The electronic configuration of an atom of Nitrogen is 1s$^2$ 2s$^2$ 2p$^1_x$ 2p$^1_y$ 2p$^1_z$ because the atom is

Analysis

The question asks why the electronic configuration of Nitrogen ($Z=7$) is written as $1s^2 2s^2 2p_x^1 2p_y^1 2p_z^1$ rather than filling one orbital completely before moving to the next.

Concept: Hund’s Rule of Maximum Multiplicity

To understand this, we look at Hund’s Rule. This rule states that for a given electron configuration, the term with the greatest multiplicity (the most unpaired electrons) has the lowest energy.

In the $2p$ subshell, there are three degenerate orbitals ($2p_x, 2p_y, 2p_z$). According to Hund's Rule, electrons will occupy these orbitals singly first with parallel spins before pairing up. This minimizes electron-electron repulsion, making the atom more stable.

Step-by-Step Solution

1. Identify the Atomic Number: Nitrogen has 7 electrons.
2. Distribute the electrons: • $1s$ orbital: 2 electrons ($1s^2$) • $2s$ orbital: 2 electrons ($2s^2$) • $2p$ subshell: 3 electrons remaining.
3. Apply the Rules: • Aufbau Principle: Electrons fill lower energy levels first. • Pauli Exclusion Principle: No two electrons can have the same four quantum numbers. • Hund’s Rule: Electrons fill degenerate orbitals (orbitals with the same energy) singly before pairing.
4. Conclusion: Writing the configuration as $2p_x^1 2p_y^1 2p_z^1$ is the standard, ground-state way of representing the distribution of electrons in the $p$-orbitals to satisfy Hund's Rule. It is not an excited state; it is the most stable arrangement for a Nitrogen atom. Therefore, the atom is simply following the established laws of quantum mechanics and electron configuration.

Evaluation of Options

• A & B: Incorrect. This is the ground state of Nitrogen, not an excited state.
• C: Incorrect. This configuration is the most stable state for Nitrogen.
• D: Correct. This notation is the standard way to represent the ground state of Nitrogen in accordance with Hund's Rule.

Exam Tip

Whenever you see $p$, $d$, or $f$ orbitals, remember the "Bus Seat Rule": Just like people entering an empty bus prefer to sit in an empty row before sitting next to someone else, electrons prefer to occupy empty orbitals before pairing up to avoid repulsion!