Problem 82 - Chemistry Matter and Change

Question

When writing the electron configuration notation for an atom, what three principles or rules should you follow?

Step-by-Step Solution

Answer

Follow the Aufbau Principle, Pauli Exclusion Principle, and Hund's Rule.

Step 1: Understand the Aufbau Principle

The Aufbau Principle states that electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels. This means you start filling from the 1s orbital and move upwards through 2s, 2p, 3s, and so on.

Step 2: Apply the Pauli Exclusion Principle

According to the Pauli Exclusion Principle, no two electrons in an atom can have the same set of four quantum numbers. This implies that each orbital can hold a maximum of two electrons with opposite spins.

Step 3: Follow Hund's Rule

Hund's Rule states that electrons must fill degenerate orbitals (orbitals with the same energy) singly before any orbital receives a second electron. This means that in a set of, for example, three p orbitals, one electron goes into each orbital before any pairing occurs.

Key Concepts

Aufbau Principle

When electrons are arranged in an atom, they fill the orbitals in order of increasing energy. This is the fundamental idea behind the Aufbau Principle. The term "Aufbau" is derived from a German word meaning "building up," and this principle is all about building up the electron configuration from the ground state.

• Electrons will first occupy the lowest available energy level, the 1s orbital, before moving to higher levels.
• After filling the 1s orbital, electrons will occupy the 2s, then 2p, and continue upwards.
• This principle provides a guideline for the filling order, making it easier to predict and write electron configurations.

Utilizing the Aufbau Principle is crucial when you're trying to determine the electron configuration of an element, as it provides the sequence in which the energy levels are filled. Remember that knowing this principle helps you ensure the electrons are placed systematically according to their energy levels.

Pauli Exclusion Principle

The Pauli Exclusion Principle is one of the cornerstones of quantum mechanics and plays a vital role in the organization of electron configurations. It states that no two electrons in the same atom can have identical sets of quantum numbers. Quantum numbers help define an electron’s unique state in an atom.

Imagine quantum numbers as a unique address for each electron:

• The principal quantum number (n) indicates the energy level.
• The azimuthal quantum number (l) specifies the shape of the orbital.
• The magnetic quantum number (m extsubscript{l}) denotes the orientation of the orbital.
• Finally, the spin quantum number (m extsubscript{s}) represents the two possible spin states of the electron.

According to this principle, an orbital can hold a maximum of two electrons, but they must have opposite spins. This opposite spin minimizes electron repulsion, allowing the electrons to exist in the confined space of an orbital. Thus, when writing electron configurations, the Pauli Exclusion Principle helps ensure that electrons are properly assigned to their respective orbitals.

Hund's Rule

Hund's Rule is an essential guideline when it comes to filling orbitals, particularly those with the same energy, known as degenerate orbitals. These include the three p orbitals, five d orbitals, or seven f orbitals. Hund's Rule tells you the most stable way to distribute electrons among these orbitals.

### Key Points of Hund's Rule:

• Electrons will populate each degenerate orbital singly before any orbital gets a second electron.
• The electrons in these singly occupied orbitals will possess the same spin.

This behavior is crucial because it minimizes electron-electron repulsions and leads to a more stable electron configuration. By ensuring that degenerate orbitals are singly occupied before pairing occurs, Hund's Rule maximizes the total spin of the electrons. In practice, following Hund's Rule allows you to determine the electron distribution, particularly in partially filled subshells, leading to a clearer picture of an element’s electron configuration.