What gives rise to the line spectra observed in atoms?

Step 1: Analyze the question The question asks what causes line spectra (the distinct, bright lines of specific wavelengths seen when an excited gas emits light).

Step 2: The concept According to Bohr's model, electrons in an atom can only occupy certain fixed (quantized) energy levels.

• When an atom is supplied with energy (heat or electric discharge), its electrons jump to higher energy levels. This is excitation.
• Excited states are unstable, so the electron falls back to a lower energy level.
• As it falls, it releases the energy difference as a photon of light:

E = E_higher − E_lower = hf

Since the energy levels are fixed, only certain values of ΔE are possible. This means only certain frequencies (wavelengths) are emitted, and each shows up as a sharp line in the spectrum.

Step 3: Evaluate the options

• A. An electron moving from a higher to a lower energy level emits a photon of definite energy, producing a spectral line. ✔
• B. Disturbed protons in the nucleus relate to nuclear processes such as gamma emission, not atomic line spectra. ✘
• C. Excitation alone only raises the electron to a higher level. Light is emitted when it returns down, so excitation by itself does not produce the line. ✘
• D. The kinetic energy of a moving atom gives a continuous range of energies, not discrete lines. ✘
• E. The potential energy of an electron is not what produces emitted radiation. ✘

Answer: A

Tip: Remember "Excite up, emit down." Absorption (going up) gives dark lines in an absorption spectrum, while emission (falling down) gives bright lines in an emission spectrum. The key phrase for line spectra is always electron transitions between energy levels.