Free Electronic Configuration MCQs with Answers

1. According to Hund's rule, electrons entering a set of degenerate orbitals will

• A. pair up in the first orbital before entering the next
• B. occupy separate orbitals with parallel spins before pairing
• C. occupy the orbital of highest energy first
• D. always pair with opposite spins immediately

Explanation: Electrons repel one another, so they spread across orbitals of equal energy singly and with parallel spins before any orbital takes a second electron. This is why nitrogen, with three 2p electrons, has three unpaired electrons rather than one pair and one single. Filling lowest energy orbitals first is the Aufbau principle, a separate rule.

2. The correct electronic configuration of chromium (Z = 24) is

• A. [Ar] 3d4 4s2
• B. [Ar] 3d5 4s1
• C. [Ar] 3d6 4s0
• D. [Ar] 3d3 4s3

Explanation: Chromium promotes one 4s electron into the 3d subshell because a half filled d5 configuration alongside a half filled 4s is more stable than d4 with a full 4s. Copper does the same thing to reach a full d10. The 4s subshell can never hold three electrons, which rules out the last option outright.

3. Which orbital is filled immediately after the 4s orbital according to the Aufbau principle?

• A. 4p
• B. 3d
• C. 4d
• D. 5s

Explanation: Orbitals fill in order of increasing n plus l, and where that sum ties the lower n goes first. For 3d the sum is 5 and for 4p it is also 5, but 3d has the lower principal quantum number and so fills first, which is why the first transition series follows calcium. The 4p orbitals are filled only after the 3d subshell is complete at zinc.

4. Hund's rule states that within a subshell, electrons

• A. pair up in the lowest orbital first
• B. occupy separate orbitals singly with parallel spins before any pairing occurs
• C. always have opposite spins
• D. fill the highest energy orbital first

Explanation: Spreading out into empty orbitals minimises electron electron repulsion, so nitrogen's three 2p electrons occupy the three separate p orbitals rather than crowding two into one. This is why so many transition metal ions are paramagnetic. Pairing only begins once every orbital in the subshell holds one electron.

5. The Pauli exclusion principle states that

• A. no two electrons in an atom can have the same set of all four quantum numbers
• B. electrons always pair up
• C. orbitals fill in order of increasing energy
• D. electrons occupy the nucleus

Explanation: Because the first three quantum numbers define an orbital, the principle limits any orbital to two electrons, which must then differ in spin. This single rule is what gives every shell its fixed capacity and ultimately why the periodic table has the shape it does. The filling order is the separate aufbau principle.

6. According to the n plus l rule, which orbital is filled first?

• A. 3d
• B. 4s
• C. 4p
• D. 4d

Explanation: For 4s the sum is 4 plus 0, that is 4, while for 3d it is 3 plus 2, that is 5, so the lower sum fills first and 4s is occupied before 3d. When two orbitals have the same sum, the one with the lower n fills first. This ordering explains why potassium and calcium come before the transition metals.

7. The electronic configuration of the chloride ion, given that chlorine has atomic number 17, is

• A. 1s2 2s2 2p6 3s2 3p5
• B. 1s2 2s2 2p6 3s2 3p6
• C. 1s2 2s2 2p6 3s2 3p4
• D. 1s2 2s2 2p6 3s1

Explanation: Chlorine gains one electron to form the chloride ion, taking its total to 18 and giving it the argon configuration with a complete octet. That stability is exactly why the ion forms so readily. Writing the neutral atom's configuration instead is the standard slip.

8. Chromium and copper have anomalous electronic configurations because

• A. their nuclei are unstable
• B. a half filled or completely filled d subshell is more stable
• C. they lose electrons easily
• D. the 4s orbital does not exist for them

Explanation: Promoting one 4s electron gives chromium a 3d5 4s1 arrangement and copper a 3d10 4s1 arrangement, both of which gain stability from the symmetry and reduced repulsion of a half filled or full subshell. These two exceptions are examined far more often than the rule itself. The energy gap between 4s and 3d is small enough for the swap to be worthwhile.

9. The energy required to remove the most loosely bound electron from a gaseous atom is called

• A. electron affinity
• B. ionisation energy
• C. electronegativity
• D. lattice energy

Explanation: Ionisation energy is always endothermic because work must be done against the nuclear attraction, and it rises steeply for each successive electron removed. Electron affinity is the energy change when an electron is added instead, and electronegativity describes the pull on shared electrons within a bond. All three are distinct and are regularly confused.

10. Number of unpaired electrons present in the ground state of Fe3+ are (Atomic number of Fe = 26).

• A. Three
• B. Four
• C. Five
• D. Six

Explanation: Neutral iron is [Ar] 3d6 4s2, and forming Fe3+ removes both 4s electrons and one 3d electron, leaving 3d5. By Hund's rule those five electrons occupy the five d orbitals singly with parallel spins, so all five are unpaired, and that half filled arrangement is what makes Fe3+ so stable. Removing d electrons before the 4s pair is the standard error.