📝 Electron energy levels shells (12 MCQs)

📖 From Campbell Biology • 2. The Chemistry of Life • 12 questions available

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What is Electron energy levels shells?

Definition:
Electron energy levels, or shells, are fixed orbits around the nucleus where electrons reside, each with a specific energy, and they are designated by quantum numbers (n = 1, 2, 3, ...), with shells closest to the nucleus having the lowest energy and the highest energy shells being farthest; the capacity of each shell is given by $2n^2$ electrons, and the distribution of electrons across shells determines an atom's chemical properties and reactivity.

Working:
Electrons occupy shells in order of increasing energy, with the lowest energy (n=1) filled first, and the outermost shell (valence shell) contains valence electrons, which are involved in chemical bonding; the electron configuration follows the Aufbau principle, and the stability of an atom is associated with a full outer shell (octet rule); the energy of an electron is quantized, and transitions between levels involve absorption or emission of photons, with energy $\Delta E = h\nu$ , and this explains the emission spectra of elements.

Example:
A simple example is oxygen, with electron configuration 2, 6 (n=1 has 2 electrons, n=2 has 6 electrons), and it needs 2 more electrons to fill its outer shell, making it reactive; another example is neon, with configuration 2, 8, which is stable and unreactive (noble gas), illustrating how electron shells determine chemical behavior.

Reason:
Understanding electron energy levels is central to chemistry and biology, as it explains chemical bonding, reactivity, and the behavior of atoms in biological systems, and it is fundamental to spectroscopy and quantum mechanics.

Easy

Medium

Hard

📝 All Electron energy levels shells MCQs

Q1. An atom absorbs energy and an electron moves from $n=2$ to $n=4$ . Which statement best explains what happens when the electron later returns to $n=2$ ?

A.The electron releases energy as a photon whose energy equals the difference between the two levels ✅

B.The electron releases exactly the same amount of energy that the atom originally contained

C.The electron permanently loses two energy levels during the transition

D.The electron releases energy continuously while moving between the two levels

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Electron energy levels are quantized, so an electron can occupy specific allowed states. When it returns from $n=4$ to $n=2$ , it releases a photon with energy equal to the difference between those two energy levels, not an arbitrary amount.

Q2. Two electrons in the same atom are observed with one electron in $n=2$ and another in $n=3$ . A student claims the $n=3$ electron must always move faster because its principal energy level is higher. What is the best evaluation?

A.The claim is correct because higher energy always means higher speed

B.The claim is incorrect because energy level describes allowed energy states, not simply the electron's speed ✅

C.The claim is correct only when both electrons have identical masses

D.The claim is incorrect because electrons in atoms have no energy

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: A principal energy level identifies an allowed energy state rather than directly specifying a simple classical speed. Atomic electrons are described by quantum states, so treating them as tiny particles moving in circular paths can produce misleading conclusions.

Q3. An atom is exposed first to low-energy radiation and then to higher-energy radiation. Only the higher-energy radiation produces an electronic transition. Which conclusion is most justified?

A.The lower-energy radiation must have a longer exposure time

B.The electron can absorb any amount of energy if exposure is sufficiently long

C.The higher-energy photons have energies matching an allowed energy difference ✅

D.The atom has stopped interacting with the lower-energy radiation

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: Electronic transitions require specific energy differences between allowed states. If the lower-energy photons do not have enough energy to match an available transition, they may not produce that transition, whereas higher-energy photons can if their energies match the required difference.

Q4. A researcher proposes that an electron can occupy an energy halfway between two allowed levels if the atom is heated gradually. Which observation would most strongly challenge this proposal?

A.The atom becomes warmer during heating

B.Spectral measurements show discrete lines rather than a continuous set of intermediate transition energies ✅

C.The electron contains negative charge

D.The atom can absorb energy from its surroundings

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: If electrons could occupy arbitrary energies between established levels, transitions would be expected at a continuous range of energies. Discrete spectral lines instead support the model that electrons occupy quantized energy states.

Q5. An electron initially occupies $n=3$ . It absorbs exactly the energy needed to reach $n=5$ , then emits one photon and ends at $n=4$ . Compared with the absorbed photon, how is the emitted photon's energy related?

A.It is greater because emission always releases more energy

B.It is equal because both events involve the same electron

C.It is smaller because the

n=5

n=4

energy difference is smaller than the

n=3

n=5

difference ✅

D.It must be zero because the electron remains in an excited state

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: The absorbed photon supplies the energy difference between $n=3$ and $n=5$ . The emitted photon corresponds only to the subsequent transition from $n=5$ to $n=4$ , which is a smaller energy change, so its energy is smaller.

