Electrons enter into orbitals in order of increasing energy as exemplified by?

To solve this question, we need to apply two fundamental principles of electronic configuration:

1. The Aufbau Principle

This principle states that electrons fill orbitals in order of increasing energy, starting from the lowest energy level (1s) and moving upwards.

2. Hund’s Rule of Maximum Multiplicity

This rule states that for degenerate orbitals (orbitals with the same energy, like the three 2p orbitals: $2p_x, 2p_y, 2p_z$), electrons fill them singly first with parallel spins before pairing begins. This is because electrons are negatively charged and repel each other; they prefer to occupy separate orbitals if space is available.

Step-by-Step Analysis

The question asks for the configuration that correctly follows the order of increasing energy and the rules of orbital filling. Let's look at the total number of electrons in the options provided:

• Option A: $2+2+2+2+0+0 = 8$ electrons. The $2p$ subshell has 4 electrons. According to Hund's Rule, these should be distributed as $2p_x^2, 2p_y^1, 2p_z^1$. Option A incorrectly pairs them all in $2p_x$ and $2p_y$ while leaving $2p_z$ empty.
• Option B: $2+2+2+1+1+0 = 8$ electrons. This represents Oxygen (atomic number 8). The configuration $1s^2 2s^2 2p_x^2 2p_y^1 2p_z^1$ perfectly follows the Aufbau principle and Hund's Rule (the 4 electrons in the $2p$ subshell are distributed to minimize repulsion).
• Option C: $2+2+1+1+1+1 = 8$ electrons. This is incorrect because it places an electron in the $3s$ orbital before the $2p$ subshell is fully filled.
• Option D: $2+2+2+2+0+1 = 9$ electrons. This is an invalid configuration for an 8-electron system and violates the Aufbau principle by jumping to $3s$ before completing the $2p$ subshell.

Conclusion

Option B is the only one that correctly follows the Aufbau principle (filling lower energy levels first) and Hund's Rule (distributing electrons singly in degenerate orbitals before pairing).

Exam Tip: When dealing with $p$-orbitals, always remember the "Bus Seat Rule": electrons act like people getting on a bus; they prefer to sit in an empty row before sitting next to someone else. Always fill the $p_x, p_y,$ and $p_z$ orbitals singly before adding a second electron to any of them.