Aufbau Principle - Definition, Example, Features, Configuration, FAQs

Aufbau Principle

Consider the electron arrangement of carbon, an atomic number of six. The 1s and 2s orbitals each have four electrons. The 2p subshell is occupied by the remaining two electrons. There is the option of pairing the electrons in one of the 2p orbitals or leaving them unpaired in two different but degenerate p orbitals. The orbitals are filled according to Hund's rule: for an atom with electrons in a set of degenerate orbitals, the lowest-energy configuration is the one with the most unpaired electrons. Thus, the n, l, and m_s quantum numbers of the two electrons in the carbon 2p orbitals are identical, but their m_l quantum numbers differ (in accordance with the Pauli exclusion principle). With an electron configuration of 1s²2s²2p², the orbital diagram for carbon is:

According to Hund's rule, nitrogen (atomic number 7) covers the 1s and 2s subshells and has one electron in each of the three 2p orbitals. The spins of these three electrons are unpaired. In any of the two 2p orbitals (the electrons have opposite spins), oxygen (atomic number 8) has a pair of electrons and a single electron in the other two. Fluorine (atomic number 9) contains only one unpaired electron in its 2p orbital. The noble gas neon (atomic number 10) has all of its electrons coupled, and all of its orbitals in the n = 1 and n = 2 shells are filled.

Features of Aufbau Principle's

1. According to the Aufbau principle, electrons inhabit the lowest-energy orbitals first. This means that electrons can only enter higher-energy orbitals after lower-energy orbitals have been entirely filled.

2. The (n+l) rule may be used to establish the sequence in which the energy of orbitals grows, with the sum of the primary and azimuthal quantum numbers determining the orbital energy level.

3. Lower orbital energies correlate to lower (n+l) values. When two orbitals have the same (n+l) values, the orbital with the lower n value is said to have less energy.

4. The orbitals are filled with electrons in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so on.

Aufbau Principle Exceptions

Chromium's electron configuration is [Ar]3d⁵4s^1, not [Ar]3d⁴4s² (as suggested by the Aufbau principle). This is due to a number of variables, including half-filled subshells, enhanced stability and the comparatively small energy gap between the 3d and 4s subshells.

Lower electron-electron repulsions in the orbitals of half-filled subshells increase stability by reducing electron-electron repulsions. Similarly, totally filled subshells improve the atom's stability. As a result, some atoms' electron configurations defy the Aufbau principle depending on the energy gap between the orbitals. Copper, for example, is an exception to this rule, having an electrical configuration that corresponds to [Ar]3d¹⁰4s¹. The stability given by a totally filled 3d subshell explains this.

Electronic Configuration According to Aufbau Principle

Electronic configuration of Sulphur

1. Sulphur has an atomic number of 16, indicating that it has a total of 16 electrons.

2. According to the Aufbau principle, two of these electrons are in the 1s subshell, eight are in the 2s and 2p subshells, and the rest are dispersed between the 3s and 3p subshells.

3. As a result, Sulphur’s electronic configuration can be expressed as $1 s^2 2 s^2 2 p^6 3 s^2 3 p^4$.

Electronic configuration of Nitrogen

• Nitrogen is a seven-electron element (since its atomic number is 7).

• The 1s, 2s, and 2p orbitals are occupied with electrons.

• Nitrogen's electronic configuration is written as $1 s^2 2 s^2 2 p^3$.

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Some Solved Examples

Question 1: $4 d, 5 p, 5 f$ and $6 p$ orbitals are arranged in the order of decreasing energy. The correct option is:
1) (correct) $5 f>6 p>5 p>4 d$

2) $6 p>5 f>5 p>4 d$

3) $6 p>5 f>4 d>5 p$

4) $5 f>6 p>4 d>5 p$

Solution:

Aufbau Principle -

In the ground state of the atoms, the orbitals are filled in order of their increasing energies.

Option 1

The energy of various orbitals is compared with $(n+l)$, the rule.

The orbitals having a lower value of (n + l) have lower energy. If the value of $(n+l)$ is the same for two orbitals, then the orbital with a lower value of 'n' would have lower energy and be filled first.

Hence, the answer is option (1).

Question 2: If $n=6$, the correct sequence for filling electrons will be:
1) (correct) $n s \rightarrow(n-2) f \rightarrow(n-1) d \rightarrow n p$

2) $n s \rightarrow(n-1) d \rightarrow(n-2) f \rightarrow n p$

3 )$n s \rightarrow(n-2) f \rightarrow n p \rightarrow(n-1) d$

4) $n s \rightarrow n p \rightarrow(n-1) d \rightarrow(n-2) f$

Solution:

As we learned in

Aufbau Principle -

In the ground state of the atoms, the orbitals are filled in order of their increasing energies.

In the ground state of atoms, the orbitals are filled in order of their increasing energies.

Clearly for n = 6, we have the order 6s, 4f, 5d, 6p or ns, (n-2)f, (n-1)d, np

Hence, the answer is option (1).

Question 3:

Identify the orbital of the excited electron in the atom with an electronic configuration of [Xe] 6s² 4f¹⁴ 5d⁹ 6p¹

1) 4f

2) 5d

3) (correct) 6p

4) 6s

Solution:

As we learn

Aufbau Principle -

In the ground state of the atoms, the orbitals are filled in order of their increasing energies.

Energy order: 4f < 5d < 6p

Hence, the answer is the option (3).

Question 4: Which of the following electron configurations violates the Aufbau Principle?
1) $1 s^2 2 s^2 2 p^6$

2) (correct) $1 s^2 2 s^2 3 s^1$

3) $1 s^2 2 s^2 2 p^3$

4) $1 s^2 2 s^1$

Solution:

he Aufbau Principle states that electrons fill orbitals in order of increasing energy, starting with the lowest energy level. In option 2, the orbital is being filled before the 2p orbital, which violates the Aufbau Principle. Hence option B is correct.

Option 1 is correct because it follows the Aufbau Principle, where the 1s, 2s, and 2p orbitals are filled before the 3s orbital. Hence option A is incorrect.

Option 3 is correct because it follows the Aufbau Principle, but it violates the Pauli Exclusion Principle because there are two electrons with the same spin in the 2p sublevel. Hence option C is incorrect.

Option 4 is correct because it follows the Aufbau Principle, but it violates the Pauli Exclusion Principle because there is only one electron in the 2s orbital. Hence option 4 is incorrect.

Hence, the answer is the option (2).

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