Great question! Since the Franck-Hertz experiment hinges on observing discrete atomic energy level transitions (and we need to avoid the messy continuous energy states that come with molecules), these three elements check all the boxes for reliable, easy-to-observe results:

Mercury #

• Vapor control simplicity: Mercury is a liquid at room temperature—heat it to around 100°C, and you get a steady, adjustable vapor pressure. No fancy high-vacuum or complex gas handling systems needed, which makes the experimental setup way less hassle.
• Well-matched energy gap: Its first excited state sits 4.9 eV above the ground state, a value that’s perfectly within the range of standard electron acceleration voltages. When electrons transfer this exact energy to mercury atoms, the atoms emit UV light (detectable via phototubes or fluorescent screens), and the resulting current dips are sharp, making the experiment’s signature peaks and valleys super easy to interpret.
• High atomic mass: Heavier atoms mean electrons collide with them in a way that transfers energy only in discrete, well-defined chunks—no random partial energy losses that would blur the experimental curve.

Neon #

• No heating required: Neon is a gas at room temperature, so you just fill the tube and go. This makes it ideal for classroom demos or quick, low-fuss experiments.
• Visible, tangible emission: Its first excited state transition emits bright orange-red light that’s visible to the naked eye. You don’t need special detectors to see the energy transfer happening—this makes the experiment’s core concept feel concrete for learners.
• Chemically inert: As a noble gas, neon doesn’t react with electrodes or tube materials, so you get consistent, repeatable results every time you run the experiment.

Argon #

• Cheap and accessible: Argon is one of the most abundant noble gases, so it’s low-cost and easy to source for labs of all sizes.
• Strictly discrete energy levels: Like all noble gases, argon has a full outer electron shell, so its atomic energy levels are completely discrete (no continuous molecular states to interfere). Its first excited state is ~11.6 eV, which is still achievable with standard lab power supplies.
• Predictable collision behavior: Electrons colliding with argon atoms transfer energy only at specific, measurable voltages, leading to clear, reproducible current vs. voltage curves that are perfect for data analysis.

All three are monatomic (mercury vapor consists of single Hg atoms, neon/argon are naturally monatomic noble gases), so they fit the experiment’s hard requirement of avoiding molecular continuous energy levels entirely.

内容的提问来源于stack exchange，提问作者Our