MCQ’s On Afbau Principle

To populate electrons in different orbitals, the aufbau principle is applied. The aufbau principle is based on orbital relative energy. It considers Pauli’s exclusion principle as well as the Hund’s rule. This principle states that electrons are filled in orbitals in order of increasing energy. The lower-energy orbitals are filled first, followed by the higher-energy orbitals.

1. Any two electrons in an atom cannot have the same four quantum numbers. The term for this principle is

1. 1. Pauli’s exclusion principle
   2. Hund’s rule
   3. Aufbau principle
   4. None of the above.

Answer: a) Pauli’s-exclusion principle, because according to Pauli’s Exclusion Principle, no two electrons in the same atom may have the same values for all four quantum numbers. To put it another way, (1) only two electrons can occupy the same orbital, and (2) the spins of two electrons in the same orbital must be opposing.

2. According to the aufbau principle

1. 1. Electrons tend to remain unpaired.
   2. Orbitals are regions where one is likely to find an electron.
   3. Only two electrons can occupy an orbital
   4. Electrons enter the lowest available energy level.

Answer: d) electrons enter the lowest available energy level, because the Aufbau principle states that electrons will inhabit the lowest-energy orbitals first. This means that electrons can only enter higher-energy orbitals after lower-energy orbitals have been entirely filled.

3. Which of the following is incorrect about ‘s’ orbitals?

1. 1. They can only hold one electron.
   2. They are spherical in shape.
   3. They are found in all principal energy levels.
   4. None of these.

Answer: a) They can hold one electron, because this is related to Pauli’s notion of exclusion. The only thing that distinguishes one electron from another in the same orbital is its spin. There can only be two electrons in an orbital since there are only two conceivable spins.

4. Which one of the following statements about s-orbitals is incorrect?

1. 1. They are spherical in shape.
   2. The maximum number of s- orbitals in any principal level is 1.
   3. They are found in all principal energy levels.
   4. They can hold only one electron.

Answer: a) They are spherical in shape, because An s orbital has zero angular momentum by definition, and l = 0. Atomic orbitals with non-spherical forms result from any non-zero angular momentum.

5. The number of d- electrons in Fe2+ (z=26) is not equal to the number of electrons among the given option is –

1. 1. S-electrons in mg
   2. P electrons in cl
   3. D electrons in fe
   4. P electrons in ne

Answer: b) p electrons in cl, because Chlorine has 17 electrons and has an atomic number of 17. As a result, there are 11 electrons in the p-orbital, i.e. 6 in 2p and 5 in 3p.

6.  What is the correct order of orbitals in which electrons are filled?

1. 1. 4s, 3d, 4p, 5s, 4d
   2. 3d, 4s, 4p, 4d, 5s
   3. 3d, 4p, 4s, 4d, 5s
   4. 5s, 4p, 3d, 4d, 5s

Answer: a) 4s, 3d, 4p, 5s, 4d, because the Aufbau principle is used to determine atoms’ electronic configuration. According to this theory, electrons should be filled in lower energy orbitals first, followed by higher energy orbitals. Furthermore, the n+l rule aids in determining the energy of orbitals.

7. What is the correct electronic configuration of an atom (Cu) with atomic number 29?

1. 1. 1s2, 2s22p6, 3s23p63d10, 4s1
   2. 1s2, 2s22p6, 3s23p6, 4s2 3d9
   3. 1s2, 2s22p6, 3s23p6, 4s24p63d.
   4. 1s2, 2s22p6, 3s23p6, 4s24p6, 5s25p1

Answer: a) 1s2,2s22p6, 3s23p63d10, 4s1, because the atomic number of Cu is 29. 1s2 2s2 2p6 3s2 3p6 4s2 3d9 should be the electronic arrangement. The electronic setup, on the other hand, is 1s2 2s2 2p6 3s2 3p6 4s1 3d10 or [Ar] 3d104s1. This is due to the lower energy or greater stability of the half-filled and fully filled structures (d5 d10 f7 f14).

8.  Which of the following ions has 18 electrons in the outermost shell?

1. 1. Th4+
   2. Cu+
   3. K+
   4. Cs+

Answer: b) Cu+, because the 3rd subshell is made up of a s orbital, a set of (3)p orbitals, and a set of (5)d orbitals. Each orbital transports two electrons, therefore a total of 18 electrons can be injected. This is due to the fact that it is the full power of the third shell, and it does not specify the sequence in which the electrons are filled.

Each shell can only hold a certain amount of electrons: the first shell can hold up to two electrons, the second shell can hold up to eight (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) electrons, and so on. In theory, the nth shell can hold up to 2(n2) electrons, according to the general formula.

9. The electrons will enter which of these orbitals after filling the 3d orbitals completely?

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

Answer: d) 4p , because according to the Aufbau’s principle, electrons fill in the orbitals with increasing levels of energy, after 3p, the energy of 4s(n+l=4) and 3d(n+l=5) is lower due to the lower (n+l) value, electrons tend to fill in the 4s orbital first. Orbital filling is determined by orbital energy, whereas orbital electron removal is determined by orbital position.The energy of 4s is lower than that of 3d. As a result, it fills up first. 4s, on the other hand, is further away from the nucleus than 3d. As a result, when electrons are removed, they are first removed from 4s. It is easier to remove electrons from the nucleus when they are further away.

10. How many unpaired electrons are present in N2+?

1. 1. 3
   2. 0
   3. 2
   4. 1

Answer: d) 1 , because The total number of electrons in a nitrogen atom is seven. Two electrons will occupy the n = 1 level, and five electrons will fill the n = 2 level.To fill up its shell with 8, nitrogen can bond three times with other electrons (8-5=3). And these are the three unpaired electrons that resided in the Nitrogen atom’s 2 p sub-shell prior to the creation of three bonds.