2 3d^9$?

This ignores the stability of a full $d$ sub-shell. The correct configuration is $[Ar] 4s^1 3d^{10}$, as promoting one $4s$ electron to the $3d$ sub-shell creates a more stable, lower-energy state.

Why is it incorrect to fill the $3d$ sub-shell before the $4s$ sub-shell in a neutral Potassium atom?

According to the Aufbau Principle, the $4s$ sub-shell is lower in energy than the $3d$ sub-shell in neutral atoms. Therefore, the 19th electron must occupy the $4s$ orbital first.

What mistake is commonly made when drawing orbital diagrams for Nitrogen ($1s^2 2s^2 2p^3$)?

Students often pair two electrons in one $p$ orbital and leave one empty. According to Hund's Rule, all three $p$ orbitals should contain one single electron with the same spin to minimize repulsion.

Define the Pauli Exclusion Principle.

It is a quantum mechanical principle stating that no two electrons in an atom can have the same set of four quantum numbers. In practical terms, an orbital can hold at most two electrons, and they must have opposite spins.

What are valence electrons?

Valence electrons are the electrons located in the outermost principal quantum shell of an atom. They are responsible for the chemical bonding and reactivity of the element.

What is 'Noble Gas Shorthand'?

It is a simplified way of writing electronic configurations where the symbol of the previous noble gas in brackets represents the core electrons. This highlights the valence electrons, which are most relevant to chemical behavior.

Why do electrons in the same orbital have opposite spins?

Electrons have opposite spins to satisfy the Pauli Exclusion Principle and to minimize the magnetic and electrostatic repulsion between them. This allows two negatively charged particles to occupy the same spatial orbital.

Library Podcasts

Courses

Referral & Rewards

Revision Notes

AS-Level

Cambridge International Examinations

Chemistry

1. Atomic Structure

Determining Electronic Configuration

Summary

Electronic configuration is the systematic arrangement of electrons within an atom's shells, sub-shells, and orbitals. This distribution follows fundamental quantum mechanical principles to ensure the atom remains in its lowest energy state, known as the ground state.

1. Definition & Core Concepts

Electronic Configuration: This term describes the specific distribution of electrons among the various available energy levels and orbitals of an atom or ion.
Principal Quantum Shells: Electrons are organized into shells designated by the principal quantum number $n$ . As $n$ increases, the shell's energy and its distance from the nucleus also increase.
Sub-shells and Orbitals: Each shell is divided into sub-shells ( $s, p, d, f$ ), which contain orbitals. An orbital is a region of space where there is a high probability of finding an electron, and each orbital can hold a maximum of two electrons.
Capacity Limits: The maximum number of electrons a shell can hold is determined by the formula $2n^2$ . For example, the third shell ( $n=3$ ) can hold up to $18$ electrons.

2. The Aufbau Principle

Diagonal rule diagram showing the order of filling sub-shells from 1s to 4f.

3. Pauli Exclusion Principle & Hund's Rule

4. Notation Methods

5. Key Distinctions

6. Exceptions to the Rule

7. Exam Strategy & Tips