Aufbau Principle Class 11: Notes, FAQs & NEET Questions

Aufbau principle in chemistry explains about the filling of electrons in lower and higher energy level orbitals. According to this principle, “the electrons are filled in orbitals in increasing energy levels.” The sum of principal and azimuthal quantum numbers is the key factor in determining energy levels of orbitals. Overall, a complete understanding of the Aufbau principle is important for writing electronic configurations and knowing the position of an atom. 

• The word “Aufbau” in the Aufbau Principle is a German word meaning “building up or construction”. 
• As per Aufbau principle, the filling of electrons starts with orbital having lowest energy to higher energy levels. 
• The n+l rule in Aufbau principle class 11 determines the increasing order of energy in the orbitals. 

Here in this article, we have mentioned some important Aufbau Principle Class 11 FAQs and NEET Questions below:

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Very Short Answers Type Questions (1 mark)

Ques. Who gave the aufbau principle definition?

Ans. Aufbau is derived from a German word ‘aufbeen’ which means build up. The definition of aufbau or build up principle was given by Neils Bohr in 1920. 

Ques. What is the order of filling electrons as per aufbau principle? 

Ans. According to aufbau principle, the order of filling electrons is given as: 

1s>2s>3s>3p>4s>3d>4p>5s>4d>5p>6s>4f>5d>6p>7s>5f>6d>7p. 

Ques. Which of the following is the correct order of filling of electrons in orbitals? 

• 2s, 3s, 4s, 3p

• 4s, 3s, 2s, 3p

• 2s, 3s, 3p, 4s

• 4s, 3p, 3s, 2s

Ans. (c) The order of orbitals in which electrons will be filled: 2s, 3s, 3p, 4s.

Ques. Are orbitals and subshells same?

Ans. An atomic orbital is a space characterised by the presence of electrons whereas the pathway in which electrons move freely around its orbit is called subshells. 

• An orbital can have a maximum of two electrons. 
• S, P, D, & F are the four subshells and the number of electrons varies in all four subshells. 

Ques. How Paulie’s exclusion principle relates to the Aufbau’s rule?

Ans. According to the Paulie’s exclusion principle, “an orbital can hold a maximum of two electrons in an opposite spin (either in -½ or in +½).

Short Answers Type Questions (2 marks)

Ques. What are the rules of aufbau principle in chemistry? 

Ans. The aufbau principle in chemistry follows three rules that are mentioned below: 

• The minimum number of electrons in an orbital is 2. 
• In the ground state, electrons are first filled in in lowest energy level orbitals. 

Ques. Which two elements do not follow aufbau rule?

• Cu & Ca

• Mg & Ca

• Cu & Cr

• None of the above

Ans. (c) Cu & Cr

Note: Copper (Cu) & Chromium (Cr) do not follow aufbau rule or principle as they have a full-filled and half-filled electronic configuration. 

Ques. An atom contains 12 electrons and 12 neutrons. Answer the following questions: 

(a) Name the element and find the number of protons.

(b) Write the electron configuration of the element using aufbau principle. 

Ans. According to the question, 

Number of electrons in the atom (X) = 12

Number of Neutrons in the atom (X) = 12

(a) To maintain the neutral charge on atom, number of protons = 12

Therefore, the unknown element (X) will be Magnesium. 

(b) Electronic configuration of Magnesium (Mg) = 1s22s22p63p2 or [Ne]3s2

Ques. What do you understand about the n/l rule in chemistry? 

Ans. The n represents the principal quantum number whereas l is the symbolic representation of azimuthal quantum number. 

• The energy levels of an orbital is dependent on the sum total of n+l value. 
• The rule n/l states that, "Electrons will be filled first in the orbital with the lowest n+l value."

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Long Answers Type Questions (3 marks)

Ques. What is the aufbau principle, explain?

Ans. The filling of electrons starts from the orbitals with the lowest energy level and once they are full-filled the electrons fill the next orbitals.

