What is electron configuration and Aufbau principle explained
Electronic Configuration of Atoms:
The number of electrons in an atom is a distinct property, and its configuration provides a clear understanding of the characteristics of that atom. In accordance with the laws of quantum mechanics, a specific number of electrons in an atom are distributed into different designated energy levels (shells) and subshells (orbitals). This arrangement of electrons in various orbitals of an atom is defined as electronic configuration.
The electronic configuration of atoms follows a few fundamental rules. These rules are:
- (a) Pauli’s Exclusion Principle
- (b) Aufbau Principle
- (c) Hund’s Rule
Aufbau Principle:
The rule that determines the sequence in which electrons enter different orbitals of various subshells during the distribution of electrons in an atom is known as the Aufbau Principle.
According to this principle, electrons enter different orbitals in an atom in the increasing order of their energy levels. That means electrons will initially occupy the lower-energy orbitals first. The energy of an orbital is determined by the combined value of the principal quantum number (n) and the azimuthal quantum number
General Rules of Electronic Configuration and Exceptions to the (n + l) Rule
According to the general rule, if the outermost subshell of an element ends with the configuration of d4, d9, f6, or f13, it does not represent the stable or correct electronic configuration of that atom. In these cases, to attain maximum stability, an electron from the adjacent s-orbital is transferred to form a stable half-filled or fully-filled d5, d10, f7, or f14 configuration respectively.
• Cr(24) → 1s2 2s2 2p6 3s2 3p6 3d4 4s2 (According to general rule – Incorrect)
• Cr(24) → 1s2 2s2 2p6 3s2 3p6 3d5 4s1 (Actual and Correct configuration)
• Cu(29) → 1s2 2s2 2p6 3s2 3p6 3d9 4s2 (According to general rule – Incorrect)
• Cu(29) → 1s2 2s2 2p6 3s2 3p6 3d10 4s1 (Actual and Correct configuration)
Reason for the Exception: Degenerate orbitals that are exactly half-filled (d5, f7) or completely filled (d10, f14) possess greater symmetry and extraordinary stability. Therefore, an atom adopts this exceptional configuration to enhance its own stability or equilibrium.
An exception to the Aufbau principle or the (n + l) rule is observed in the electronic configurations of Lanthanum (57La) and Actinium (89Ac).
• For 5f and 6d subshells: The values of (n + l) are (5 + 3 = 8) and (6 + 2 = 8) respectively, which means they are equal.
According to the rule, when the value of (n + l) is equal for two orbitals, the orbital with the lower principal quantum number (n) possesses lower energy. Based on this, the order of energy should be: 4f < 5d and 5f < 6d.
Consequently, according to the Aufbau or (n + l) rule, the electronic configurations of Lanthanum and Actinium should have been:
• La(57) → [Xe] 4f1 5d0 6s2
• Ac(89) → [Rn] 5f1 6d0 7s2
• La(57) → [Xe] 4f0 5d1 6s2
• Ac(89) → [Rn] 5f0 6d1 7s2
That means, in the case of Lanthanum and Actinium, the entering electron bypasses the lower-energy f-orbital and directly enters the d-orbital. This is how the exception to the (n + l) rule occurs in these elements.
Aufbau Principle — Questions and Answers
Question 1: State the Aufbau Principle.
Answer: During the electronic configuration of an atom, electrons initially enter the orbital of the lowest energy level and, after completely filling this lowest-energy orbital, progressively occupy the orbitals of higher energy levels.
Question 2: From which language is the word ‘Aufbau’ derived, and what does it mean?
Answer: ‘Aufbau’ is a German word, which means— “building up” or “constructing step-by-step”.
Question 3: What is the rule or principle for measuring the energy of an orbital?
Answer: The energy of an orbital is determined by the sum of the principal quantum number (n) and the azimuthal quantum number (l), which is represented by the (n + l) value.
Question 4: Which orbital structure is the most stable?
Answer: An electronic configuration is most stable when the degenerate orbitals are either completely filled (e.g., d10, f14) or exactly half-filled (e.g., d5, f7).
Question 1: Why does the 19th electron of Potassium (19K) enter the 4s orbital instead of the 3d orbital?
• For 4s orbital: n = 4, l = 0 → (n + l) = 4 + 0 = 4
Question 2: Between 3d and 4p orbitals, which one will an electron enter first and why?
• In the case of 4p orbital: (n + l) = 4 + 1 = 5
Question 3: Why does the electronic configuration of Chromium (24Cr) exhibit an exception to the Aufbau principle? Explain.
Answer: According to the Aufbau principle, the expected electronic configuration of Chromium should be [Ar] 3d4 4s2. However, it is well established that when degenerate orbitals (such as the d orbitals) are exactly half-filled (d5) or completely filled (d10), the atomic structure attains maximum stability due to electronic symmetry.
In the case of Chromium, an electron from the 4s orbital easily shifts to the 3d orbital, resulting in the actual configuration of [Ar] 3d5 4s1. Here, since both the d and s orbitals are half-filled, the atom achieves maximum stability. This exceptional stability is the reason why the electronic configuration of Chromium deviates from the Aufbau principle.
Question 4: Why is there an exception to the Aufbau or (n + l) rule in the electronic configuration of Lanthanum (57La)?
• For 5d orbital: (n + l) = 5 + 2 = 7
In reality, however, the energy difference between the 5d and 4f orbitals is extremely small, and for Lanthanum, the 5d orbital temporarily gains greater stability. Consequently, the electron completely bypasses the 4f orbital, leaving it vacant, and directly enters the 5d orbital to form the actual stable configuration: [Xe] 4f0 5d1 6s2. This is why Lanthanum exhibits an exception to the Aufbau or (n + l) rule.