Q6. A student draws an energy-level diagram with $n=1$ , $n=2$ , $n=3$ , and $n=4$ equally spaced vertically. The student argues that each successive transition therefore releases the same energy. What is the main error?

A.Energy levels cannot be represented using diagrams

B.Energy-level spacing is generally not required to be equal, so equal visual spacing cannot establish equal energy differences ✅

C.Electrons can only occupy

n=1

D.Photons never carry energy during transitions

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: An energy-level diagram is a model, and its visual spacing should represent actual relative energies only when deliberately scaled that way. For many atoms, higher levels become closer together, so equal drawing distances cannot justify equal transition energies.

Q7. A detector records photons from an atom at wavelengths $400\text{ nm}$ , $500\text{ nm}$ , and $700\text{ nm}$ . Which photon has the greatest energy and therefore represents the largest energy difference between two allowed levels?

A.The

700\text{ nm}

photon

B.The

500\text{ nm}

photon

C.The

400\text{ nm}

photon ✅

D.All three have equal energy because they come from the same atom

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: Photon energy is inversely related to wavelength. Therefore, among $400\text{ nm}$ , $500\text{ nm}$ , and $700\text{ nm}$ , the $400\text{ nm}$ photon has the greatest energy and corresponds to the largest energy difference among the transitions represented.

Q8. An energy-level experiment gives the following simplified data: transition A produces 22 energy units, transition B produces 55 units, and transition C produces 88 units. Which transition corresponds to the largest separation between the initial and final energy levels?

A.A

B.B

C.C ✅

D.They cannot be compared because all transitions involve electrons

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: The energy carried by an emitted or absorbed photon corresponds to the energy difference between the relevant electronic states. Therefore, the transition associated with 88 energy units has the largest separation among the listed transitions.

Q9. A graph of photon energy versus wavelength shows a decreasing curve: approximately $3.1$ eV at $400\text{ nm}$ , $2.5$ eV at $500\text{ nm}$ , and $1.8$ eV at $700\text{ nm}$ . A student concludes that longer wavelengths correspond to larger electronic transitions. Which correction is best?

A.Longer wavelengths correspond to lower photon energies and therefore smaller energy differences ✅

B.Longer wavelengths always indicate higher principal energy levels

C.Wavelength has no relationship to photon energy

D.The graph proves that all electronic transitions have equal energy

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: The graph shows an inverse relationship between wavelength and photon energy. Because transition energy equals photon energy for the associated absorption or emission, longer wavelengths correspond to smaller energy differences between electronic states.

Q10. An atom has allowed energy states labeled $E_1<E_2<E_3$ . An electron moves from $E_1$ to $E_3$ directly. Another electron moves from $E_1$ to $E_2$ , then from $E_2$ to $E_3$ . Which statement correctly compares the total energy absorbed?

A.The two-step path absorbs more total energy

B.The direct path absorbs more total energy

C.Both paths require the same total energy, although the two-step path can involve different photons ✅

D.The two-step path requires no energy because each step is smaller

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: The total energy change depends on the initial and final states, not on whether the transition occurs in one step or multiple steps. Thus both routes from $E_1$ to $E_3$ require the same net energy, although photon energies can differ.

Q11. A student says, 'If an electron absorbs a photon with twice the energy required for a transition, it will simply move halfway beyond the next allowed energy level.' Which reasoning best identifies the flaw?

A.Electrons cannot absorb photons

B.Energy levels are quantized, so excess photon energy does not automatically place an electron at an intermediate allowed state ✅

C.Photons have no measurable energy

D.All energy levels contain exactly the same energy

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Electrons do not normally occupy arbitrary intermediate energies between quantized states. A photon must satisfy the energy requirements of an allowed transition, so doubling the photon energy does not mean the electron simply stops halfway between two permitted states.

Q12. An electron absorbs energy and reaches an excited state. It then emits two photons in sequence before returning to its original state. Which conclusion must be true about the two emitted photon energies?

A.They must have identical energies

B.Their combined energy equals the electron's total energy in the excited state

C.Their combined energy equals the energy difference between the original and final states ✅

D.Each photon must contain half the atom's total energy

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: When an electron undergoes sequential transitions, each photon carries the energy associated with one particular energy-level difference. The sum of the emitted photon energies equals the overall energy difference between the starting and ending states.

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