Atomic Structure and electron transition

1. Which of the following best explains why Rutherford’s gold foil experiment disproved the plum pudding model of the atom?

• Most alpha particles were deflected, suggesting the atom has a dense, positively charged center.

• A small number of alpha particles were deflected at large angles, indicating a central nucleus.

• Electrons were shown to orbit the nucleus in fixed energy levels.

• The mass of the atom was evenly distributed, consistent with a uniform positive charge.

Answer explanation

Explanation: Rutherford’s experiment showed that some alpha particles bounced back, which would be impossible in the plum pudding model. This led to the nuclear model with a dense, central nucleus.

2. Why did Bohr modify Rutherford’s atomic model, and what was the most revolutionary aspect of Bohr’s proposal?

• To explain the photoelectric effect; he introduced wave-particle duality.

• To incorporate quantum theory; he proposed that electrons occupy fixed energy levels.

• To address atomic mass; he added neutrons to the nucleus.

• To explain atomic bonding; he proposed electron sharing in outer shells.

Answer explanation

Explanation: Bohr added quantum ideas to Rutherford’s model by suggesting that electrons orbit the nucleus in fixed energy levels, explaining discrete spectral lines.

3. Which of the following statements about Dalton’s atomic theory is inconsistent with modern understanding of the atom?

• Atoms of the same element are identical in mass and properties.

• Atoms cannot be subdivided or destroyed in chemical reactions.

• Atoms of different elements combine in simple whole-number ratios.

• Chemical reactions involve rearrangement of atoms.

Answer explanation

Explanation: We now know that isotopes exist — atoms of the same element with different masses, so not all atoms of an element are identical.

4. 4. Which statement correctly describes the energy level labeled n = 1 in a hydrogen atom?

• It is the energy level furthest from the nucleus and has the highest energy.

• It is the energy level closest to the nucleus and has the lowest energy.

• It corresponds to the ionized state of the atom.

• It is the most unstable energy level.

Answer explanation

Explanation:

In hydrogen, the n = 1 level is the lowest energy level (most negative value, −13.6 eV) and is closest to the nucleus. As n increases, the electron is further away and less tightly bound (energy approaches 0 from below).

5. 5. What happens when an electron in a hydrogen atom transitions from n = 3 to n = 1?

• The atom absorbs a photon of energy 12.09 eV.

• A photon is emitted with energy equal to the difference between energy levels.

•  The electron is ionized from the atom.

• No energy change occurs because both states are bound

Answer explanation

Explanation:

When an electron falls from a higher to a lower energy level, it emits a photon. The energy of this photon equals the difference between the two energy levels.

6. Which transition represents the ionization of a hydrogen atom from its ground state?

• From n = 1 to n = 2

• From n = 2 to n = 3

•  From n = 1 to n = ∞ 

• From n = ∞ to n = 1

Answer explanation

Explanation:

Ionization occurs when the electron gains enough energy to leave the atom completely. In hydrogen, this means going from n = 1 (−13.6 eV) to n = ∞ (0 eV).

7. Which of the following correctly ranks the energy levels of a hydrogen atom in increasing order?

•  n = ∞ < n = 2 < n = 1

• n = 1 < n = 2 < n = 3

• n = 3 < n = 2 < n = 1

• n = 1 = n = 2 = n = 3

Answer explanation

Explanation:

In hydrogen, energy levels are negative and become less negative (closer to 0) as n increases.

So, E₁ < E₂ < E₃ (numerically: −13.6 < −3.40 < −1.51 eV).

8. 8. The energy of the n = 2 level in hydrogen is −3.40 eV. What is the energy of the photon emitted when the electron falls from n = 3 to n = 2?

•  1.89 eV

• 10.2 eV

• 5.10 eV

• 0.85 eV

Answer explanation

Explanation:

The energy at n = 3 is −1.51 eV.

Photon energy = E₃ − E₂ = (−1.51) − (−3.40) = 1.89 eV.

 9. In the hydrogen atom, which electron transition emits the highest energy photon?

•  n = 3 → n = 2

• n = 2 → n = 1 

•  n = 4 → n = 3

• n = 5 → n = 4

Answer explanation

Explanation:

The energy difference is greatest between n = 2 and n = 1:

−3.40 eV to −13.6 eV = 10.2 eV, which is higher than all the other options.

10. An electron in a hydrogen atom makes a transition from energy level n = 4 to n = 2. Which of the following is true about the photon emitted?

• The photon has higher energy than a transition from n = 3 to n = 2

• The photon’s frequency is lower than that from a n = 3 to n = 2 transition

•  The emitted photon corresponds to the absorption spectrum line from n = 2 to n = 4

• No photon is emitted in this transition

Answer explanation

In a transition from n = 4 to n = 2, the energy change is greater than from n = 3 to n = 2, so the photon has higher energy and frequency. Option C is tempting but refers to absorption, not emission.