Which of the following gases is used to retard the vaporization of the filament in a light bulb?

Step 1: Analyze the question We need a gas placed inside a light bulb that slows the evaporation (vaporization) of the hot tungsten filament. The gas must be unreactive, because the filament runs at about 2500°C or more.

Step 2: Concept

• When a tungsten filament glows, it gets extremely hot and tungsten atoms tend to evaporate, which thins the filament and eventually breaks it.
• Oxygen would make the filament burn (oxidize) and is therefore unsuitable.
• Hydrogen is flammable and reactive, and it conducts heat away well, so it is also unsuitable.
• An inert gas at fairly high pressure reduces the evaporation of the metal, because gas atoms collide with the evaporating tungsten atoms and send some back to the filament.
• Heavier inert gases are better because they slow evaporation more and conduct less heat away. Argon (atomic mass 40) is much heavier than helium (atomic mass 4), and it is cheap and abundant in air (about 0.9%). Bulbs are usually filled with argon, often mixed with a little nitrogen.

Step 3: Eliminate options

• A. Argon: inert, dense, cheap, so it is suitable.
• B. Helium: inert, but light and a good heat conductor, so it carries away more heat and is less effective and more expensive.
• C. Hydrogen: reactive and flammable.
• D. Oxygen: supports combustion and would burn the filament.

Step 4: Answer The correct option is **A.

Tip: Remember "light bulb = lazy gas." Noble gases are the lazy (unreactive) ones, and argon is the one commonly used in bulbs. Neon is used in advertising signs, and helium is used in balloons and airships.