• This rule of filling electrons is based on a principle called the Aufbau principle in chemistry.
• Atoms in their ground state fill orbitals in increasing energy levels.
• In one orbital, two electrons can be filled at most. 

Ques. Explain the n+l rule in aufbau principle briefly. 

Ans. The importance of the n+l rule in aufbau principle is in determining the orbital energy levels. Here, n represents the principal quantum number whereas l signifies azimuthal quantum number. 

• According to this rule, energy decreases with decrease in the total n+l value and vice versa. 
• The filling of electrons using n+l rule can be explained as: 

• Since, electrons are filled in the orbitals based on the increasing order of energy levels, the order of filling is given as: 

1s,2s,2p,3s,3p,4s,3d,4p.

Ques. How to learn aufbau principle in class 11?

Ans. The basic rule of aufbau principle is that electrons are filled from lower energy to higher energy orbitals. This can be explained by the help of figure given below: 

 

Ques. What is the filling capacity of electrons in s,p,d, and f subshells?

Ans. The filling capacity of electrons varies in all four subshells. Maximum number of electrons in any orbital is calculated by the formula- 2(2l+1)

Where l = number of sub-shells. 

• S-orbital (l=0) can have a maximum of 2 electrons. 

• 6 electrons can be filled in p-orbital (l=1). 

• For d-orbital (l=2), the maximum number of electrons is equal to 10. 

• For f-orbital (l=3), the maximum number of electrons is 14. 

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Very Long Answers Type Questions (5 marks)

Ques. Which elements do not follow aufbau principle?

Ans. The Aufbau principle is followed by almost all elements with some exceptions.

• The electronic configuration of copper (Cu), and chromium (Cr)as per aufbau principle is [Ar]3d44s2, and [Ar]3d94s2. 
• However, Cr and Cu do not follow this principle because these elements become half-filled and the fulfilled electronic configuration of these elements are more stable . 
• Therefore, the actual configuration of these exceptional elements are given as: 
• Cr (A.No-24): [Ar]3d54s1

• Cu (A.No-29): [Ar]3d104s2

Ques. What is the relevance of the aufbau principle in class 11? 

Ans. In chemistry, the aufbau principle is very important in determining the electronic configuration and location of atoms or ions in the periodic table. 

• The orbitals that take part in the electron configuration are s,p,d,and f. 
• Based on the different energy levels of these orbitals, electrons are filled. 

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Aufbau Principle Class 11 NEET Questions

Ques. How many orbitals can be fit if n=3, l=1, m=0?  (NEET 2014)

1. 4
2. 3
3. 1
4. 2

Ans. (c) 1

Explanation: If n=3, l=1, m=0, the orbital will be 1 that makes 3p orbital. 

Ques. Which of the following statements is incorrect about Afbau rule? (NEET 2018)

1. The “s” orbital has zero angular angular momentum in total. 
2. Three quantum numbers determines an orbital, whereas electrons in an atom are determined by four quantum numbers. 
3. The electronic configuration of nitrogen (₇N¹⁴) is 1S²2S²2Px¹2Py¹2Pz¹. 
4. The value of m for dz² is zero. 

Ans. (c) The electronic configuration of nitrogen (₇N¹⁴) is 1S²2S²2Px¹2Py¹2Pz¹. 

Explanation: In electrons having same energy levels i.e, degenerate orbitals, spin of all unpaired electrons will be same. Therefore, the electronic configuration of ₇N¹⁴ is: 1S²2S²2Px¹2Py¹2Pz¹.

Ques. Which of the following is the correct order of filling of electrons in orbitals?(NEET 2019)

1. 5f > 6p > 4d > 5p
2. 5f > 6p > 5p > 4d
3. 6p > 5f > 4d > 5p
4. 6p > 5f > 5p > 4d

Ans. (b) 5f > 6p > 5p > 4d

Explanation: As per Aufbau principle, energy increases with increase in the n+l value. So, n+l values of above orbitals are:

5f= 5+3=8

6p= 6+1= 7

5p= 5+1= 6

4d=4+2=6

Thus, the order of filling electrons will be-5f > 6p > 5p > 4d